The Linux-USB Host Side API¶

Introduction to USB on Linux¶

A Universal Serial Bus (USB) is used to connect a host, such as a PC or workstation, to a number of peripheral devices. USB uses a tree structure, with the host as the root (the system’s master), hubs as interior nodes, and peripherals as leaves (and slaves). Modern PCs support several such trees of USB devices, usually a few USB 3.0 (5 GBit/s) or USB 3.1 (10 GBit/s) and some legacy USB 2.0 (480 MBit/s) busses just in case.

That master/slave asymmetry was designed-in for a number of reasons, one being ease of use. It is not physically possible to mistake upstream and downstream or it does not matter with a type C plug (or they are built into the peripheral). Also, the host software doesn’t need to deal with distributed auto-configuration since the pre-designated master node manages all that.

Kernel developers added USB support to Linux early in the 2.2 kernel series and have been developing it further since then. Besides support for each new generation of USB, various host controllers gained support, new drivers for peripherals have been added and advanced features for latency measurement and improved power management introduced.

Linux can run inside USB devices as well as on the hosts that control the devices. But USB device drivers running inside those peripherals don’t do the same things as the ones running inside hosts, so they’ve been given a different name: gadget drivers. This document does not cover gadget drivers.

USB Host-Side API Model¶

Host-side drivers for USB devices talk to the “usbcore” APIs. There are two. One is intended for general-purpose drivers (exposed through driver frameworks), and the other is for drivers that are part of the core. Such core drivers include the hub driver (which manages trees of USB devices) and several different kinds of host controller drivers, which control individual busses.

The device model seen by USB drivers is relatively complex.

USB supports four kinds of data transfers (control, bulk, interrupt, and isochronous). Two of them (control and bulk) use bandwidth as it’s available, while the other two (interrupt and isochronous) are scheduled to provide guaranteed bandwidth.
The device description model includes one or more “configurations” per device, only one of which is active at a time. Devices are supposed to be capable of operating at lower than their top speeds and may provide a BOS descriptor showing the lowest speed they remain fully operational at.
From USB 3.0 on configurations have one or more “functions”, which provide a common functionality and are grouped together for purposes of power management.
Configurations or functions have one or more “interfaces”, each of which may have “alternate settings”. Interfaces may be standardized by USB “Class” specifications, or may be specific to a vendor or device.

USB device drivers actually bind to interfaces, not devices. Think of them as “interface drivers”, though you may not see many devices where the distinction is important. Most USB devices are simple, with only one function, one configuration, one interface, and one alternate setting.
Interfaces have one or more “endpoints”, each of which supports one type and direction of data transfer such as “bulk out” or “interrupt in”. The entire configuration may have up to sixteen endpoints in each direction, allocated as needed among all the interfaces.
Data transfer on USB is packetized; each endpoint has a maximum packet size. Drivers must often be aware of conventions such as flagging the end of bulk transfers using “short” (including zero length) packets.
The Linux USB API supports synchronous calls for control and bulk messages. It also supports asynchronous calls for all kinds of data transfer, using request structures called “URBs” (USB Request Blocks).

Accordingly, the USB Core API exposed to device drivers covers quite a lot of territory. You’ll probably need to consult the USB 3.0 specification, available online from www.usb.org at no cost, as well as class or device specifications.

The only host-side drivers that actually touch hardware (reading/writing registers, handling IRQs, and so on) are the HCDs. In theory, all HCDs provide the same functionality through the same API. In practice, that’s becoming more true, but there are still differences that crop up especially with fault handling on the less common controllers. Different controllers don’t necessarily report the same aspects of failures, and recovery from faults (including software-induced ones like unlinking an URB) isn’t yet fully consistent. Device driver authors should make a point of doing disconnect testing (while the device is active) with each different host controller driver, to make sure drivers don’t have bugs of their own as well as to make sure they aren’t relying on some HCD-specific behavior.

USB-Standard Types¶

In <linux/usb/ch9.h> you will find the USB data types defined in chapter 9 of the USB specification. These data types are used throughout USB, and in APIs including this host side API, gadget APIs, usb character devices and debugfs interfaces.

const char *usb_ep_type_string(int ep_type)¶: Returns human readable-name of the endpoint type.

Parameters

int ep_type: The endpoint type to return human-readable name for. If it’s not any of the types: USB_ENDPOINT_XFER_{CONTROL, ISOC, BULK, INT}, usually got by usb_endpoint_type(), the string ‘unknown’ will be returned.

const char *usb_speed_string(enum usb_device_speed speed)¶: Returns human readable-name of the speed.

Parameters

enum usb_device_speed speed: The speed to return human-readable name for. If it’s not any of the speeds defined in usb_device_speed enum, string for USB_SPEED_UNKNOWN will be returned.

enum usb_device_speed usb_get_maximum_speed(struct device *dev)¶: Get maximum requested speed for a given USB controller.

Parameters

struct device * dev: Pointer to the given USB controller device

Description

The function gets the maximum speed string from property “maximum-speed”, and returns the corresponding enum usb_device_speed.

const char *usb_state_string(enum usb_device_state state)¶: Returns human readable name for the state.

Parameters

enum usb_device_state state: The state to return a human-readable name for. If it’s not any of the states devices in usb_device_state_string enum, the string UNKNOWN will be returned.

const char *usb_decode_ctrl(char *str, size_t size, __u8 bRequestType, __u8 bRequest, __u16 wValue, __u16 wIndex, __u16 wLength)¶: Returns human readable representation of control request.

Parameters

char * str: buffer to return a human-readable representation of control request. This buffer should have about 200 bytes.
size_t size: size of str buffer.
__u8 bRequestType: matches the USB bmRequestType field
__u8 bRequest: matches the USB bRequest field
__u16 wValue: matches the USB wValue field (CPU byte order)
__u16 wIndex: matches the USB wIndex field (CPU byte order)
__u16 wLength: matches the USB wLength field (CPU byte order)

Description

Function returns decoded, formatted and human-readable description of control request packet.

The usage scenario for this is for tracepoints, so function as a return use the same value as in parameters. This approach allows to use this function in TP_printk

Important: wValue, wIndex, wLength parameters before invoking this function should be processed by le16_to_cpu macro.

Host-Side Data Types and Macros¶

The host side API exposes several layers to drivers, some of which are more necessary than others. These support lifecycle models for host side drivers and devices, and support passing buffers through usbcore to some HCD that performs the I/O for the device driver.

struct usb_host_endpoint: host-side endpoint descriptor and queue

Definition

struct usb_host_endpoint {
  struct usb_endpoint_descriptor          desc;
  struct usb_ss_ep_comp_descriptor        ss_ep_comp;
  struct usb_ssp_isoc_ep_comp_descriptor  ssp_isoc_ep_comp;
  struct list_head                urb_list;
  void *hcpriv;
  struct ep_device                *ep_dev;
  unsigned char *extra;
  int extralen;
  int enabled;
  int streams;
};

Members

desc: descriptor for this endpoint, wMaxPacketSize in native byteorder
ss_ep_comp: SuperSpeed companion descriptor for this endpoint
ssp_isoc_ep_comp: SuperSpeedPlus isoc companion descriptor for this endpoint
urb_list: urbs queued to this endpoint; maintained by usbcore
hcpriv: for use by HCD; typically holds hardware dma queue head (QH) with one or more transfer descriptors (TDs) per urb
ep_dev: ep_device for sysfs info
extra: descriptors following this endpoint in the configuration
extralen: how many bytes of “extra” are valid
enabled: URBs may be submitted to this endpoint
streams: number of USB-3 streams allocated on the endpoint

Description

USB requests are always queued to a given endpoint, identified by a descriptor within an active interface in a given USB configuration.

struct usb_interface: what usb device drivers talk to

Definition

struct usb_interface {
  struct usb_host_interface *altsetting;
  struct usb_host_interface *cur_altsetting;
  unsigned num_altsetting;
  struct usb_interface_assoc_descriptor *intf_assoc;
  int minor;
  enum usb_interface_condition condition;
  unsigned sysfs_files_created:1;
  unsigned ep_devs_created:1;
  unsigned unregistering:1;
  unsigned needs_remote_wakeup:1;
  unsigned needs_altsetting0:1;
  unsigned needs_binding:1;
  unsigned resetting_device:1;
  unsigned authorized:1;
  struct device dev;
  struct device *usb_dev;
  struct work_struct reset_ws;
};

Members

altsetting: array of interface structures, one for each alternate setting that may be selected. Each one includes a set of endpoint configurations. They will be in no particular order.
cur_altsetting: the current altsetting.
num_altsetting: number of altsettings defined.
intf_assoc: interface association descriptor
minor: the minor number assigned to this interface, if this interface is bound to a driver that uses the USB major number. If this interface does not use the USB major, this field should be unused. The driver should set this value in the probe() function of the driver, after it has been assigned a minor number from the USB core by calling usb_register_dev().
condition: binding state of the interface: not bound, binding (in probe()), bound to a driver, or unbinding (in disconnect())
sysfs_files_created: sysfs attributes exist
ep_devs_created: endpoint child pseudo-devices exist
unregistering: flag set when the interface is being unregistered
needs_remote_wakeup: flag set when the driver requires remote-wakeup capability during autosuspend.
needs_altsetting0: flag set when a set-interface request for altsetting 0 has been deferred.
needs_binding: flag set when the driver should be re-probed or unbound following a reset or suspend operation it doesn’t support.
resetting_device: USB core reset the device, so use alt setting 0 as current; needs bandwidth alloc after reset.
authorized: This allows to (de)authorize individual interfaces instead a whole device in contrast to the device authorization.
dev: driver model’s view of this device
usb_dev: if an interface is bound to the USB major, this will point to the sysfs representation for that device.
reset_ws: Used for scheduling resets from atomic context.

Description

USB device drivers attach to interfaces on a physical device. Each interface encapsulates a single high level function, such as feeding an audio stream to a speaker or reporting a change in a volume control. Many USB devices only have one interface. The protocol used to talk to an interface’s endpoints can be defined in a usb “class” specification, or by a product’s vendor. The (default) control endpoint is part of every interface, but is never listed among the interface’s descriptors.

The driver that is bound to the interface can use standard driver model calls such as dev_get_drvdata() on the dev member of this structure.

Each interface may have alternate settings. The initial configuration of a device sets altsetting 0, but the device driver can change that setting using usb_set_interface(). Alternate settings are often used to control the use of periodic endpoints, such as by having different endpoints use different amounts of reserved USB bandwidth. All standards-conformant USB devices that use isochronous endpoints will use them in non-default settings.

The USB specification says that alternate setting numbers must run from 0 to one less than the total number of alternate settings. But some devices manage to mess this up, and the structures aren’t necessarily stored in numerical order anyhow. Use usb_altnum_to_altsetting() to look up an alternate setting in the altsetting array based on its number.

struct usb_interface_cache: long-term representation of a device interface

Definition

struct usb_interface_cache {
  unsigned num_altsetting;
  struct kref ref;
  struct usb_host_interface altsetting[];
};

Members

num_altsetting: number of altsettings defined.
ref: reference counter.
altsetting: variable-length array of interface structures, one for each alternate setting that may be selected. Each one includes a set of endpoint configurations. They will be in no particular order.

Description

These structures persist for the lifetime of a usb_device, unlike struct usb_interface (which persists only as long as its configuration is installed). The altsetting arrays can be accessed through these structures at any time, permitting comparison of configurations and providing support for the /sys/kernel/debug/usb/devices pseudo-file.

struct usb_host_config: representation of a device’s configuration

Definition

struct usb_host_config {
  struct usb_config_descriptor    desc;
  char *string;
  struct usb_interface_assoc_descriptor *intf_assoc[USB_MAXIADS];
  struct usb_interface *interface[USB_MAXINTERFACES];
  struct usb_interface_cache *intf_cache[USB_MAXINTERFACES];
  unsigned char *extra;
  int extralen;
};

Members

desc: the device’s configuration descriptor.
string: pointer to the cached version of the iConfiguration string, if present for this configuration.
intf_assoc: list of any interface association descriptors in this config
interface: array of pointers to usb_interface structures, one for each interface in the configuration. The number of interfaces is stored in desc.bNumInterfaces. These pointers are valid only while the the configuration is active.
intf_cache: array of pointers to usb_interface_cache structures, one for each interface in the configuration. These structures exist for the entire life of the device.
extra: pointer to buffer containing all extra descriptors associated with this configuration (those preceding the first interface descriptor).
extralen: length of the extra descriptors buffer.

Description

USB devices may have multiple configurations, but only one can be active at any time. Each encapsulates a different operational environment; for example, a dual-speed device would have separate configurations for full-speed and high-speed operation. The number of configurations available is stored in the device descriptor as bNumConfigurations.

A configuration can contain multiple interfaces. Each corresponds to a different function of the USB device, and all are available whenever the configuration is active. The USB standard says that interfaces are supposed to be numbered from 0 to desc.bNumInterfaces-1, but a lot of devices get this wrong. In addition, the interface array is not guaranteed to be sorted in numerical order. Use usb_ifnum_to_if() to look up an interface entry based on its number.

Device drivers should not attempt to activate configurations. The choice of which configuration to install is a policy decision based on such considerations as available power, functionality provided, and the user’s desires (expressed through userspace tools). However, drivers can call usb_reset_configuration() to reinitialize the current configuration and all its interfaces.

struct usb_device: kernel’s representation of a USB device

Definition

struct usb_device {
  int devnum;
  char devpath[16];
  u32 route;
  enum usb_device_state   state;
  enum usb_device_speed   speed;
  unsigned int            rx_lanes;
  unsigned int            tx_lanes;
  struct usb_tt   *tt;
  int ttport;
  unsigned int toggle[2];
  struct usb_device *parent;
  struct usb_bus *bus;
  struct usb_host_endpoint ep0;
  struct device dev;
  struct usb_device_descriptor descriptor;
  struct usb_host_bos *bos;
  struct usb_host_config *config;
  struct usb_host_config *actconfig;
  struct usb_host_endpoint *ep_in[16];
  struct usb_host_endpoint *ep_out[16];
  char **rawdescriptors;
  unsigned short bus_mA;
  u8 portnum;
  u8 level;
  u8 devaddr;
  unsigned can_submit:1;
  unsigned persist_enabled:1;
  unsigned have_langid:1;
  unsigned authorized:1;
  unsigned authenticated:1;
  unsigned wusb:1;
  unsigned lpm_capable:1;
  unsigned usb2_hw_lpm_capable:1;
  unsigned usb2_hw_lpm_besl_capable:1;
  unsigned usb2_hw_lpm_enabled:1;
  unsigned usb2_hw_lpm_allowed:1;
  unsigned usb3_lpm_u1_enabled:1;
  unsigned usb3_lpm_u2_enabled:1;
  int string_langid;
  char *product;
  char *manufacturer;
  char *serial;
  struct list_head filelist;
  int maxchild;
  u32 quirks;
  atomic_t urbnum;
  unsigned long active_duration;
#ifdef CONFIG_PM;
  unsigned long connect_time;
  unsigned do_remote_wakeup:1;
  unsigned reset_resume:1;
  unsigned port_is_suspended:1;
#endif;
  struct wusb_dev *wusb_dev;
  int slot_id;
  enum usb_device_removable removable;
  struct usb2_lpm_parameters l1_params;
  struct usb3_lpm_parameters u1_params;
  struct usb3_lpm_parameters u2_params;
  unsigned lpm_disable_count;
  u16 hub_delay;
  unsigned use_generic_driver:1;
};

Members

devnum: device number; address on a USB bus
devpath: device ID string for use in messages (e.g., /port/…)
route: tree topology hex string for use with xHCI
state: device state: configured, not attached, etc.
speed: device speed: high/full/low (or error)
rx_lanes: number of rx lanes in use, USB 3.2 adds dual-lane support
tx_lanes: number of tx lanes in use, USB 3.2 adds dual-lane support
tt: Transaction Translator info; used with low/full speed dev, highspeed hub
ttport: device port on that tt hub
toggle: one bit for each endpoint, with ([0] = IN, [1] = OUT) endpoints
parent: our hub, unless we’re the root
bus: bus we’re part of
ep0: endpoint 0 data (default control pipe)
dev: generic device interface
descriptor: USB device descriptor
bos: USB device BOS descriptor set
config: all of the device’s configs
actconfig: the active configuration
ep_in: array of IN endpoints
ep_out: array of OUT endpoints
rawdescriptors: raw descriptors for each config
bus_mA: Current available from the bus
portnum: parent port number (origin 1)
level: number of USB hub ancestors
devaddr: device address, XHCI: assigned by HW, others: same as devnum
can_submit: URBs may be submitted
persist_enabled: USB_PERSIST enabled for this device
have_langid: whether string_langid is valid
authorized: policy has said we can use it; (user space) policy determines if we authorize this device to be used or not. By default, wired USB devices are authorized. WUSB devices are not, until we authorize them from user space. FIXME – complete doc
authenticated: Crypto authentication passed
wusb: device is Wireless USB
lpm_capable: device supports LPM
usb2_hw_lpm_capable: device can perform USB2 hardware LPM
usb2_hw_lpm_besl_capable: device can perform USB2 hardware BESL LPM
usb2_hw_lpm_enabled: USB2 hardware LPM is enabled
usb2_hw_lpm_allowed: Userspace allows USB 2.0 LPM to be enabled
usb3_lpm_u1_enabled: USB3 hardware U1 LPM enabled
usb3_lpm_u2_enabled: USB3 hardware U2 LPM enabled
string_langid: language ID for strings
product: iProduct string, if present (static)
manufacturer: iManufacturer string, if present (static)
serial: iSerialNumber string, if present (static)
filelist: usbfs files that are open to this device
maxchild: number of ports if hub
quirks: quirks of the whole device
urbnum: number of URBs submitted for the whole device
active_duration: total time device is not suspended
connect_time: time device was first connected
do_remote_wakeup: remote wakeup should be enabled
reset_resume: needs reset instead of resume
port_is_suspended: the upstream port is suspended (L2 or U3)
wusb_dev: if this is a Wireless USB device, link to the WUSB specific data for the device.
slot_id: Slot ID assigned by xHCI
removable: Device can be physically removed from this port
l1_params: best effor service latency for USB2 L1 LPM state, and L1 timeout.
u1_params: exit latencies for USB3 U1 LPM state, and hub-initiated timeout.
u2_params: exit latencies for USB3 U2 LPM state, and hub-initiated timeout.
lpm_disable_count: Ref count used by usb_disable_lpm() and usb_enable_lpm() to keep track of the number of functions that require USB 3.0 Link Power Management to be disabled for this usb_device. This count should only be manipulated by those functions, with the bandwidth_mutex is held.
hub_delay: cached value consisting of: parent->hub_delay + wHubDelay + tTPTransmissionDelay (40ns)

Description

Will be used as wValue for SetIsochDelay requests.

Notes

Usbcore drivers should not set usbdev->state directly. Instead use usb_set_device_state().

usb_hub_for_each_child ( hdev, port1, child): iterate over all child devices on the hub

Parameters

hdev: USB device belonging to the usb hub
port1: portnum associated with child device
child: child device pointer

int usb_interface_claimed(struct usb_interface *iface)¶: returns true iff an interface is claimed

Parameters

struct usb_interface * iface: the interface being checked

Return

true (nonzero) iff the interface is claimed, else false (zero).

Note

Callers must own the driver model’s usb bus readlock. So driver probe() entries don’t need extra locking, but other call contexts may need to explicitly claim that lock.

int usb_make_path(struct usb_device *dev, char *buf, size_t size)¶: returns stable device path in the usb tree

Parameters

struct usb_device * dev: the device whose path is being constructed
char * buf: where to put the string
size_t size: how big is “buf”?

Return

Length of the string (> 0) or negative if size was too small.

Note

This identifier is intended to be “stable”, reflecting physical paths in hardware such as physical bus addresses for host controllers or ports on USB hubs. That makes it stay the same until systems are physically reconfigured, by re-cabling a tree of USB devices or by moving USB host controllers. Adding and removing devices, including virtual root hubs in host controller driver modules, does not change these path identifiers; neither does rebooting or re-enumerating. These are more useful identifiers than changeable (“unstable”) ones like bus numbers or device addresses.

With a partial exception for devices connected to USB 2.0 root hubs, these identifiers are also predictable. So long as the device tree isn’t changed, plugging any USB device into a given hub port always gives it the same path. Because of the use of “companion” controllers, devices connected to ports on USB 2.0 root hubs (EHCI host controllers) will get one path ID if they are high speed, and a different one if they are full or low speed.

USB_DEVICE ( vend, prod): macro used to describe a specific usb device

Parameters

vend: the 16 bit USB Vendor ID
prod: the 16 bit USB Product ID

Description

This macro is used to create a struct usb_device_id that matches a specific device.

USB_DEVICE_VER ( vend, prod, lo, hi): describe a specific usb device with a version range

Parameters

vend: the 16 bit USB Vendor ID
prod: the 16 bit USB Product ID
lo: the bcdDevice_lo value
hi: the bcdDevice_hi value

Description

This macro is used to create a struct usb_device_id that matches a specific device, with a version range.

USB_DEVICE_INTERFACE_CLASS ( vend, prod, cl): describe a usb device with a specific interface class

Parameters

vend: the 16 bit USB Vendor ID
prod: the 16 bit USB Product ID
cl: bInterfaceClass value

Description

This macro is used to create a struct usb_device_id that matches a specific interface class of devices.

USB_DEVICE_INTERFACE_PROTOCOL ( vend, prod, pr): describe a usb device with a specific interface protocol

Parameters

vend: the 16 bit USB Vendor ID
prod: the 16 bit USB Product ID
pr: bInterfaceProtocol value

Description

This macro is used to create a struct usb_device_id that matches a specific interface protocol of devices.

USB_DEVICE_INTERFACE_NUMBER ( vend, prod, num): describe a usb device with a specific interface number

Parameters

vend: the 16 bit USB Vendor ID
prod: the 16 bit USB Product ID
num: bInterfaceNumber value

Description

This macro is used to create a struct usb_device_id that matches a specific interface number of devices.

USB_DEVICE_INFO ( cl, sc, pr): macro used to describe a class of usb devices

Parameters

cl: bDeviceClass value
sc: bDeviceSubClass value
pr: bDeviceProtocol value

Description

This macro is used to create a struct usb_device_id that matches a specific class of devices.

USB_INTERFACE_INFO ( cl, sc, pr): macro used to describe a class of usb interfaces

Parameters

cl: bInterfaceClass value
sc: bInterfaceSubClass value
pr: bInterfaceProtocol value

Description

This macro is used to create a struct usb_device_id that matches a specific class of interfaces.

USB_DEVICE_AND_INTERFACE_INFO ( vend, prod, cl, sc, pr): describe a specific usb device with a class of usb interfaces

Parameters

vend: the 16 bit USB Vendor ID
prod: the 16 bit USB Product ID
cl: bInterfaceClass value
sc: bInterfaceSubClass value
pr: bInterfaceProtocol value

Description

This macro is used to create a struct usb_device_id that matches a specific device with a specific class of interfaces.

This is especially useful when explicitly matching devices that have vendor specific bDeviceClass values, but standards-compliant interfaces.

USB_VENDOR_AND_INTERFACE_INFO ( vend, cl, sc, pr): describe a specific usb vendor with a class of usb interfaces

Parameters

vend: the 16 bit USB Vendor ID
cl: bInterfaceClass value
sc: bInterfaceSubClass value
pr: bInterfaceProtocol value

Description

This macro is used to create a struct usb_device_id that matches a specific vendor with a specific class of interfaces.

This is especially useful when explicitly matching devices that have vendor specific bDeviceClass values, but standards-compliant interfaces.

struct usbdrv_wrap: wrapper for driver-model structure

Definition

struct usbdrv_wrap {
  struct device_driver driver;
  int for_devices;
};

Members

driver: The driver-model core driver structure.
for_devices: Non-zero for device drivers, 0 for interface drivers.

struct usb_driver: identifies USB interface driver to usbcore

Definition

struct usb_driver {
  const char *name;
  int (*probe) (struct usb_interface *intf, const struct usb_device_id *id);
  void (*disconnect) (struct usb_interface *intf);
  int (*unlocked_ioctl) (struct usb_interface *intf, unsigned int code, void *buf);
  int (*suspend) (struct usb_interface *intf, pm_message_t message);
  int (*resume) (struct usb_interface *intf);
  int (*reset_resume)(struct usb_interface *intf);
  int (*pre_reset)(struct usb_interface *intf);
  int (*post_reset)(struct usb_interface *intf);
  const struct usb_device_id *id_table;
  const struct attribute_group **dev_groups;
  struct usb_dynids dynids;
  struct usbdrv_wrap drvwrap;
  unsigned int no_dynamic_id:1;
  unsigned int supports_autosuspend:1;
  unsigned int disable_hub_initiated_lpm:1;
  unsigned int soft_unbind:1;
};

Members

name: The driver name should be unique among USB drivers, and should normally be the same as the module name.
probe: Called to see if the driver is willing to manage a particular interface on a device. If it is, probe returns zero and uses usb_set_intfdata() to associate driver-specific data with the interface. It may also use usb_set_interface() to specify the appropriate altsetting. If unwilling to manage the interface, return -ENODEV, if genuine IO errors occurred, an appropriate negative errno value.
disconnect: Called when the interface is no longer accessible, usually because its device has been (or is being) disconnected or the driver module is being unloaded.
unlocked_ioctl: Used for drivers that want to talk to userspace through the “usbfs” filesystem. This lets devices provide ways to expose information to user space regardless of where they do (or don’t) show up otherwise in the filesystem.
suspend: Called when the device is going to be suspended by the system either from system sleep or runtime suspend context. The return value will be ignored in system sleep context, so do NOT try to continue using the device if suspend fails in this case. Instead, let the resume or reset-resume routine recover from the failure.
resume: Called when the device is being resumed by the system.
reset_resume: Called when the suspended device has been reset instead of being resumed.
pre_reset: Called by usb_reset_device() when the device is about to be reset. This routine must not return until the driver has no active URBs for the device, and no more URBs may be submitted until the post_reset method is called.
post_reset: Called by usb_reset_device() after the device has been reset
id_table: USB drivers use ID table to support hotplugging. Export this with MODULE_DEVICE_TABLE(usb,…). This must be set or your driver’s probe function will never get called.
dev_groups: Attributes attached to the device that will be created once it is bound to the driver.
dynids: used internally to hold the list of dynamically added device ids for this driver.
drvwrap: Driver-model core structure wrapper.
no_dynamic_id: if set to 1, the USB core will not allow dynamic ids to be added to this driver by preventing the sysfs file from being created.
supports_autosuspend: if set to 0, the USB core will not allow autosuspend for interfaces bound to this driver.
disable_hub_initiated_lpm: if set to 1, the USB core will not allow hubs to initiate lower power link state transitions when an idle timeout occurs. Device-initiated USB 3.0 link PM will still be allowed.
soft_unbind: if set to 1, the USB core will not kill URBs and disable endpoints before calling the driver’s disconnect method.

Description

USB interface drivers must provide a name, probe() and disconnect() methods, and an id_table. Other driver fields are optional.

The id_table is used in hotplugging. It holds a set of descriptors, and specialized data may be associated with each entry. That table is used by both user and kernel mode hotplugging support.

The probe() and disconnect() methods are called in a context where they can sleep, but they should avoid abusing the privilege. Most work to connect to a device should be done when the device is opened, and undone at the last close. The disconnect code needs to address concurrency issues with respect to open() and close() methods, as well as forcing all pending I/O requests to complete (by unlinking them as necessary, and blocking until the unlinks complete).

struct usb_device_driver: identifies USB device driver to usbcore

Definition

struct usb_device_driver {
  const char *name;
  bool (*match) (struct usb_device *udev);
  int (*probe) (struct usb_device *udev);
  void (*disconnect) (struct usb_device *udev);
  int (*suspend) (struct usb_device *udev, pm_message_t message);
  int (*resume) (struct usb_device *udev, pm_message_t message);
  const struct attribute_group **dev_groups;
  struct usbdrv_wrap drvwrap;
  const struct usb_device_id *id_table;
  unsigned int supports_autosuspend:1;
  unsigned int generic_subclass:1;
};

Members

name: The driver name should be unique among USB drivers, and should normally be the same as the module name.
probe: Called to see if the driver is willing to manage a particular device. If it is, probe returns zero and uses dev_set_drvdata() to associate driver-specific data with the device. If unwilling to manage the device, return a negative errno value.
disconnect: Called when the device is no longer accessible, usually because it has been (or is being) disconnected or the driver’s module is being unloaded.
suspend: Called when the device is going to be suspended by the system.
resume: Called when the device is being resumed by the system.
dev_groups: Attributes attached to the device that will be created once it is bound to the driver.
drvwrap: Driver-model core structure wrapper.
supports_autosuspend: if set to 0, the USB core will not allow autosuspend for devices bound to this driver.
generic_subclass: if set to 1, the generic USB driver’s probe, disconnect, resume and suspend functions will be called in addition to the driver’s own, so this part of the setup does not need to be replicated.

Description

USB drivers must provide all the fields listed above except drvwrap.

struct usb_class_driver: identifies a USB driver that wants to use the USB major number

Definition

struct usb_class_driver {
  char *name;
  char *(*devnode)(struct device *dev, umode_t *mode);
  const struct file_operations *fops;
  int minor_base;
};

Members

name: the usb class device name for this driver. Will show up in sysfs.
devnode: Callback to provide a naming hint for a possible device node to create.
fops: pointer to the struct file_operations of this driver.
minor_base: the start of the minor range for this driver.

Description

This structure is used for the usb_register_dev() and usb_deregister_dev() functions, to consolidate a number of the parameters used for them.

module_usb_driver ( __usb_driver): Helper macro for registering a USB driver

Parameters

__usb_driver: usb_driver struct

Description

Helper macro for USB drivers which do not do anything special in module init/exit. This eliminates a lot of boilerplate. Each module may only use this macro once, and calling it replaces module_init() and module_exit()

struct urb: USB Request Block

Definition

struct urb {
  struct list_head urb_list;
  struct list_head anchor_list;
  struct usb_anchor *anchor;
  struct usb_device *dev;
  struct usb_host_endpoint *ep;
  unsigned int pipe;
  unsigned int stream_id;
  int status;
  unsigned int transfer_flags;
  void *transfer_buffer;
  dma_addr_t transfer_dma;
  struct scatterlist *sg;
  int num_mapped_sgs;
  int num_sgs;
  u32 transfer_buffer_length;
  u32 actual_length;
  unsigned char *setup_packet;
  dma_addr_t setup_dma;
  int start_frame;
  int number_of_packets;
  int interval;
  int error_count;
  void *context;
  usb_complete_t complete;
  struct usb_iso_packet_descriptor iso_frame_desc[];
};

Members

urb_list: For use by current owner of the URB.
anchor_list: membership in the list of an anchor
anchor: to anchor URBs to a common mooring
dev: Identifies the USB device to perform the request.
ep: Points to the endpoint’s data structure. Will eventually replace pipe.
pipe: Holds endpoint number, direction, type, and more. Create these values with the eight macros available; usb_{snd,rcv}TYPEpipe(dev,endpoint), where the TYPE is “ctrl” (control), “bulk”, “int” (interrupt), or “iso” (isochronous). For example usb_sndbulkpipe() or usb_rcvintpipe(). Endpoint numbers range from zero to fifteen. Note that “in” endpoint two is a different endpoint (and pipe) from “out” endpoint two. The current configuration controls the existence, type, and maximum packet size of any given endpoint.
stream_id: the endpoint’s stream ID for bulk streams
status: This is read in non-iso completion functions to get the status of the particular request. ISO requests only use it to tell whether the URB was unlinked; detailed status for each frame is in the fields of the iso_frame-desc.
transfer_flags: A variety of flags may be used to affect how URB submission, unlinking, or operation are handled. Different kinds of URB can use different flags.
transfer_buffer: This identifies the buffer to (or from) which the I/O request will be performed unless URB_NO_TRANSFER_DMA_MAP is set (however, do not leave garbage in transfer_buffer even then). This buffer must be suitable for DMA; allocate it with kmalloc() or equivalent. For transfers to “in” endpoints, contents of this buffer will be modified. This buffer is used for the data stage of control transfers.
transfer_dma: When transfer_flags includes URB_NO_TRANSFER_DMA_MAP, the device driver is saying that it provided this DMA address, which the host controller driver should use in preference to the transfer_buffer.
sg: scatter gather buffer list, the buffer size of each element in the list (except the last) must be divisible by the endpoint’s max packet size if no_sg_constraint isn’t set in ‘struct usb_bus’
num_mapped_sgs: (internal) number of mapped sg entries
num_sgs: number of entries in the sg list
transfer_buffer_length: How big is transfer_buffer. The transfer may be broken up into chunks according to the current maximum packet size for the endpoint, which is a function of the configuration and is encoded in the pipe. When the length is zero, neither transfer_buffer nor transfer_dma is used.
actual_length: This is read in non-iso completion functions, and it tells how many bytes (out of transfer_buffer_length) were transferred. It will normally be the same as requested, unless either an error was reported or a short read was performed. The URB_SHORT_NOT_OK transfer flag may be used to make such short reads be reported as errors.
setup_packet: Only used for control transfers, this points to eight bytes of setup data. Control transfers always start by sending this data to the device. Then transfer_buffer is read or written, if needed.
setup_dma: DMA pointer for the setup packet. The caller must not use this field; setup_packet must point to a valid buffer.
start_frame: Returns the initial frame for isochronous transfers.
number_of_packets: Lists the number of ISO transfer buffers.
interval: Specifies the polling interval for interrupt or isochronous transfers. The units are frames (milliseconds) for full and low speed devices, and microframes (1/8 millisecond) for highspeed and SuperSpeed devices.
error_count: Returns the number of ISO transfers that reported errors.
context: For use in completion functions. This normally points to request-specific driver context.
complete: Completion handler. This URB is passed as the parameter to the completion function. The completion function may then do what it likes with the URB, including resubmitting or freeing it.
iso_frame_desc: Used to provide arrays of ISO transfer buffers and to collect the transfer status for each buffer.

Description

This structure identifies USB transfer requests. URBs must be allocated by calling usb_alloc_urb() and freed with a call to usb_free_urb(). Initialization may be done using various usb_fill_*_urb() functions. URBs are submitted using usb_submit_urb(), and pending requests may be canceled using usb_unlink_urb() or usb_kill_urb().

Data Transfer Buffers:

Normally drivers provide I/O buffers allocated with kmalloc() or otherwise taken from the general page pool. That is provided by transfer_buffer (control requests also use setup_packet), and host controller drivers perform a dma mapping (and unmapping) for each buffer transferred. Those mapping operations can be expensive on some platforms (perhaps using a dma bounce buffer or talking to an IOMMU), although they’re cheap on commodity x86 and ppc hardware.

Alternatively, drivers may pass the URB_NO_TRANSFER_DMA_MAP transfer flag, which tells the host controller driver that no such mapping is needed for the transfer_buffer since the device driver is DMA-aware. For example, a device driver might allocate a DMA buffer with usb_alloc_coherent() or call usb_buffer_map(). When this transfer flag is provided, host controller drivers will attempt to use the dma address found in the transfer_dma field rather than determining a dma address themselves.

Note that transfer_buffer must still be set if the controller does not support DMA (as indicated by hcd_uses_dma()) and when talking to root hub. If you have to trasfer between highmem zone and the device on such controller, create a bounce buffer or bail out with an error. If transfer_buffer cannot be set (is in highmem) and the controller is DMA capable, assign NULL to it, so that usbmon knows not to use the value. The setup_packet must always be set, so it cannot be located in highmem.

Initialization:

All URBs submitted must initialize the dev, pipe, transfer_flags (may be zero), and complete fields. All URBs must also initialize transfer_buffer and transfer_buffer_length. They may provide the URB_SHORT_NOT_OK transfer flag, indicating that short reads are to be treated as errors; that flag is invalid for write requests.

Bulk URBs may use the URB_ZERO_PACKET transfer flag, indicating that bulk OUT transfers should always terminate with a short packet, even if it means adding an extra zero length packet.

Control URBs must provide a valid pointer in the setup_packet field. Unlike the transfer_buffer, the setup_packet may not be mapped for DMA beforehand.

Interrupt URBs must provide an interval, saying how often (in milliseconds or, for highspeed devices, 125 microsecond units) to poll for transfers. After the URB has been submitted, the interval field reflects how the transfer was actually scheduled. The polling interval may be more frequent than requested. For example, some controllers have a maximum interval of 32 milliseconds, while others support intervals of up to 1024 milliseconds. Isochronous URBs also have transfer intervals. (Note that for isochronous endpoints, as well as high speed interrupt endpoints, the encoding of the transfer interval in the endpoint descriptor is logarithmic. Device drivers must convert that value to linear units themselves.)

If an isochronous endpoint queue isn’t already running, the host controller will schedule a new URB to start as soon as bandwidth utilization allows. If the queue is running then a new URB will be scheduled to start in the first transfer slot following the end of the preceding URB, if that slot has not already expired. If the slot has expired (which can happen when IRQ delivery is delayed for a long time), the scheduling behavior depends on the URB_ISO_ASAP flag. If the flag is clear then the URB will be scheduled to start in the expired slot, implying that some of its packets will not be transferred; if the flag is set then the URB will be scheduled in the first unexpired slot, breaking the queue’s synchronization. Upon URB completion, the start_frame field will be set to the (micro)frame number in which the transfer was scheduled. Ranges for frame counter values are HC-specific and can go from as low as 256 to as high as 65536 frames.

Isochronous URBs have a different data transfer model, in part because the quality of service is only “best effort”. Callers provide specially allocated URBs, with number_of_packets worth of iso_frame_desc structures at the end. Each such packet is an individual ISO transfer. Isochronous URBs are normally queued, submitted by drivers to arrange that transfers are at least double buffered, and then explicitly resubmitted in completion handlers, so that data (such as audio or video) streams at as constant a rate as the host controller scheduler can support.

Completion Callbacks:

The completion callback is made in_interrupt(), and one of the first things that a completion handler should do is check the status field. The status field is provided for all URBs. It is used to report unlinked URBs, and status for all non-ISO transfers. It should not be examined before the URB is returned to the completion handler.

The context field is normally used to link URBs back to the relevant driver or request state.

When the completion callback is invoked for non-isochronous URBs, the actual_length field tells how many bytes were transferred. This field is updated even when the URB terminated with an error or was unlinked.

ISO transfer status is reported in the status and actual_length fields of the iso_frame_desc array, and the number of errors is reported in error_count. Completion callbacks for ISO transfers will normally (re)submit URBs to ensure a constant transfer rate.

Note that even fields marked “public” should not be touched by the driver when the urb is owned by the hcd, that is, since the call to usb_submit_urb() till the entry into the completion routine.

void usb_fill_control_urb(struct urb *urb, struct usb_device *dev, unsigned int pipe, unsigned char *setup_packet, void *transfer_buffer, int buffer_length, usb_complete_t complete_fn, void *context)¶: initializes a control urb

Parameters

struct urb * urb: pointer to the urb to initialize.
struct usb_device * dev: pointer to the struct usb_device for this urb.
unsigned int pipe: the endpoint pipe
unsigned char * setup_packet: pointer to the setup_packet buffer
void * transfer_buffer: pointer to the transfer buffer
int buffer_length: length of the transfer buffer
usb_complete_t complete_fn: pointer to the usb_complete_t function
void * context: what to set the urb context to.

Description

Initializes a control urb with the proper information needed to submit it to a device.

void usb_fill_bulk_urb(struct urb *urb, struct usb_device *dev, unsigned int pipe, void *transfer_buffer, int buffer_length, usb_complete_t complete_fn, void *context)¶: macro to help initialize a bulk urb

Parameters

struct urb * urb: pointer to the urb to initialize.
struct usb_device * dev: pointer to the struct usb_device for this urb.
unsigned int pipe: the endpoint pipe
void * transfer_buffer: pointer to the transfer buffer
int buffer_length: length of the transfer buffer
usb_complete_t complete_fn: pointer to the usb_complete_t function
void * context: what to set the urb context to.

Description

Initializes a bulk urb with the proper information needed to submit it to a device.

void usb_fill_int_urb(struct urb *urb, struct usb_device *dev, unsigned int pipe, void *transfer_buffer, int buffer_length, usb_complete_t complete_fn, void *context, int interval)¶: macro to help initialize a interrupt urb

Parameters

struct urb * urb: pointer to the urb to initialize.
struct usb_device * dev: pointer to the struct usb_device for this urb.
unsigned int pipe: the endpoint pipe
void * transfer_buffer: pointer to the transfer buffer
int buffer_length: length of the transfer buffer
usb_complete_t complete_fn: pointer to the usb_complete_t function
void * context: what to set the urb context to.
int interval: what to set the urb interval to, encoded like the endpoint descriptor’s bInterval value.

Description

Initializes a interrupt urb with the proper information needed to submit it to a device.

Note that High Speed and SuperSpeed(+) interrupt endpoints use a logarithmic encoding of the endpoint interval, and express polling intervals in microframes (eight per millisecond) rather than in frames (one per millisecond).

Wireless USB also uses the logarithmic encoding, but specifies it in units of 128us instead of 125us. For Wireless USB devices, the interval is passed through to the host controller, rather than being translated into microframe units.

int usb_urb_dir_in(struct urb *urb)¶: check if an URB describes an IN transfer

Parameters

struct urb * urb: URB to be checked

Return

1 if urb describes an IN transfer (device-to-host), otherwise 0.

int usb_urb_dir_out(struct urb *urb)¶: check if an URB describes an OUT transfer

Parameters

struct urb * urb: URB to be checked

Return

1 if urb describes an OUT transfer (host-to-device), otherwise 0.

struct usb_sg_request: support for scatter/gather I/O

Definition

struct usb_sg_request {
  int status;
  size_t bytes;
};

Members

status: zero indicates success, else negative errno
bytes: counts bytes transferred.

Description

These requests are initialized using usb_sg_init(), and then are used as request handles passed to usb_sg_wait() or usb_sg_cancel(). Most members of the request object aren’t for driver access.

The status and bytecount values are valid only after usb_sg_wait() returns. If the status is zero, then the bytecount matches the total from the request.

After an error completion, drivers may need to clear a halt condition on the endpoint.

USB Core APIs¶

There are two basic I/O models in the USB API. The most elemental one is asynchronous: drivers submit requests in the form of an URB, and the URB’s completion callback handles the next step. All USB transfer types support that model, although there are special cases for control URBs (which always have setup and status stages, but may not have a data stage) and isochronous URBs (which allow large packets and include per-packet fault reports). Built on top of that is synchronous API support, where a driver calls a routine that allocates one or more URBs, submits them, and waits until they complete. There are synchronous wrappers for single-buffer control and bulk transfers (which are awkward to use in some driver disconnect scenarios), and for scatterlist based streaming i/o (bulk or interrupt).

USB drivers need to provide buffers that can be used for DMA, although they don’t necessarily need to provide the DMA mapping themselves. There are APIs to use used when allocating DMA buffers, which can prevent use of bounce buffers on some systems. In some cases, drivers may be able to rely on 64bit DMA to eliminate another kind of bounce buffer.

void usb_init_urb(struct urb *urb)¶: initializes a urb so that it can be used by a USB driver

Parameters

struct urb * urb: pointer to the urb to initialize

Description

Initializes a urb so that the USB subsystem can use it properly.

If a urb is created with a call to usb_alloc_urb() it is not necessary to call this function. Only use this if you allocate the space for a struct urb on your own. If you call this function, be careful when freeing the memory for your urb that it is no longer in use by the USB core.

Only use this function if you _really_ understand what you are doing.

struct urb *usb_alloc_urb(int iso_packets, gfp_t mem_flags)¶: creates a new urb for a USB driver to use

Parameters

int iso_packets: number of iso packets for this urb
gfp_t mem_flags: the type of memory to allocate, see kmalloc() for a list of valid options for this.

Description

Creates an urb for the USB driver to use, initializes a few internal structures, increments the usage counter, and returns a pointer to it.

If the driver want to use this urb for interrupt, control, or bulk endpoints, pass ‘0’ as the number of iso packets.

The driver must call usb_free_urb() when it is finished with the urb.

Return

A pointer to the new urb, or NULL if no memory is available.

void usb_free_urb(struct urb *urb)¶: frees the memory used by a urb when all users of it are finished

Parameters

struct urb * urb: pointer to the urb to free, may be NULL

Description

Must be called when a user of a urb is finished with it. When the last user of the urb calls this function, the memory of the urb is freed.

Note

The transfer buffer associated with the urb is not freed unless the URB_FREE_BUFFER transfer flag is set.

struct urb *usb_get_urb(struct urb *urb)¶: increments the reference count of the urb

Parameters

struct urb * urb: pointer to the urb to modify, may be NULL

Description

This must be called whenever a urb is transferred from a device driver to a host controller driver. This allows proper reference counting to happen for urbs.

Return

A pointer to the urb with the incremented reference counter.

void usb_anchor_urb(struct urb *urb, struct usb_anchor *anchor)¶: anchors an URB while it is processed

Parameters

struct urb * urb: pointer to the urb to anchor
struct usb_anchor * anchor: pointer to the anchor

Description

This can be called to have access to URBs which are to be executed without bothering to track them

void usb_unanchor_urb(struct urb *urb)¶: unanchors an URB

Parameters

struct urb * urb: pointer to the urb to anchor

Description

Call this to stop the system keeping track of this URB

int usb_urb_ep_type_check(const struct urb *urb)¶: sanity check of endpoint in the given urb

Parameters

const struct urb * urb: urb to be checked

Description

This performs a light-weight sanity check for the endpoint in the given urb. It returns 0 if the urb contains a valid endpoint, otherwise a negative error code.

int usb_submit_urb(struct urb *urb, gfp_t mem_flags)¶: issue an asynchronous transfer request for an endpoint

Parameters

struct urb * urb: pointer to the urb describing the request
gfp_t mem_flags: the type of memory to allocate, see kmalloc() for a list of valid options for this.

Description

This submits a transfer request, and transfers control of the URB describing that request to the USB subsystem. Request completion will be indicated later, asynchronously, by calling the completion handler. The three types of completion are success, error, and unlink (a software-induced fault, also called “request cancellation”).

URBs may be submitted in interrupt context.

The caller must have correctly initialized the URB before submitting it. Functions such as usb_fill_bulk_urb() and usb_fill_control_urb() are available to ensure that most fields are correctly initialized, for the particular kind of transfer, although they will not initialize any transfer flags.

If the submission is successful, the complete() callback from the URB will be called exactly once, when the USB core and Host Controller Driver (HCD) are finished with the URB. When the completion function is called, control of the URB is returned to the device driver which issued the request. The completion handler may then immediately free or reuse that URB.

With few exceptions, USB device drivers should never access URB fields provided by usbcore or the HCD until its complete() is called. The exceptions relate to periodic transfer scheduling. For both interrupt and isochronous urbs, as part of successful URB submission urb->interval is modified to reflect the actual transfer period used (normally some power of two units). And for isochronous urbs, urb->start_frame is modified to reflect when the URB’s transfers were scheduled to start.

Not all isochronous transfer scheduling policies will work, but most host controller drivers should easily handle ISO queues going from now until 10-200 msec into the future. Drivers should try to keep at least one or two msec of data in the queue; many controllers require that new transfers start at least 1 msec in the future when they are added. If the driver is unable to keep up and the queue empties out, the behavior for new submissions is governed by the URB_ISO_ASAP flag. If the flag is set, or if the queue is idle, then the URB is always assigned to the first available (and not yet expired) slot in the endpoint’s schedule. If the flag is not set and the queue is active then the URB is always assigned to the next slot in the schedule following the end of the endpoint’s previous URB, even if that slot is in the past. When a packet is assigned in this way to a slot that has already expired, the packet is not transmitted and the corresponding usb_iso_packet_descriptor’s status field will return -EXDEV. If this would happen to all the packets in the URB, submission fails with a -EXDEV error code.

For control endpoints, the synchronous usb_control_msg() call is often used (in non-interrupt context) instead of this call. That is often used through convenience wrappers, for the requests that are standardized in the USB 2.0 specification. For bulk endpoints, a synchronous usb_bulk_msg() call is available.

Return

0 on successful submissions. A negative error number otherwise.

Request Queuing:

URBs may be submitted to endpoints before previous ones complete, to minimize the impact of interrupt latencies and system overhead on data throughput. With that queuing policy, an endpoint’s queue would never be empty. This is required for continuous isochronous data streams, and may also be required for some kinds of interrupt transfers. Such queuing also maximizes bandwidth utilization by letting USB controllers start work on later requests before driver software has finished the completion processing for earlier (successful) requests.

As of Linux 2.6, all USB endpoint transfer queues support depths greater than one. This was previously a HCD-specific behavior, except for ISO transfers. Non-isochronous endpoint queues are inactive during cleanup after faults (transfer errors or cancellation).

Reserved Bandwidth Transfers:

Periodic transfers (interrupt or isochronous) are performed repeatedly, using the interval specified in the urb. Submitting the first urb to the endpoint reserves the bandwidth necessary to make those transfers. If the USB subsystem can’t allocate sufficient bandwidth to perform the periodic request, submitting such a periodic request should fail.

For devices under xHCI, the bandwidth is reserved at configuration time, or when the alt setting is selected. If there is not enough bus bandwidth, the configuration/alt setting request will fail. Therefore, submissions to periodic endpoints on devices under xHCI should never fail due to bandwidth constraints.

Device drivers must explicitly request that repetition, by ensuring that some URB is always on the endpoint’s queue (except possibly for short periods during completion callbacks). When there is no longer an urb queued, the endpoint’s bandwidth reservation is canceled. This means drivers can use their completion handlers to ensure they keep bandwidth they need, by reinitializing and resubmitting the just-completed urb until the driver longer needs that periodic bandwidth.

Memory Flags:

The general rules for how to decide which mem_flags to use are the same as for kmalloc. There are four different possible values; GFP_KERNEL, GFP_NOFS, GFP_NOIO and GFP_ATOMIC.

GFP_NOFS is not ever used, as it has not been implemented yet.

GFP_ATOMIC is used when

you are inside a completion handler, an interrupt, bottom half, tasklet or timer, or
you are holding a spinlock or rwlock (does not apply to semaphores), or
current->state != TASK_RUNNING, this is the case only after you’ve changed it.

GFP_NOIO is used in the block io path and error handling of storage devices.

All other situations use GFP_KERNEL.

Some more specific rules for mem_flags can be inferred, such as

start_xmit, timeout, and receive methods of network drivers must use GFP_ATOMIC (they are called with a spinlock held);
queuecommand methods of scsi drivers must use GFP_ATOMIC (also called with a spinlock held);
If you use a kernel thread with a network driver you must use GFP_NOIO, unless (b) or (c) apply;
after you have done a down() you can use GFP_KERNEL, unless (b) or (c) apply or your are in a storage driver’s block io path;
USB probe and disconnect can use GFP_KERNEL unless (b) or (c) apply; and
changing firmware on a running storage or net device uses GFP_NOIO, unless b) or c) apply

int usb_unlink_urb(struct urb *urb)¶: abort/cancel a transfer request for an endpoint

Parameters

struct urb * urb: pointer to urb describing a previously submitted request, may be NULL

Description

This routine cancels an in-progress request. URBs complete only once per submission, and may be canceled only once per submission. Successful cancellation means termination of urb will be expedited and the completion handler will be called with a status code indicating that the request has been canceled (rather than any other code).

Drivers should not call this routine or related routines, such as usb_kill_urb() or usb_unlink_anchored_urbs(), after their disconnect method has returned. The disconnect function should synchronize with a driver’s I/O routines to insure that all URB-related activity has completed before it returns.

This request is asynchronous, however the HCD might call the ->complete() callback during unlink. Therefore when drivers call usb_unlink_urb(), they must not hold any locks that may be taken by the completion function. Success is indicated by returning -EINPROGRESS, at which time the URB will probably not yet have been given back to the device driver. When it is eventually called, the completion function will see urb->status == -ECONNRESET. Failure is indicated by usb_unlink_urb() returning any other value. Unlinking will fail when urb is not currently “linked” (i.e., it was never submitted, or it was unlinked before, or the hardware is already finished with it), even if the completion handler has not yet run.

The URB must not be deallocated while this routine is running. In particular, when a driver calls this routine, it must insure that the completion handler cannot deallocate the URB.

Return

-EINPROGRESS on success. See description for other values on failure.

Unlinking and Endpoint Queues:

[The behaviors and guarantees described below do not apply to virtual root hubs but only to endpoint queues for physical USB devices.]

Host Controller Drivers (HCDs) place all the URBs for a particular endpoint in a queue. Normally the queue advances as the controller hardware processes each request. But when an URB terminates with an error its queue generally stops (see below), at least until that URB’s completion routine returns. It is guaranteed that a stopped queue will not restart until all its unlinked URBs have been fully retired, with their completion routines run, even if that’s not until some time after the original completion handler returns. The same behavior and guarantee apply when an URB terminates because it was unlinked.

Bulk and interrupt endpoint queues are guaranteed to stop whenever an URB terminates with any sort of error, including -ECONNRESET, -ENOENT, and -EREMOTEIO. Control endpoint queues behave the same way except that they are not guaranteed to stop for -EREMOTEIO errors. Queues for isochronous endpoints are treated differently, because they must advance at fixed rates. Such queues do not stop when an URB encounters an error or is unlinked. An unlinked isochronous URB may leave a gap in the stream of packets; it is undefined whether such gaps can be filled in.

Note that early termination of an URB because a short packet was received will generate a -EREMOTEIO error if and only if the URB_SHORT_NOT_OK flag is set. By setting this flag, USB device drivers can build deep queues for large or complex bulk transfers and clean them up reliably after any sort of aborted transfer by unlinking all pending URBs at the first fault.

When a control URB terminates with an error other than -EREMOTEIO, it is quite likely that the status stage of the transfer will not take place.

void usb_kill_urb(struct urb *urb)¶: cancel a transfer request and wait for it to finish

Parameters

struct urb * urb: pointer to URB describing a previously submitted request, may be NULL

Description

This routine cancels an in-progress request. It is guaranteed that upon return all completion handlers will have finished and the URB will be totally idle and available for reuse. These features make this an ideal way to stop I/O in a disconnect() callback or close() function. If the request has not already finished or been unlinked the completion handler will see urb->status == -ENOENT.

While the routine is running, attempts to resubmit the URB will fail with error -EPERM. Thus even if the URB’s completion handler always tries to resubmit, it will not succeed and the URB will become idle.

The URB must not be deallocated while this routine is running. In particular, when a driver calls this routine, it must insure that the completion handler cannot deallocate the URB.

This routine may not be used in an interrupt context (such as a bottom half or a completion handler), or when holding a spinlock, or in other situations where the caller can’t schedule().

This routine should not be called by a driver after its disconnect method has returned.

void usb_poison_urb(struct urb *urb)¶: reliably kill a transfer and prevent further use of an URB

Parameters

struct urb * urb: pointer to URB describing a previously submitted request, may be NULL

Description

This routine cancels an in-progress request. It is guaranteed that upon return all completion handlers will have finished and the URB will be totally idle and cannot be reused. These features make this an ideal way to stop I/O in a disconnect() callback. If the request has not already finished or been unlinked the completion handler will see urb->status == -ENOENT.

After and while the routine runs, attempts to resubmit the URB will fail with error -EPERM. Thus even if the URB’s completion handler always tries to resubmit, it will not succeed and the URB will become idle.

The URB must not be deallocated while this routine is running. In particular, when a driver calls this routine, it must insure that the completion handler cannot deallocate the URB.

This routine may not be used in an interrupt context (such as a bottom half or a completion handler), or when holding a spinlock, or in other situations where the caller can’t schedule().

This routine should not be called by a driver after its disconnect method has returned.

void usb_block_urb(struct urb *urb)¶: reliably prevent further use of an URB

Parameters

struct urb * urb: pointer to URB to be blocked, may be NULL

Description

After the routine has run, attempts to resubmit the URB will fail with error -EPERM. Thus even if the URB’s completion handler always tries to resubmit, it will not succeed and the URB will become idle.

The URB must not be deallocated while this routine is running. In particular, when a driver calls this routine, it must insure that the completion handler cannot deallocate the URB.

void usb_kill_anchored_urbs(struct usb_anchor *anchor)¶: cancel transfer requests en masse

Parameters

struct usb_anchor * anchor: anchor the requests are bound to

Description

this allows all outstanding URBs to be killed starting from the back of the queue

This routine should not be called by a driver after its disconnect method has returned.

void usb_poison_anchored_urbs(struct usb_anchor *anchor)¶: cease all traffic from an anchor

Parameters

struct usb_anchor * anchor: anchor the requests are bound to

Description

this allows all outstanding URBs to be poisoned starting from the back of the queue. Newly added URBs will also be poisoned

This routine should not be called by a driver after its disconnect method has returned.

void usb_unpoison_anchored_urbs(struct usb_anchor *anchor)¶: let an anchor be used successfully again

Parameters

struct usb_anchor * anchor: anchor the requests are bound to

Description

Reverses the effect of usb_poison_anchored_urbs the anchor can be used normally after it returns

void usb_unlink_anchored_urbs(struct usb_anchor *anchor)¶: asynchronously cancel transfer requests en masse

Parameters

struct usb_anchor * anchor: anchor the requests are bound to

Description

this allows all outstanding URBs to be unlinked starting from the back of the queue. This function is asynchronous. The unlinking is just triggered. It may happen after this function has returned.

This routine should not be called by a driver after its disconnect method has returned.

void usb_anchor_suspend_wakeups(struct usb_anchor *anchor)¶

Parameters

struct usb_anchor * anchor: the anchor you want to suspend wakeups on

Description

Call this to stop the last urb being unanchored from waking up any usb_wait_anchor_empty_timeout waiters. This is used in the hcd urb give- back path to delay waking up until after the completion handler has run.

void usb_anchor_resume_wakeups(struct usb_anchor *anchor)¶

Parameters

struct usb_anchor * anchor: the anchor you want to resume wakeups on

Description

Allow usb_wait_anchor_empty_timeout waiters to be woken up again, and wake up any current waiters if the anchor is empty.

int usb_wait_anchor_empty_timeout(struct usb_anchor *anchor, unsigned int timeout)¶: wait for an anchor to be unused

Parameters

struct usb_anchor * anchor: the anchor you want to become unused
unsigned int timeout: how long you are willing to wait in milliseconds

Description

Call this is you want to be sure all an anchor’s URBs have finished

Return

Non-zero if the anchor became unused. Zero on timeout.

struct urb *usb_get_from_anchor(struct usb_anchor *anchor)¶: get an anchor’s oldest urb

Parameters

struct usb_anchor * anchor: the anchor whose urb you want

Description

This will take the oldest urb from an anchor, unanchor and return it

Return

The oldest urb from anchor, or NULL if anchor has no urbs associated with it.

void usb_scuttle_anchored_urbs(struct usb_anchor *anchor)¶: unanchor all an anchor’s urbs

Parameters

struct usb_anchor * anchor: the anchor whose urbs you want to unanchor

Description

use this to get rid of all an anchor’s urbs

int usb_anchor_empty(struct usb_anchor *anchor)¶: is an anchor empty

Parameters

struct usb_anchor * anchor: the anchor you want to query

Return

1 if the anchor has no urbs associated with it.

int usb_control_msg(struct usb_device *dev, unsigned int pipe, __u8 request, __u8 requesttype, __u16 value, __u16 index, void *data, __u16 size, int timeout)¶: Builds a control urb, sends it off and waits for completion

Parameters

struct usb_device * dev: pointer to the usb device to send the message to
unsigned int pipe: endpoint “pipe” to send the message to
__u8 request: USB message request value
__u8 requesttype: USB message request type value
__u16 value: USB message value
__u16 index: USB message index value
void * data: pointer to the data to send
__u16 size: length in bytes of the data to send
int timeout: time in msecs to wait for the message to complete before timing out (if 0 the wait is forever)

Context

!in_interrupt ()

Description

This function sends a simple control message to a specified endpoint and waits for the message to complete, or timeout.

Don’t use this function from within an interrupt context. If you need an asynchronous message, or need to send a message from within interrupt context, use usb_submit_urb(). If a thread in your driver uses this call, make sure your disconnect() method can wait for it to complete. Since you don’t have a handle on the URB used, you can’t cancel the request.

Return

If successful, the number of bytes transferred. Otherwise, a negative error number.

int usb_interrupt_msg(struct usb_device *usb_dev, unsigned int pipe, void *data, int len, int *actual_length, int timeout)¶: Builds an interrupt urb, sends it off and waits for completion

Parameters

struct usb_device * usb_dev: pointer to the usb device to send the message to
unsigned int pipe: endpoint “pipe” to send the message to
void * data: pointer to the data to send
int len: length in bytes of the data to send
int * actual_length: pointer to a location to put the actual length transferred in bytes
int timeout: time in msecs to wait for the message to complete before timing out (if 0 the wait is forever)

Context

!in_interrupt ()

Description

This function sends a simple interrupt message to a specified endpoint and waits for the message to complete, or timeout.

Don’t use this function from within an interrupt context. If you need an asynchronous message, or need to send a message from within interrupt context, use usb_submit_urb() If a thread in your driver uses this call, make sure your disconnect() method can wait for it to complete. Since you don’t have a handle on the URB used, you can’t cancel the request.

Return

If successful, 0. Otherwise a negative error number. The number of actual bytes transferred will be stored in the actual_length parameter.

int usb_bulk_msg(struct usb_device *usb_dev, unsigned int pipe, void *data, int len, int *actual_length, int timeout)¶: Builds a bulk urb, sends it off and waits for completion

Parameters

struct usb_device * usb_dev: pointer to the usb device to send the message to
unsigned int pipe: endpoint “pipe” to send the message to
void * data: pointer to the data to send
int len: length in bytes of the data to send
int * actual_length: pointer to a location to put the actual length transferred in bytes
int timeout: time in msecs to wait for the message to complete before timing out (if 0 the wait is forever)

Context

!in_interrupt ()

Description

This function sends a simple bulk message to a specified endpoint and waits for the message to complete, or timeout.

Don’t use this function from within an interrupt context. If you need an asynchronous message, or need to send a message from within interrupt context, use usb_submit_urb() If a thread in your driver uses this call, make sure your disconnect() method can wait for it to complete. Since you don’t have a handle on the URB used, you can’t cancel the request.

Because there is no usb_interrupt_msg() and no USBDEVFS_INTERRUPT ioctl, users are forced to abuse this routine by using it to submit URBs for interrupt endpoints. We will take the liberty of creating an interrupt URB (with the default interval) if the target is an interrupt endpoint.

Return

If successful, 0. Otherwise a negative error number. The number of actual bytes transferred will be stored in the actual_length parameter.

int usb_sg_init(struct usb_sg_request *io, struct usb_device *dev, unsigned pipe, unsigned period, struct scatterlist *sg, int nents, size_t length, gfp_t mem_flags)¶: initializes scatterlist-based bulk/interrupt I/O request

Parameters

struct usb_sg_request * io: request block being initialized. until usb_sg_wait() returns, treat this as a pointer to an opaque block of memory,
struct usb_device * dev: the usb device that will send or receive the data
unsigned pipe: endpoint “pipe” used to transfer the data
unsigned period: polling rate for interrupt endpoints, in frames or (for high speed endpoints) microframes; ignored for bulk
struct scatterlist * sg: scatterlist entries
int nents: how many entries in the scatterlist
size_t length: how many bytes to send from the scatterlist, or zero to send every byte identified in the list.
gfp_t mem_flags: SLAB_* flags affecting memory allocations in this call

Description

This initializes a scatter/gather request, allocating resources such as I/O mappings and urb memory (except maybe memory used by USB controller drivers).

The request must be issued using usb_sg_wait(), which waits for the I/O to complete (or to be canceled) and then cleans up all resources allocated by usb_sg_init().

The request may be canceled with usb_sg_cancel(), either before or after usb_sg_wait() is called.

Return

Zero for success, else a negative errno value.

void usb_sg_wait(struct usb_sg_request *io)¶: synchronously execute scatter/gather request

Parameters

struct usb_sg_request * io: request block handle, as initialized with usb_sg_init(). some fields become accessible when this call returns.

Context

!in_interrupt ()

Description

This function blocks until the specified I/O operation completes. It leverages the grouping of the related I/O requests to get good transfer rates, by queueing the requests. At higher speeds, such queuing can significantly improve USB throughput.

There are three kinds of completion for this function.

success, where io->status is zero. The number of io->bytes transferred is as requested.
error, where io->status is a negative errno value. The number of io->bytes transferred before the error is usually less than requested, and can be nonzero.
cancellation, a type of error with status -ECONNRESET that is initiated by usb_sg_cancel().

When this function returns, all memory allocated through usb_sg_init() or this call will have been freed. The request block parameter may still be passed to usb_sg_cancel(), or it may be freed. It could also be reinitialized and then reused.

Data Transfer Rates:

Bulk transfers are valid for full or high speed endpoints. The best full speed data rate is 19 packets of 64 bytes each per frame, or 1216 bytes per millisecond. The best high speed data rate is 13 packets of 512 bytes each per microframe, or 52 KBytes per millisecond.

The reason to use interrupt transfers through this API would most likely be to reserve high speed bandwidth, where up to 24 KBytes per millisecond could be transferred. That capability is less useful for low or full speed interrupt endpoints, which allow at most one packet per millisecond, of at most 8 or 64 bytes (respectively).

It is not necessary to call this function to reserve bandwidth for devices under an xHCI host controller, as the bandwidth is reserved when the configuration or interface alt setting is selected.

void usb_sg_cancel(struct usb_sg_request *io)¶: stop scatter/gather i/o issued by usb_sg_wait()

Parameters

struct usb_sg_request * io: request block, initialized with usb_sg_init()

Description

This stops a request after it has been started by usb_sg_wait(). It can also prevents one initialized by usb_sg_init() from starting, so that call just frees resources allocated to the request.

int usb_get_descriptor(struct usb_device *dev, unsigned char type, unsigned char index, void *buf, int size)¶: issues a generic GET_DESCRIPTOR request

Parameters

struct usb_device * dev: the device whose descriptor is being retrieved
unsigned char type: the descriptor type (USB_DT_*)
unsigned char index: the number of the descriptor
void * buf: where to put the descriptor
int size: how big is “buf”?

Context

!in_interrupt ()

Description

Gets a USB descriptor. Convenience functions exist to simplify getting some types of descriptors. Use usb_get_string() or usb_string() for USB_DT_STRING. Device (USB_DT_DEVICE) and configuration descriptors (USB_DT_CONFIG) are part of the device structure. In addition to a number of USB-standard descriptors, some devices also use class-specific or vendor-specific descriptors.

This call is synchronous, and may not be used in an interrupt context.

Return

The number of bytes received on success, or else the status code returned by the underlying usb_control_msg() call.

int usb_string(struct usb_device *dev, int index, char *buf, size_t size)¶: returns UTF-8 version of a string descriptor

Parameters

struct usb_device * dev: the device whose string descriptor is being retrieved
int index: the number of the descriptor
char * buf: where to put the string
size_t size: how big is “buf”?

Context

!in_interrupt ()

Description

This converts the UTF-16LE encoded strings returned by devices, from usb_get_string_descriptor(), to null-terminated UTF-8 encoded ones that are more usable in most kernel contexts. Note that this function chooses strings in the first language supported by the device.

This call is synchronous, and may not be used in an interrupt context.

Return

length of the string (>= 0) or usb_control_msg status (< 0).

int usb_get_status(struct usb_device *dev, int recip, int type, int target, void *data)¶: issues a GET_STATUS call

Parameters

struct usb_device * dev: the device whose status is being checked
int recip: USB_RECIP_*; for device, interface, or endpoint
int type: USB_STATUS_TYPE_*; for standard or PTM status types
int target: zero (for device), else interface or endpoint number
void * data: pointer to two bytes of bitmap data

Context

!in_interrupt ()

Description

Returns device, interface, or endpoint status. Normally only of interest to see if the device is self powered, or has enabled the remote wakeup facility; or whether a bulk or interrupt endpoint is halted (“stalled”).

Bits in these status bitmaps are set using the SET_FEATURE request, and cleared using the CLEAR_FEATURE request. The usb_clear_halt() function should be used to clear halt (“stall”) status.

This call is synchronous, and may not be used in an interrupt context.

Returns 0 and the status value in *data (in host byte order) on success, or else the status code from the underlying usb_control_msg() call.

int usb_clear_halt(struct usb_device *dev, int pipe)¶: tells device to clear endpoint halt/stall condition

Parameters

struct usb_device * dev: device whose endpoint is halted
int pipe: endpoint “pipe” being cleared

Context

!in_interrupt ()

Description

This is used to clear halt conditions for bulk and interrupt endpoints, as reported by URB completion status. Endpoints that are halted are sometimes referred to as being “stalled”. Such endpoints are unable to transmit or receive data until the halt status is cleared. Any URBs queued for such an endpoint should normally be unlinked by the driver before clearing the halt condition, as described in sections 5.7.5 and 5.8.5 of the USB 2.0 spec.

Note that control and isochronous endpoints don’t halt, although control endpoints report “protocol stall” (for unsupported requests) using the same status code used to report a true stall.

This call is synchronous, and may not be used in an interrupt context.

Return

Zero on success, or else the status code returned by the underlying usb_control_msg() call.

void usb_reset_endpoint(struct usb_device *dev, unsigned int epaddr)¶: Reset an endpoint’s state.

Parameters

struct usb_device * dev: the device whose endpoint is to be reset
unsigned int epaddr: the endpoint’s address. Endpoint number for output, endpoint number + USB_DIR_IN for input

Description

Resets any host-side endpoint state such as the toggle bit, sequence number or current window.

int usb_set_interface(struct usb_device *dev, int interface, int alternate)¶: Makes a particular alternate setting be current

Parameters

struct usb_device * dev: the device whose interface is being updated
int interface: the interface being updated
int alternate: the setting being chosen.

Context

!in_interrupt ()

Description

This is used to enable data transfers on interfaces that may not be enabled by default. Not all devices support such configurability. Only the driver bound to an interface may change its setting.

Within any given configuration, each interface may have several alternative settings. These are often used to control levels of bandwidth consumption. For example, the default setting for a high speed interrupt endpoint may not send more than 64 bytes per microframe, while interrupt transfers of up to 3KBytes per microframe are legal. Also, isochronous endpoints may never be part of an interface’s default setting. To access such bandwidth, alternate interface settings must be made current.

Note that in the Linux USB subsystem, bandwidth associated with an endpoint in a given alternate setting is not reserved until an URB is submitted that needs that bandwidth. Some other operating systems allocate bandwidth early, when a configuration is chosen.

xHCI reserves bandwidth and configures the alternate setting in usb_hcd_alloc_bandwidth(). If it fails the original interface altsetting may be disabled. Drivers cannot rely on any particular alternate setting being in effect after a failure.

This call is synchronous, and may not be used in an interrupt context. Also, drivers must not change altsettings while urbs are scheduled for endpoints in that interface; all such urbs must first be completed (perhaps forced by unlinking).

Return

Zero on success, or else the status code returned by the underlying usb_control_msg() call.

int usb_reset_configuration(struct usb_device *dev)¶: lightweight device reset

Parameters

struct usb_device * dev: the device whose configuration is being reset

Description

This issues a standard SET_CONFIGURATION request to the device using the current configuration. The effect is to reset most USB-related state in the device, including interface altsettings (reset to zero), endpoint halts (cleared), and endpoint state (only for bulk and interrupt endpoints). Other usbcore state is unchanged, including bindings of usb device drivers to interfaces.

Because this affects multiple interfaces, avoid using this with composite (multi-interface) devices. Instead, the driver for each interface may use usb_set_interface() on the interfaces it claims. Be careful though; some devices don’t support the SET_INTERFACE request, and others won’t reset all the interface state (notably endpoint state). Resetting the whole configuration would affect other drivers’ interfaces.

The caller must own the device lock.

Return

Zero on success, else a negative error code.

int usb_driver_set_configuration(struct usb_device *udev, int config)¶: Provide a way for drivers to change device configurations

Parameters

struct usb_device * udev: the device whose configuration is being updated
int config: the configuration being chosen.

Context

In process context, must be able to sleep

Description

Device interface drivers are not allowed to change device configurations. This is because changing configurations will destroy the interface the driver is bound to and create new ones; it would be like a floppy-disk driver telling the computer to replace the floppy-disk drive with a tape drive!

Still, in certain specialized circumstances the need may arise. This routine gets around the normal restrictions by using a work thread to submit the change-config request.

Return

0 if the request was successfully queued, error code otherwise. The caller has no way to know whether the queued request will eventually succeed.

int cdc_parse_cdc_header(struct usb_cdc_parsed_header *hdr, struct usb_interface *intf, u8 *buffer, int buflen)¶: parse the extra headers present in CDC devices

Parameters

struct usb_cdc_parsed_header * hdr: the place to put the results of the parsing
struct usb_interface * intf: the interface for which parsing is requested
u8 * buffer: pointer to the extra headers to be parsed
int buflen: length of the extra headers

Description

This evaluates the extra headers present in CDC devices which bind the interfaces for data and control and provide details about the capabilities of the device.

Return

number of descriptors parsed or -EINVAL if the header is contradictory beyond salvage

int usb_register_dev(struct usb_interface *intf, struct usb_class_driver *class_driver)¶: register a USB device, and ask for a minor number

Parameters

struct usb_interface * intf: pointer to the usb_interface that is being registered
struct usb_class_driver * class_driver: pointer to the usb_class_driver for this device

Description

This should be called by all USB drivers that use the USB major number. If CONFIG_USB_DYNAMIC_MINORS is enabled, the minor number will be dynamically allocated out of the list of available ones. If it is not enabled, the minor number will be based on the next available free minor, starting at the class_driver->minor_base.

This function also creates a usb class device in the sysfs tree.

usb_deregister_dev() must be called when the driver is done with the minor numbers given out by this function.

Return

-EINVAL if something bad happens with trying to register a device, and 0 on success.

void usb_deregister_dev(struct usb_interface *intf, struct usb_class_driver *class_driver)¶: deregister a USB device’s dynamic minor.

Parameters

struct usb_interface * intf: pointer to the usb_interface that is being deregistered
struct usb_class_driver * class_driver: pointer to the usb_class_driver for this device

Description

Used in conjunction with usb_register_dev(). This function is called when the USB driver is finished with the minor numbers gotten from a call to usb_register_dev() (usually when the device is disconnected from the system.)

This function also removes the usb class device from the sysfs tree.

This should be called by all drivers that use the USB major number.

int usb_driver_claim_interface(struct usb_driver *driver, struct usb_interface *iface, void *priv)¶: bind a driver to an interface

Parameters

struct usb_driver * driver: the driver to be bound
struct usb_interface * iface: the interface to which it will be bound; must be in the usb device’s active configuration
void * priv: driver data associated with that interface

Description

This is used by usb device drivers that need to claim more than one interface on a device when probing (audio and acm are current examples). No device driver should directly modify internal usb_interface or usb_device structure members.

Few drivers should need to use this routine, since the most natural way to bind to an interface is to return the private data from the driver’s probe() method.

Callers must own the device lock, so driver probe() entries don’t need extra locking, but other call contexts may need to explicitly claim that lock.

Return

0 on success.

void usb_driver_release_interface(struct usb_driver *driver, struct usb_interface *iface)¶: unbind a driver from an interface

Parameters

struct usb_driver * driver: the driver to be unbound
struct usb_interface * iface: the interface from which it will be unbound

Description

This can be used by drivers to release an interface without waiting for their disconnect() methods to be called. In typical cases this also causes the driver disconnect() method to be called.

This call is synchronous, and may not be used in an interrupt context. Callers must own the device lock, so driver disconnect() entries don’t need extra locking, but other call contexts may need to explicitly claim that lock.

const struct usb_device_id *usb_match_id(struct usb_interface *interface, const struct usb_device_id *id)¶: find first usb_device_id matching device or interface

Parameters

struct usb_interface * interface: the interface of interest
const struct usb_device_id * id: array of usb_device_id structures, terminated by zero entry

Description

usb_match_id searches an array of usb_device_id’s and returns the first one matching the device or interface, or null. This is used when binding (or rebinding) a driver to an interface. Most USB device drivers will use this indirectly, through the usb core, but some layered driver frameworks use it directly. These device tables are exported with MODULE_DEVICE_TABLE, through modutils, to support the driver loading functionality of USB hotplugging.

Return

The first matching usb_device_id, or NULL.

What Matches:

The “match_flags” element in a usb_device_id controls which members are used. If the corresponding bit is set, the value in the device_id must match its corresponding member in the device or interface descriptor, or else the device_id does not match.

“driver_info” is normally used only by device drivers, but you can create a wildcard “matches anything” usb_device_id as a driver’s “modules.usbmap” entry if you provide an id with only a nonzero “driver_info” field. If you do this, the USB device driver’s probe() routine should use additional intelligence to decide whether to bind to the specified interface.

What Makes Good usb_device_id Tables:

The match algorithm is very simple, so that intelligence in driver selection must come from smart driver id records. Unless you have good reasons to use another selection policy, provide match elements only in related groups, and order match specifiers from specific to general. Use the macros provided for that purpose if you can.

The most specific match specifiers use device descriptor data. These are commonly used with product-specific matches; the USB_DEVICE macro lets you provide vendor and product IDs, and you can also match against ranges of product revisions. These are widely used for devices with application or vendor specific bDeviceClass values.

Matches based on device class/subclass/protocol specifications are slightly more general; use the USB_DEVICE_INFO macro, or its siblings. These are used with single-function devices where bDeviceClass doesn’t specify that each interface has its own class.

Matches based on interface class/subclass/protocol are the most general; they let drivers bind to any interface on a multiple-function device. Use the USB_INTERFACE_INFO macro, or its siblings, to match class-per-interface style devices (as recorded in bInterfaceClass).

Note that an entry created by USB_INTERFACE_INFO won’t match any interface if the device class is set to Vendor-Specific. This is deliberate; according to the USB spec the meanings of the interface class/subclass/protocol for these devices are also vendor-specific, and hence matching against a standard product class wouldn’t work anyway. If you really want to use an interface-based match for such a device, create a match record that also specifies the vendor ID. (Unforunately there isn’t a standard macro for creating records like this.)

Within those groups, remember that not all combinations are meaningful. For example, don’t give a product version range without vendor and product IDs; or specify a protocol without its associated class and subclass.

int usb_register_device_driver(struct usb_device_driver *new_udriver, struct module *owner)¶: register a USB device (not interface) driver

Parameters

struct usb_device_driver * new_udriver: USB operations for the device driver
struct module * owner: module owner of this driver.

Description

Registers a USB device driver with the USB core. The list of unattached devices will be rescanned whenever a new driver is added, allowing the new driver to attach to any recognized devices.

Return

A negative error code on failure and 0 on success.

void usb_deregister_device_driver(struct usb_device_driver *udriver)¶: unregister a USB device (not interface) driver

Parameters

struct usb_device_driver * udriver: USB operations of the device driver to unregister

Context

must be able to sleep

Description

Unlinks the specified driver from the internal USB driver list.

int usb_register_driver(struct usb_driver *new_driver, struct module *owner, const char *mod_name)¶: register a USB interface driver

Parameters

struct usb_driver * new_driver: USB operations for the interface driver
struct module * owner: module owner of this driver.
const char * mod_name: module name string

Description

Registers a USB interface driver with the USB core. The list of unattached interfaces will be rescanned whenever a new driver is added, allowing the new driver to attach to any recognized interfaces.

Return

A negative error code on failure and 0 on success.

NOTE

if you want your driver to use the USB major number, you must call usb_register_dev() to enable that functionality. This function no longer takes care of that.

void usb_deregister(struct usb_driver *driver)¶: unregister a USB interface driver

Parameters

struct usb_driver * driver: USB operations of the interface driver to unregister

Context

must be able to sleep

Description

Unlinks the specified driver from the internal USB driver list.

NOTE

If you called usb_register_dev(), you still need to call usb_deregister_dev() to clean up your driver’s allocated minor numbers, this * call will no longer do it for you.

void usb_enable_autosuspend(struct usb_device *udev)¶: allow a USB device to be autosuspended

Parameters

struct usb_device * udev: the USB device which may be autosuspended

Description

This routine allows udev to be autosuspended. An autosuspend won’t take place until the autosuspend_delay has elapsed and all the other necessary conditions are satisfied.

The caller must hold udev’s device lock.

void usb_disable_autosuspend(struct usb_device *udev)¶: prevent a USB device from being autosuspended

Parameters

struct usb_device * udev: the USB device which may not be autosuspended

Description

This routine prevents udev from being autosuspended and wakes it up if it is already autosuspended.

The caller must hold udev’s device lock.

void usb_autopm_put_interface(struct usb_interface *intf)¶: decrement a USB interface’s PM-usage counter

Parameters

struct usb_interface * intf: the usb_interface whose counter should be decremented

Description

This routine should be called by an interface driver when it is finished using intf and wants to allow it to autosuspend. A typical example would be a character-device driver when its device file is closed.

The routine decrements intf’s usage counter. When the counter reaches 0, a delayed autosuspend request for intf’s device is attempted. The attempt may fail (see autosuspend_check()).

This routine can run only in process context.

void usb_autopm_put_interface_async(struct usb_interface *intf)¶: decrement a USB interface’s PM-usage counter

Parameters

struct usb_interface * intf: the usb_interface whose counter should be decremented

Description

This routine does much the same thing as usb_autopm_put_interface(): It decrements intf’s usage counter and schedules a delayed autosuspend request if the counter is <= 0. The difference is that it does not perform any synchronization; callers should hold a private lock and handle all synchronization issues themselves.

Typically a driver would call this routine during an URB’s completion handler, if no more URBs were pending.

This routine can run in atomic context.

void usb_autopm_put_interface_no_suspend(struct usb_interface *intf)¶: decrement a USB interface’s PM-usage counter

Parameters

struct usb_interface * intf: the usb_interface whose counter should be decremented

Description

This routine decrements intf’s usage counter but does not carry out an autosuspend.

This routine can run in atomic context.

int usb_autopm_get_interface(struct usb_interface *intf)¶: increment a USB interface’s PM-usage counter

Parameters

struct usb_interface * intf: the usb_interface whose counter should be incremented

Description

This routine should be called by an interface driver when it wants to use intf and needs to guarantee that it is not suspended. In addition, the routine prevents intf from being autosuspended subsequently. (Note that this will not prevent suspend events originating in the PM core.) This prevention will persist until usb_autopm_put_interface() is called or intf is unbound. A typical example would be a character-device driver when its device file is opened.

intf’s usage counter is incremented to prevent subsequent autosuspends. However if the autoresume fails then the counter is re-decremented.

This routine can run only in process context.

Return

0 on success.

int usb_autopm_get_interface_async(struct usb_interface *intf)¶: increment a USB interface’s PM-usage counter

Parameters

struct usb_interface * intf: the usb_interface whose counter should be incremented

Description

This routine does much the same thing as usb_autopm_get_interface(): It increments intf’s usage counter and queues an autoresume request if the device is suspended. The differences are that it does not perform any synchronization (callers should hold a private lock and handle all synchronization issues themselves), and it does not autoresume the device directly (it only queues a request). After a successful call, the device may not yet be resumed.

This routine can run in atomic context.

Return

0 on success. A negative error code otherwise.

void usb_autopm_get_interface_no_resume(struct usb_interface *intf)¶: increment a USB interface’s PM-usage counter

Parameters

struct usb_interface * intf: the usb_interface whose counter should be incremented

Description

This routine increments intf’s usage counter but does not carry out an autoresume.

This routine can run in atomic context.

int usb_find_common_endpoints(struct usb_host_interface *alt, struct usb_endpoint_descriptor **bulk_in, struct usb_endpoint_descriptor **bulk_out, struct usb_endpoint_descriptor **int_in, struct usb_endpoint_descriptor **int_out)¶

look up common endpoint descriptors

Parameters

struct usb_host_interface * alt: alternate setting to search
struct usb_endpoint_descriptor ** bulk_in: pointer to descriptor pointer, or NULL
struct usb_endpoint_descriptor ** bulk_out: pointer to descriptor pointer, or NULL
struct usb_endpoint_descriptor ** int_in: pointer to descriptor pointer, or NULL
struct usb_endpoint_descriptor ** int_out: pointer to descriptor pointer, or NULL

Description

Search the alternate setting’s endpoint descriptors for the first bulk-in, bulk-out, interrupt-in and interrupt-out endpoints and return them in the provided pointers (unless they are NULL).

If a requested endpoint is not found, the corresponding pointer is set to NULL.

Return

Zero if all requested descriptors were found, or -ENXIO otherwise.

int usb_find_common_endpoints_reverse(struct usb_host_interface *alt, struct usb_endpoint_descriptor **bulk_in, struct usb_endpoint_descriptor **bulk_out, struct usb_endpoint_descriptor **int_in, struct usb_endpoint_descriptor **int_out)¶

look up common endpoint descriptors

Parameters

struct usb_host_interface * alt: alternate setting to search
struct usb_endpoint_descriptor ** bulk_in: pointer to descriptor pointer, or NULL
struct usb_endpoint_descriptor ** bulk_out: pointer to descriptor pointer, or NULL
struct usb_endpoint_descriptor ** int_in: pointer to descriptor pointer, or NULL
struct usb_endpoint_descriptor ** int_out: pointer to descriptor pointer, or NULL

Description

Search the alternate setting’s endpoint descriptors for the last bulk-in, bulk-out, interrupt-in and interrupt-out endpoints and return them in the provided pointers (unless they are NULL).

If a requested endpoint is not found, the corresponding pointer is set to NULL.

Return

Zero if all requested descriptors were found, or -ENXIO otherwise.

struct usb_host_interface *usb_find_alt_setting(struct usb_host_config *config, unsigned int iface_num, unsigned int alt_num)¶: Given a configuration, find the alternate setting for the given interface.

Parameters

struct usb_host_config * config: the configuration to search (not necessarily the current config).
unsigned int iface_num: interface number to search in
unsigned int alt_num: alternate interface setting number to search for.

Description

Search the configuration’s interface cache for the given alt setting.

Return

The alternate setting, if found. NULL otherwise.

struct usb_interface *usb_ifnum_to_if(const struct usb_device *dev, unsigned ifnum)¶: get the interface object with a given interface number

Parameters

const struct usb_device * dev: the device whose current configuration is considered
unsigned ifnum: the desired interface

Description

This walks the device descriptor for the currently active configuration to find the interface object with the particular interface number.

Note that configuration descriptors are not required to assign interface numbers sequentially, so that it would be incorrect to assume that the first interface in that descriptor corresponds to interface zero. This routine helps device drivers avoid such mistakes. However, you should make sure that you do the right thing with any alternate settings available for this interfaces.

Don’t call this function unless you are bound to one of the interfaces on this device or you have locked the device!

Return

A pointer to the interface that has ifnum as interface number, if found. NULL otherwise.

struct usb_host_interface *usb_altnum_to_altsetting(const struct usb_interface *intf, unsigned int altnum)¶: get the altsetting structure with a given alternate setting number.

Parameters

const struct usb_interface * intf: the interface containing the altsetting in question
unsigned int altnum: the desired alternate setting number

Description

This searches the altsetting array of the specified interface for an entry with the correct bAlternateSetting value.

Note that altsettings need not be stored sequentially by number, so it would be incorrect to assume that the first altsetting entry in the array corresponds to altsetting zero. This routine helps device drivers avoid such mistakes.

Don’t call this function unless you are bound to the intf interface or you have locked the device!

Return

A pointer to the entry of the altsetting array of intf that has altnum as the alternate setting number. NULL if not found.

struct usb_interface *usb_find_interface(struct usb_driver *drv, int minor)¶: find usb_interface pointer for driver and device

Parameters

struct usb_driver * drv: the driver whose current configuration is considered
int minor: the minor number of the desired device

Description

This walks the bus device list and returns a pointer to the interface with the matching minor and driver. Note, this only works for devices that share the USB major number.

Return

A pointer to the interface with the matching major and minor.

int usb_for_each_dev(void *data, int (*fn)(struct usb_device*, void*))¶: iterate over all USB devices in the system

Parameters

void * data: data pointer that will be handed to the callback function
int (*)(struct usb_device *, void *) fn: callback function to be called for each USB device

Description

Iterate over all USB devices and call fn for each, passing it data. If it returns anything other than 0, we break the iteration prematurely and return that value.

struct usb_device *usb_alloc_dev(struct usb_device *parent, struct usb_bus *bus, unsigned port1)¶: usb device constructor (usbcore-internal)

Parameters

struct usb_device * parent: hub to which device is connected; null to allocate a root hub
struct usb_bus * bus: bus used to access the device
unsigned port1: one-based index of port; ignored for root hubs

Context

!in_interrupt()

Description

Only hub drivers (including virtual root hub drivers for host controllers) should ever call this.

This call may not be used in a non-sleeping context.

Return

On success, a pointer to the allocated usb device. NULL on failure.

struct usb_device *usb_get_dev(struct usb_device *dev)¶: increments the reference count of the usb device structure

Parameters

struct usb_device * dev: the device being referenced

Description

Each live reference to a device should be refcounted.

Drivers for USB interfaces should normally record such references in their probe() methods, when they bind to an interface, and release them by calling usb_put_dev(), in their disconnect() methods.

Return

A pointer to the device with the incremented reference counter.

void usb_put_dev(struct usb_device *dev)¶: release a use of the usb device structure

Parameters

struct usb_device * dev: device that’s been disconnected

Description

Must be called when a user of a device is finished with it. When the last user of the device calls this function, the memory of the device is freed.

struct usb_interface *usb_get_intf(struct usb_interface *intf)¶: increments the reference count of the usb interface structure

Parameters

struct usb_interface * intf: the interface being referenced

Description

Each live reference to a interface must be refcounted.

Drivers for USB interfaces should normally record such references in their probe() methods, when they bind to an interface, and release them by calling usb_put_intf(), in their disconnect() methods.

Return

A pointer to the interface with the incremented reference counter.

void usb_put_intf(struct usb_interface *intf)¶: release a use of the usb interface structure

Parameters

struct usb_interface * intf: interface that’s been decremented

Description

Must be called when a user of an interface is finished with it. When the last user of the interface calls this function, the memory of the interface is freed.

int usb_lock_device_for_reset(struct usb_device *udev, const struct usb_interface *iface)¶: cautiously acquire the lock for a usb device structure

Parameters

struct usb_device * udev: device that’s being locked
const struct usb_interface * iface: interface bound to the driver making the request (optional)

Description

Attempts to acquire the device lock, but fails if the device is NOTATTACHED or SUSPENDED, or if iface is specified and the interface is neither BINDING nor BOUND. Rather than sleeping to wait for the lock, the routine polls repeatedly. This is to prevent deadlock with disconnect; in some drivers (such as usb-storage) the disconnect() or suspend() method will block waiting for a device reset to complete.

Return

A negative error code for failure, otherwise 0.

int usb_get_current_frame_number(struct usb_device *dev)¶: return current bus frame number

Parameters

struct usb_device * dev: the device whose bus is being queried

Return

The current frame number for the USB host controller used with the given USB device. This can be used when scheduling isochronous requests.

Note

Different kinds of host controller have different “scheduling horizons”. While one type might support scheduling only 32 frames into the future, others could support scheduling up to 1024 frames into the future.

void *usb_alloc_coherent(struct usb_device *dev, size_t size, gfp_t mem_flags, dma_addr_t *dma)¶: allocate dma-consistent buffer for URB_NO_xxx_DMA_MAP

Parameters

struct usb_device * dev: device the buffer will be used with
size_t size: requested buffer size
gfp_t mem_flags: affect whether allocation may block
dma_addr_t * dma: used to return DMA address of buffer

Return

Either null (indicating no buffer could be allocated), or the cpu-space pointer to a buffer that may be used to perform DMA to the specified device. Such cpu-space buffers are returned along with the DMA address (through the pointer provided).

Note

These buffers are used with URB_NO_xxx_DMA_MAP set in urb->transfer_flags to avoid behaviors like using “DMA bounce buffers”, or thrashing IOMMU hardware during URB completion/resubmit. The implementation varies between platforms, depending on details of how DMA will work to this device. Using these buffers also eliminates cacheline sharing problems on architectures where CPU caches are not DMA-coherent. On systems without bus-snooping caches, these buffers are uncached.

When the buffer is no longer used, free it with usb_free_coherent().

void usb_free_coherent(struct usb_device *dev, size_t size, void *addr, dma_addr_t dma)¶: free memory allocated with usb_alloc_coherent()

Parameters

struct usb_device * dev: device the buffer was used with
size_t size: requested buffer size
void * addr: CPU address of buffer
dma_addr_t dma: DMA address of buffer

Description

This reclaims an I/O buffer, letting it be reused. The memory must have been allocated using usb_alloc_coherent(), and the parameters must match those provided in that allocation request.

int usb_hub_clear_tt_buffer(struct urb *urb)¶: clear control/bulk TT state in high speed hub

Parameters

struct urb * urb: an URB associated with the failed or incomplete split transaction

Description

High speed HCDs use this to tell the hub driver that some split control or bulk transaction failed in a way that requires clearing internal state of a transaction translator. This is normally detected (and reported) from interrupt context.

It may not be possible for that hub to handle additional full (or low) speed transactions until that state is fully cleared out.

Return

0 if successful. A negative error code otherwise.

void usb_set_device_state(struct usb_device *udev, enum usb_device_state new_state)¶: change a device’s current state (usbcore, hcds)

Parameters

struct usb_device * udev: pointer to device whose state should be changed
enum usb_device_state new_state: new state value to be stored

Description

udev->state is _not_ fully protected by the device lock. Although most transitions are made only while holding the lock, the state can can change to USB_STATE_NOTATTACHED at almost any time. This is so that devices can be marked as disconnected as soon as possible, without having to wait for any semaphores to be released. As a result, all changes to any device’s state must be protected by the device_state_lock spinlock.

Once a device has been added to the device tree, all changes to its state should be made using this routine. The state should _not_ be set directly.

If udev->state is already USB_STATE_NOTATTACHED then no change is made. Otherwise udev->state is set to new_state, and if new_state is USB_STATE_NOTATTACHED then all of udev’s descendants’ states are also set to USB_STATE_NOTATTACHED.

void usb_root_hub_lost_power(struct usb_device *rhdev)¶: called by HCD if the root hub lost Vbus power

Parameters

struct usb_device * rhdev: struct usb_device for the root hub

Description

The USB host controller driver calls this function when its root hub is resumed and Vbus power has been interrupted or the controller has been reset. The routine marks rhdev as having lost power. When the hub driver is resumed it will take notice and carry out power-session recovery for all the “USB-PERSIST”-enabled child devices; the others will be disconnected.

int usb_reset_device(struct usb_device *udev)¶: warn interface drivers and perform a USB port reset

Parameters

struct usb_device * udev: device to reset (not in NOTATTACHED state)

Description

Warns all drivers bound to registered interfaces (using their pre_reset method), performs the port reset, and then lets the drivers know that the reset is over (using their post_reset method).

Return

The same as for usb_reset_and_verify_device().

Note

The caller must own the device lock. For example, it’s safe to use this from a driver probe() routine after downloading new firmware. For calls that might not occur during probe(), drivers should lock the device using usb_lock_device_for_reset().

If an interface is currently being probed or disconnected, we assume its driver knows how to handle resets. For all other interfaces, if the driver doesn’t have pre_reset and post_reset methods then we attempt to unbind it and rebind afterward.

void usb_queue_reset_device(struct usb_interface *iface)¶: Reset a USB device from an atomic context

Parameters

struct usb_interface * iface: USB interface belonging to the device to reset

Description

This function can be used to reset a USB device from an atomic context, where usb_reset_device() won’t work (as it blocks).

Doing a reset via this method is functionally equivalent to calling usb_reset_device(), except for the fact that it is delayed to a workqueue. This means that any drivers bound to other interfaces might be unbound, as well as users from usbfs in user space.

Corner cases:

Scheduling two resets at the same time from two different drivers attached to two different interfaces of the same device is possible; depending on how the driver attached to each interface handles ->pre_reset(), the second reset might happen or not.
If the reset is delayed so long that the interface is unbound from its driver, the reset will be skipped.
This function can be called during .probe(). It can also be called during .disconnect(), but doing so is pointless because the reset will not occur. If you really want to reset the device during .disconnect(), call usb_reset_device() directly – but watch out for nested unbinding issues!

struct usb_device *usb_hub_find_child(struct usb_device *hdev, int port1)¶: Get the pointer of child device attached to the port which is specified by port1.

Parameters

struct usb_device * hdev: USB device belonging to the usb hub
int port1: port num to indicate which port the child device is attached to.

Description

USB drivers call this function to get hub’s child device pointer.

Return

NULL if input param is invalid and child’s usb_device pointer if non-NULL.

Host Controller APIs¶

These APIs are only for use by host controller drivers, most of which implement standard register interfaces such as XHCI, EHCI, OHCI, or UHCI. UHCI was one of the first interfaces, designed by Intel and also used by VIA; it doesn’t do much in hardware. OHCI was designed later, to have the hardware do more work (bigger transfers, tracking protocol state, and so on). EHCI was designed with USB 2.0; its design has features that resemble OHCI (hardware does much more work) as well as UHCI (some parts of ISO support, TD list processing). XHCI was designed with USB 3.0. It continues to shift support for functionality into hardware.

There are host controllers other than the “big three”, although most PCI based controllers (and a few non-PCI based ones) use one of those interfaces. Not all host controllers use DMA; some use PIO, and there is also a simulator and a virtual host controller to pipe USB over the network.

The same basic APIs are available to drivers for all those controllers. For historical reasons they are in two layers: struct usb_bus is a rather thin layer that became available in the 2.2 kernels, while struct usb_hcd is a more featureful layer that lets HCDs share common code, to shrink driver size and significantly reduce hcd-specific behaviors.

long usb_calc_bus_time(int speed, int is_input, int isoc, int bytecount)¶: approximate periodic transaction time in nanoseconds

Parameters

int speed: from dev->speed; USB_SPEED_{LOW,FULL,HIGH}
int is_input: true iff the transaction sends data to the host
int isoc: true for isochronous transactions, false for interrupt ones
int bytecount: how many bytes in the transaction.

Return

Approximate bus time in nanoseconds for a periodic transaction.

Note

See USB 2.0 spec section 5.11.3; only periodic transfers need to be scheduled in software, this function is only used for such scheduling.

int usb_hcd_link_urb_to_ep(struct usb_hcd *hcd, struct urb *urb)¶: add an URB to its endpoint queue

Parameters

struct usb_hcd * hcd: host controller to which urb was submitted
struct urb * urb: URB being submitted

Description

Host controller drivers should call this routine in their enqueue() method. The HCD’s private spinlock must be held and interrupts must be disabled. The actions carried out here are required for URB submission, as well as for endpoint shutdown and for usb_kill_urb.

Return

0 for no error, otherwise a negative error code (in which case the enqueue() method must fail). If no error occurs but enqueue() fails anyway, it must call usb_hcd_unlink_urb_from_ep() before releasing the private spinlock and returning.

int usb_hcd_check_unlink_urb(struct usb_hcd *hcd, struct urb *urb, int status)¶: check whether an URB may be unlinked

Parameters

struct usb_hcd * hcd: host controller to which urb was submitted
struct urb * urb: URB being checked for unlinkability
int status: error code to store in urb if the unlink succeeds

Description

Host controller drivers should call this routine in their dequeue() method. The HCD’s private spinlock must be held and interrupts must be disabled. The actions carried out here are required for making sure than an unlink is valid.

Return

0 for no error, otherwise a negative error code (in which case the dequeue() method must fail). The possible error codes are:

-EIDRM: urb was not submitted or has already completed.
The completion function may not have been called yet.

-EBUSY: urb has already been unlinked.

void usb_hcd_unlink_urb_from_ep(struct usb_hcd *hcd, struct urb *urb)¶: remove an URB from its endpoint queue

Parameters

struct usb_hcd * hcd: host controller to which urb was submitted
struct urb * urb: URB being unlinked

Description

Host controller drivers should call this routine before calling usb_hcd_giveback_urb(). The HCD’s private spinlock must be held and interrupts must be disabled. The actions carried out here are required for URB completion.

void usb_hcd_giveback_urb(struct usb_hcd *hcd, struct urb *urb, int status)¶: return URB from HCD to device driver

Parameters

struct usb_hcd * hcd: host controller returning the URB
struct urb * urb: urb being returned to the USB device driver.
int status: completion status code for the URB.

Context

in_interrupt()

Description

This hands the URB from HCD to its USB device driver, using its completion function. The HCD has freed all per-urb resources (and is done using urb->hcpriv). It also released all HCD locks; the device driver won’t cause problems if it frees, modifies, or resubmits this URB.

If urb was unlinked, the value of status will be overridden by urb->unlinked. Erroneous short transfers are detected in case the HCD hasn’t checked for them.

int usb_alloc_streams(struct usb_interface *interface, struct usb_host_endpoint **eps, unsigned int num_eps, unsigned int num_streams, gfp_t mem_flags)¶: allocate bulk endpoint stream IDs.

Parameters

struct usb_interface * interface: alternate setting that includes all endpoints.
struct usb_host_endpoint ** eps: array of endpoints that need streams.
unsigned int num_eps: number of endpoints in the array.
unsigned int num_streams: number of streams to allocate.
gfp_t mem_flags: flags hcd should use to allocate memory.

Description

Sets up a group of bulk endpoints to have num_streams stream IDs available. Drivers may queue multiple transfers to different stream IDs, which may complete in a different order than they were queued.

Return

On success, the number of allocated streams. On failure, a negative error code.

int usb_free_streams(struct usb_interface *interface, struct usb_host_endpoint **eps, unsigned int num_eps, gfp_t mem_flags)¶: free bulk endpoint stream IDs.

Parameters

struct usb_interface * interface: alternate setting that includes all endpoints.
struct usb_host_endpoint ** eps: array of endpoints to remove streams from.
unsigned int num_eps: number of endpoints in the array.
gfp_t mem_flags: flags hcd should use to allocate memory.

Description

Reverts a group of bulk endpoints back to not using stream IDs. Can fail if we are given bad arguments, or HCD is broken.

Return

0 on success. On failure, a negative error code.

void usb_hcd_resume_root_hub(struct usb_hcd *hcd)¶: called by HCD to resume its root hub

Parameters

struct usb_hcd * hcd: host controller for this root hub

Description

The USB host controller calls this function when its root hub is suspended (with the remote wakeup feature enabled) and a remote wakeup request is received. The routine submits a workqueue request to resume the root hub (that is, manage its downstream ports again).

int usb_bus_start_enum(struct usb_bus *bus, unsigned port_num)¶: start immediate enumeration (for OTG)

Parameters

struct usb_bus * bus: the bus (must use hcd framework)
unsigned port_num: 1-based number of port; usually bus->otg_port

Context

in_interrupt()

Description

Starts enumeration, with an immediate reset followed later by hub_wq identifying and possibly configuring the device. This is needed by OTG controller drivers, where it helps meet HNP protocol timing requirements for starting a port reset.

Return

0 if successful.

irqreturn_t usb_hcd_irq(int irq, void *__hcd)¶: hook IRQs to HCD framework (bus glue)

Parameters

int irq: the IRQ being raised
void * __hcd: pointer to the HCD whose IRQ is being signaled

Description

If the controller isn’t HALTed, calls the driver’s irq handler. Checks whether the controller is now dead.

Return

IRQ_HANDLED if the IRQ was handled. IRQ_NONE otherwise.

void usb_hc_died(struct usb_hcd *hcd)¶: report abnormal shutdown of a host controller (bus glue)

Parameters

struct usb_hcd * hcd: pointer to the HCD representing the controller

Description

This is called by bus glue to report a USB host controller that died while operations may still have been pending. It’s called automatically by the PCI glue, so only glue for non-PCI busses should need to call it.

Only call this function with the primary HCD.

struct usb_hcd *usb_create_shared_hcd(const struct hc_driver *driver, struct device *dev, const char *bus_name, struct usb_hcd *primary_hcd)¶: create and initialize an HCD structure

Parameters

const struct hc_driver * driver: HC driver that will use this hcd
struct device * dev: device for this HC, stored in hcd->self.controller
const char * bus_name: value to store in hcd->self.bus_name
struct usb_hcd * primary_hcd: a pointer to the usb_hcd structure that is sharing the PCI device. Only allocate certain resources for the primary HCD

Context

!in_interrupt()

Description

Allocate a struct usb_hcd, with extra space at the end for the HC driver’s private data. Initialize the generic members of the hcd structure.

Return

On success, a pointer to the created and initialized HCD structure. On failure (e.g. if memory is unavailable), NULL.

struct usb_hcd *usb_create_hcd(const struct hc_driver *driver, struct device *dev, const char *bus_name)¶: create and initialize an HCD structure

Parameters

const struct hc_driver * driver: HC driver that will use this hcd
struct device * dev: device for this HC, stored in hcd->self.controller
const char * bus_name: value to store in hcd->self.bus_name

Context

!in_interrupt()

Description

Allocate a struct usb_hcd, with extra space at the end for the HC driver’s private data. Initialize the generic members of the hcd structure.

Return

On success, a pointer to the created and initialized HCD structure. On failure (e.g. if memory is unavailable), NULL.

int usb_add_hcd(struct usb_hcd *hcd, unsigned int irqnum, unsigned long irqflags)¶: finish generic HCD structure initialization and register

Parameters

struct usb_hcd * hcd: the usb_hcd structure to initialize
unsigned int irqnum: Interrupt line to allocate
unsigned long irqflags: Interrupt type flags

Description

Finish the remaining parts of generic HCD initialization: allocate the buffers of consistent memory, register the bus, request the IRQ line, and call the driver’s reset() and start() routines.

void usb_remove_hcd(struct usb_hcd *hcd)¶: shutdown processing for generic HCDs

Parameters

struct usb_hcd * hcd: the usb_hcd structure to remove

Context

!in_interrupt()

Description

Disconnects the root hub, then reverses the effects of usb_add_hcd(), invoking the HCD’s stop() method.

int usb_hcd_pci_probe(struct pci_dev *dev, const struct pci_device_id *id)¶: initialize PCI-based HCDs

Parameters

struct pci_dev * dev: USB Host Controller being probed
const struct pci_device_id * id: pci hotplug id connecting controller to HCD framework

Context

!in_interrupt()

Description

Allocates basic PCI resources for this USB host controller, and then invokes the start() method for the HCD associated with it through the hotplug entry’s driver_data.

Store this function in the HCD’s struct pci_driver as probe().

Return

0 if successful.

void usb_hcd_pci_remove(struct pci_dev *dev)¶: shutdown processing for PCI-based HCDs

Parameters

struct pci_dev * dev: USB Host Controller being removed

Context

!in_interrupt()

Description

Reverses the effect of usb_hcd_pci_probe(), first invoking the HCD’s stop() method. It is always called from a thread context, normally “rmmod”, “apmd”, or something similar.

Store this function in the HCD’s struct pci_driver as remove().

void usb_hcd_pci_shutdown(struct pci_dev *dev)¶: shutdown host controller

Parameters

struct pci_dev * dev: USB Host Controller being shutdown

int hcd_buffer_create(struct usb_hcd *hcd)¶: initialize buffer pools

Parameters

struct usb_hcd * hcd: the bus whose buffer pools are to be initialized

Context

!in_interrupt()

Description

Call this as part of initializing a host controller that uses the dma memory allocators. It initializes some pools of dma-coherent memory that will be shared by all drivers using that controller.

Call hcd_buffer_destroy() to clean up after using those pools.

Return

0 if successful. A negative errno value otherwise.

void hcd_buffer_destroy(struct usb_hcd *hcd)¶: deallocate buffer pools

Parameters

struct usb_hcd * hcd: the bus whose buffer pools are to be destroyed

Context

!in_interrupt()

Description

This frees the buffer pools created by hcd_buffer_create().

The USB character device nodes¶

This chapter presents the Linux character device nodes. You may prefer to avoid writing new kernel code for your USB driver. User mode device drivers are usually packaged as applications or libraries, and may use character devices through some programming library that wraps it. Such libraries include:

libusb for C/C++, and

jUSB for Java.

Some old information about it can be seen at the “USB Device Filesystem” section of the USB Guide. The latest copy of the USB Guide can be found at http://www.linux-usb.org/

Note

They were used to be implemented via usbfs, but this is not part of the sysfs debug interface.

This particular documentation is incomplete, especially with respect to the asynchronous mode. As of kernel 2.5.66 the code and this (new) documentation need to be cross-reviewed.

What files are in “devtmpfs”?¶

Conventionally mounted at /dev/bus/usb/, usbfs features include:

/dev/bus/usb/BBB/DDD … magic files exposing the each device’s configuration descriptors, and supporting a series of ioctls for making device requests, including I/O to devices. (Purely for access by programs.)

Each bus is given a number (BBB) based on when it was enumerated; within each bus, each device is given a similar number (DDD). Those BBB/DDD paths are not “stable” identifiers; expect them to change even if you always leave the devices plugged in to the same hub port. Don’t even think of saving these in application configuration files. Stable identifiers are available, for user mode applications that want to use them. HID and networking devices expose these stable IDs, so that for example you can be sure that you told the right UPS to power down its second server. Pleast note that it doesn’t (yet) expose those IDs.

/dev/bus/usb/BBB/DDD¶

Use these files in one of these basic ways:

They can be read, producing first the device descriptor (18 bytes) and then the descriptors for the current configuration. See the USB 2.0 spec for details about those binary data formats. You’ll need to convert most multibyte values from little endian format to your native host byte order, although a few of the fields in the device descriptor (both of the BCD-encoded fields, and the vendor and product IDs) will be byteswapped for you. Note that configuration descriptors include descriptors for interfaces, altsettings, endpoints, and maybe additional class descriptors.
Perform USB operations using ioctl() requests to make endpoint I/O requests (synchronously or asynchronously) or manage the device. These requests need the CAP_SYS_RAWIO capability, as well as filesystem access permissions. Only one ioctl request can be made on one of these device files at a time. This means that if you are synchronously reading an endpoint from one thread, you won’t be able to write to a different endpoint from another thread until the read completes. This works for half duplex protocols, but otherwise you’d use asynchronous i/o requests.

Each connected USB device has one file. The BBB indicates the bus number. The DDD indicates the device address on that bus. Both of these numbers are assigned sequentially, and can be reused, so you can’t rely on them for stable access to devices. For example, it’s relatively common for devices to re-enumerate while they are still connected (perhaps someone jostled their power supply, hub, or USB cable), so a device might be 002/027 when you first connect it and 002/048 sometime later.

These files can be read as binary data. The binary data consists of first the device descriptor, then the descriptors for each configuration of the device. Multi-byte fields in the device descriptor are converted to host endianness by the kernel. The configuration descriptors are in bus endian format! The configuration descriptor are wTotalLength bytes apart. If a device returns less configuration descriptor data than indicated by wTotalLength there will be a hole in the file for the missing bytes. This information is also shown in text form by the /sys/kernel/debug/usb/devices file, described later.

These files may also be used to write user-level drivers for the USB devices. You would open the /dev/bus/usb/BBB/DDD file read/write, read its descriptors to make sure it’s the device you expect, and then bind to an interface (or perhaps several) using an ioctl call. You would issue more ioctls to the device to communicate to it using control, bulk, or other kinds of USB transfers. The IOCTLs are listed in the <linux/usbdevice_fs.h> file, and at this writing the source code (linux/drivers/usb/core/devio.c) is the primary reference for how to access devices through those files.

Note that since by default these BBB/DDD files are writable only by root, only root can write such user mode drivers. You can selectively grant read/write permissions to other users by using chmod. Also, usbfs mount options such as devmode=0666 may be helpful.

Life Cycle of User Mode Drivers¶

Such a driver first needs to find a device file for a device it knows how to handle. Maybe it was told about it because a /sbin/hotplug event handling agent chose that driver to handle the new device. Or maybe it’s an application that scans all the /dev/bus/usb device files, and ignores most devices. In either case, it should read() all the descriptors from the device file, and check them against what it knows how to handle. It might just reject everything except a particular vendor and product ID, or need a more complex policy.

Never assume there will only be one such device on the system at a time! If your code can’t handle more than one device at a time, at least detect when there’s more than one, and have your users choose which device to use.

Once your user mode driver knows what device to use, it interacts with it in either of two styles. The simple style is to make only control requests; some devices don’t need more complex interactions than those. (An example might be software using vendor-specific control requests for some initialization or configuration tasks, with a kernel driver for the rest.)

More likely, you need a more complex style driver: one using non-control endpoints, reading or writing data and claiming exclusive use of an interface. Bulk transfers are easiest to use, but only their sibling interrupt transfers work with low speed devices. Both interrupt and isochronous transfers offer service guarantees because their bandwidth is reserved. Such “periodic” transfers are awkward to use through usbfs, unless you’re using the asynchronous calls. However, interrupt transfers can also be used in a synchronous “one shot” style.

Your user-mode driver should never need to worry about cleaning up request state when the device is disconnected, although it should close its open file descriptors as soon as it starts seeing the ENODEV errors.

The ioctl() Requests¶

To use these ioctls, you need to include the following headers in your userspace program:

#include <linux/usb.h>
#include <linux/usbdevice_fs.h>
#include <asm/byteorder.h>

The standard USB device model requests, from “Chapter 9” of the USB 2.0 specification, are automatically included from the <linux/usb/ch9.h> header.

Unless noted otherwise, the ioctl requests described here will update the modification time on the usbfs file to which they are applied (unless they fail). A return of zero indicates success; otherwise, a standard USB error code is returned (These are documented in USB Error codes).

Each of these files multiplexes access to several I/O streams, one per endpoint. Each device has one control endpoint (endpoint zero) which supports a limited RPC style RPC access. Devices are configured by hub_wq (in the kernel) setting a device-wide configuration that affects things like power consumption and basic functionality. The endpoints are part of USB interfaces, which may have altsettings affecting things like which endpoints are available. Many devices only have a single configuration and interface, so drivers for them will ignore configurations and altsettings.

Management/Status Requests¶

A number of usbfs requests don’t deal very directly with device I/O. They mostly relate to device management and status. These are all synchronous requests.

USBDEVFS_CLAIMINTERFACE

This is used to force usbfs to claim a specific interface, which has not previously been claimed by usbfs or any other kernel driver. The ioctl parameter is an integer holding the number of the interface (bInterfaceNumber from descriptor).

Note that if your driver doesn’t claim an interface before trying to use one of its endpoints, and no other driver has bound to it, then the interface is automatically claimed by usbfs.

This claim will be released by a RELEASEINTERFACE ioctl, or by closing the file descriptor. File modification time is not updated by this request.

USBDEVFS_CONNECTINFO

Says whether the device is lowspeed. The ioctl parameter points to a structure like this:

struct usbdevfs_connectinfo {
        unsigned int   devnum;
        unsigned char  slow;
};

File modification time is not updated by this request.

You can’t tell whether a “not slow” device is connected at high speed (480 MBit/sec) or just full speed (12 MBit/sec). You should know the devnum value already, it’s the DDD value of the device file name.

USBDEVFS_GETDRIVER

Returns the name of the kernel driver bound to a given interface (a string). Parameter is a pointer to this structure, which is modified:

struct usbdevfs_getdriver {
        unsigned int  interface;
        char          driver[USBDEVFS_MAXDRIVERNAME + 1];
};

File modification time is not updated by this request.

USBDEVFS_IOCTL

Passes a request from userspace through to a kernel driver that has an ioctl entry in the struct usb_driver it registered:

struct usbdevfs_ioctl {
        int     ifno;
        int     ioctl_code;
        void    *data;
};

/* user mode call looks like this.
 * 'request' becomes the driver->ioctl() 'code' parameter.
 * the size of 'param' is encoded in 'request', and that data
 * is copied to or from the driver->ioctl() 'buf' parameter.
 */
static int
usbdev_ioctl (int fd, int ifno, unsigned request, void *param)
{
        struct usbdevfs_ioctl   wrapper;

        wrapper.ifno = ifno;
        wrapper.ioctl_code = request;
        wrapper.data = param;

        return ioctl (fd, USBDEVFS_IOCTL, &wrapper);
}

File modification time is not updated by this request.

This request lets kernel drivers talk to user mode code through filesystem operations even when they don’t create a character or block special device. It’s also been used to do things like ask devices what device special file should be used. Two pre-defined ioctls are used to disconnect and reconnect kernel drivers, so that user mode code can completely manage binding and configuration of devices.

USBDEVFS_RELEASEINTERFACE

This is used to release the claim usbfs made on interface, either implicitly or because of a USBDEVFS_CLAIMINTERFACE call, before the file descriptor is closed. The ioctl parameter is an integer holding the number of the interface (bInterfaceNumber from descriptor); File modification time is not updated by this request.

Warning

No security check is made to ensure that the task which made the claim is the one which is releasing it. This means that user mode driver may interfere other ones.

USBDEVFS_RESETEP

Resets the data toggle value for an endpoint (bulk or interrupt) to DATA0. The ioctl parameter is an integer endpoint number (1 to 15, as identified in the endpoint descriptor), with USB_DIR_IN added if the device’s endpoint sends data to the host.

Warning

Avoid using this request. It should probably be removed. Using it typically means the device and driver will lose toggle synchronization. If you really lost synchronization, you likely need to completely handshake with the device, using a request like CLEAR_HALT or SET_INTERFACE.

USBDEVFS_DROP_PRIVILEGES

This is used to relinquish the ability to do certain operations which are considered to be privileged on a usbfs file descriptor. This includes claiming arbitrary interfaces, resetting a device on which there are currently claimed interfaces from other users, and issuing USBDEVFS_IOCTL calls. The ioctl parameter is a 32 bit mask of interfaces the user is allowed to claim on this file descriptor. You may issue this ioctl more than one time to narrow said mask.

Synchronous I/O Support¶

Synchronous requests involve the kernel blocking until the user mode request completes, either by finishing successfully or by reporting an error. In most cases this is the simplest way to use usbfs, although as noted above it does prevent performing I/O to more than one endpoint at a time.

USBDEVFS_BULK

Issues a bulk read or write request to the device. The ioctl parameter is a pointer to this structure:

struct usbdevfs_bulktransfer {
        unsigned int  ep;
        unsigned int  len;
        unsigned int  timeout; /* in milliseconds */
        void          *data;
};

The ep value identifies a bulk endpoint number (1 to 15, as identified in an endpoint descriptor), masked with USB_DIR_IN when referring to an endpoint which sends data to the host from the device. The length of the data buffer is identified by len; Recent kernels support requests up to about 128KBytes. FIXME say how read length is returned, and how short reads are handled..

USBDEVFS_CLEAR_HALT

Clears endpoint halt (stall) and resets the endpoint toggle. This is only meaningful for bulk or interrupt endpoints. The ioctl parameter is an integer endpoint number (1 to 15, as identified in an endpoint descriptor), masked with USB_DIR_IN when referring to an endpoint which sends data to the host from the device.

Use this on bulk or interrupt endpoints which have stalled, returning -EPIPE status to a data transfer request. Do not issue the control request directly, since that could invalidate the host’s record of the data toggle.

USBDEVFS_CONTROL

Issues a control request to the device. The ioctl parameter points to a structure like this:

struct usbdevfs_ctrltransfer {
        __u8   bRequestType;
        __u8   bRequest;
        __u16  wValue;
        __u16  wIndex;
        __u16  wLength;
        __u32  timeout;  /* in milliseconds */
        void   *data;
};

The first eight bytes of this structure are the contents of the SETUP packet to be sent to the device; see the USB 2.0 specification for details. The bRequestType value is composed by combining a USB_TYPE_* value, a USB_DIR_* value, and a USB_RECIP_* value (from linux/usb.h). If wLength is nonzero, it describes the length of the data buffer, which is either written to the device (USB_DIR_OUT) or read from the device (USB_DIR_IN).

At this writing, you can’t transfer more than 4 KBytes of data to or from a device; usbfs has a limit, and some host controller drivers have a limit. (That’s not usually a problem.) Also there’s no way to say it’s not OK to get a short read back from the device.

USBDEVFS_RESET

Does a USB level device reset. The ioctl parameter is ignored. After the reset, this rebinds all device interfaces. File modification time is not updated by this request.

Warning

Avoid using this call until some usbcore bugs get fixed, since it does not fully synchronize device, interface, and driver (not just usbfs) state.

USBDEVFS_SETINTERFACE

Sets the alternate setting for an interface. The ioctl parameter is a pointer to a structure like this:

struct usbdevfs_setinterface {
        unsigned int  interface;
        unsigned int  altsetting;
};

File modification time is not updated by this request.

Those struct members are from some interface descriptor applying to the current configuration. The interface number is the bInterfaceNumber value, and the altsetting number is the bAlternateSetting value. (This resets each endpoint in the interface.)

USBDEVFS_SETCONFIGURATION

Issues the usb_set_configuration() call for the device. The parameter is an integer holding the number of a configuration (bConfigurationValue from descriptor). File modification time is not updated by this request.

Warning

Avoid using this call until some usbcore bugs get fixed, since it does not fully synchronize device, interface, and driver (not just usbfs) state.

Asynchronous I/O Support¶

As mentioned above, there are situations where it may be important to initiate concurrent operations from user mode code. This is particularly important for periodic transfers (interrupt and isochronous), but it can be used for other kinds of USB requests too. In such cases, the asynchronous requests described here are essential. Rather than submitting one request and having the kernel block until it completes, the blocking is separate.

These requests are packaged into a structure that resembles the URB used by kernel device drivers. (No POSIX Async I/O support here, sorry.) It identifies the endpoint type (USBDEVFS_URB_TYPE_*), endpoint (number, masked with USB_DIR_IN as appropriate), buffer and length, and a user “context” value serving to uniquely identify each request. (It’s usually a pointer to per-request data.) Flags can modify requests (not as many as supported for kernel drivers).

Each request can specify a realtime signal number (between SIGRTMIN and SIGRTMAX, inclusive) to request a signal be sent when the request completes.

When usbfs returns these urbs, the status value is updated, and the buffer may have been modified. Except for isochronous transfers, the actual_length is updated to say how many bytes were transferred; if the USBDEVFS_URB_DISABLE_SPD flag is set (“short packets are not OK”), if fewer bytes were read than were requested then you get an error report:

struct usbdevfs_iso_packet_desc {
        unsigned int                     length;
        unsigned int                     actual_length;
        unsigned int                     status;
};

struct usbdevfs_urb {
        unsigned char                    type;
        unsigned char                    endpoint;
        int                              status;
        unsigned int                     flags;
        void                             *buffer;
        int                              buffer_length;
        int                              actual_length;
        int                              start_frame;
        int                              number_of_packets;
        int                              error_count;
        unsigned int                     signr;
        void                             *usercontext;
        struct usbdevfs_iso_packet_desc  iso_frame_desc[];
};

For these asynchronous requests, the file modification time reflects when the request was initiated. This contrasts with their use with the synchronous requests, where it reflects when requests complete.

USBDEVFS_DISCARDURB: TBS File modification time is not updated by this request.
USBDEVFS_DISCSIGNAL: TBS File modification time is not updated by this request.
USBDEVFS_REAPURB: TBS File modification time is not updated by this request.
USBDEVFS_REAPURBNDELAY: TBS File modification time is not updated by this request.
USBDEVFS_SUBMITURB: TBS

The USB devices¶

The USB devices are now exported via debugfs:

/sys/kernel/debug/usb/devices … a text file showing each of the USB devices on known to the kernel, and their configuration descriptors. You can also poll() this to learn about new devices.

/sys/kernel/debug/usb/devices¶

This file is handy for status viewing tools in user mode, which can scan the text format and ignore most of it. More detailed device status (including class and vendor status) is available from device-specific files. For information about the current format of this file, see below.

This file, in combination with the poll() system call, can also be used to detect when devices are added or removed:

int fd;
struct pollfd pfd;

fd = open("/sys/kernel/debug/usb/devices", O_RDONLY);
pfd = { fd, POLLIN, 0 };
for (;;) {
    /* The first time through, this call will return immediately. */
    poll(&pfd, 1, -1);

    /* To see what's changed, compare the file's previous and current
       contents or scan the filesystem.  (Scanning is more precise.) */
}

Note that this behavior is intended to be used for informational and debug purposes. It would be more appropriate to use programs such as udev or HAL to initialize a device or start a user-mode helper program, for instance.

In this file, each device’s output has multiple lines of ASCII output.

I made it ASCII instead of binary on purpose, so that someone can obtain some useful data from it without the use of an auxiliary program. However, with an auxiliary program, the numbers in the first 4 columns of each T: line (topology info: Lev, Prnt, Port, Cnt) can be used to build a USB topology diagram.

Each line is tagged with a one-character ID for that line:

T = Topology (etc.)
B = Bandwidth (applies only to USB host controllers, which are
virtualized as root hubs)
D = Device descriptor info.
P = Product ID info. (from Device descriptor, but they won't fit
together on one line)
S = String descriptors.
C = Configuration descriptor info. (* = active configuration)
I = Interface descriptor info.
E = Endpoint descriptor info.

/sys/kernel/debug/usb/devices output format¶

Legend::: d = decimal number (may have leading spaces or 0’s) x = hexadecimal number (may have leading spaces or 0’s) s = string

Topology info¶

T:  Bus=dd Lev=dd Prnt=dd Port=dd Cnt=dd Dev#=ddd Spd=dddd MxCh=dd
|   |      |      |       |       |      |        |        |__MaxChildren
|   |      |      |       |       |      |        |__Device Speed in Mbps
|   |      |      |       |       |      |__DeviceNumber
|   |      |      |       |       |__Count of devices at this level
|   |      |      |       |__Connector/Port on Parent for this device
|   |      |      |__Parent DeviceNumber
|   |      |__Level in topology for this bus
|   |__Bus number
|__Topology info tag

Speed may be:

1.5

Mbit/s for low speed USB

12

Mbit/s for full speed USB

480

Mbit/s for high speed USB (added for USB 2.0); also used for Wireless USB, which has no fixed speed

5000

Mbit/s for SuperSpeed USB (added for USB 3.0)

For reasons lost in the mists of time, the Port number is always too low by 1. For example, a device plugged into port 4 will show up with Port=03.

Bandwidth info¶

B:  Alloc=ddd/ddd us (xx%), #Int=ddd, #Iso=ddd
|   |                       |         |__Number of isochronous requests
|   |                       |__Number of interrupt requests
|   |__Total Bandwidth allocated to this bus
|__Bandwidth info tag

Bandwidth allocation is an approximation of how much of one frame (millisecond) is in use. It reflects only periodic transfers, which are the only transfers that reserve bandwidth. Control and bulk transfers use all other bandwidth, including reserved bandwidth that is not used for transfers (such as for short packets).

The percentage is how much of the “reserved” bandwidth is scheduled by those transfers. For a low or full speed bus (loosely, “USB 1.1”), 90% of the bus bandwidth is reserved. For a high speed bus (loosely, “USB 2.0”) 80% is reserved.

Device descriptor info & Product ID info¶

D:  Ver=x.xx Cls=xx(s) Sub=xx Prot=xx MxPS=dd #Cfgs=dd
P:  Vendor=xxxx ProdID=xxxx Rev=xx.xx

where:

D:  Ver=x.xx Cls=xx(sssss) Sub=xx Prot=xx MxPS=dd #Cfgs=dd
|   |        |             |      |       |       |__NumberConfigurations
|   |        |             |      |       |__MaxPacketSize of Default Endpoint
|   |        |             |      |__DeviceProtocol
|   |        |             |__DeviceSubClass
|   |        |__DeviceClass
|   |__Device USB version
|__Device info tag #1

where:

P:  Vendor=xxxx ProdID=xxxx Rev=xx.xx
|   |           |           |__Product revision number
|   |           |__Product ID code
|   |__Vendor ID code
|__Device info tag #2

String descriptor info¶

S:  Manufacturer=ssss
|   |__Manufacturer of this device as read from the device.
|      For USB host controller drivers (virtual root hubs) this may
|      be omitted, or (for newer drivers) will identify the kernel
|      version and the driver which provides this hub emulation.
|__String info tag

S:  Product=ssss
|   |__Product description of this device as read from the device.
|      For older USB host controller drivers (virtual root hubs) this
|      indicates the driver; for newer ones, it's a product (and vendor)
|      description that often comes from the kernel's PCI ID database.
|__String info tag

S:  SerialNumber=ssss
|   |__Serial Number of this device as read from the device.
|      For USB host controller drivers (virtual root hubs) this is
|      some unique ID, normally a bus ID (address or slot name) that
|      can't be shared with any other device.
|__String info tag

Configuration descriptor info¶

C:* #Ifs=dd Cfg#=dd Atr=xx MPwr=dddmA
| | |       |       |      |__MaxPower in mA
| | |       |       |__Attributes
| | |       |__ConfiguratioNumber
| | |__NumberOfInterfaces
| |__ "*" indicates the active configuration (others are " ")
|__Config info tag

USB devices may have multiple configurations, each of which act rather differently. For example, a bus-powered configuration might be much less capable than one that is self-powered. Only one device configuration can be active at a time; most devices have only one configuration.

Each configuration consists of one or more interfaces. Each interface serves a distinct “function”, which is typically bound to a different USB device driver. One common example is a USB speaker with an audio interface for playback, and a HID interface for use with software volume control.

Interface descriptor info (can be multiple per Config)¶

I:* If#=dd Alt=dd #EPs=dd Cls=xx(sssss) Sub=xx Prot=xx Driver=ssss
| | |      |      |       |             |      |       |__Driver name
| | |      |      |       |             |      |          or "(none)"
| | |      |      |       |             |      |__InterfaceProtocol
| | |      |      |       |             |__InterfaceSubClass
| | |      |      |       |__InterfaceClass
| | |      |      |__NumberOfEndpoints
| | |      |__AlternateSettingNumber
| | |__InterfaceNumber
| |__ "*" indicates the active altsetting (others are " ")
|__Interface info tag

A given interface may have one or more “alternate” settings. For example, default settings may not use more than a small amount of periodic bandwidth. To use significant fractions of bus bandwidth, drivers must select a non-default altsetting.

Only one setting for an interface may be active at a time, and only one driver may bind to an interface at a time. Most devices have only one alternate setting per interface.

Endpoint descriptor info (can be multiple per Interface)¶

E:  Ad=xx(s) Atr=xx(ssss) MxPS=dddd Ivl=dddss
|   |        |            |         |__Interval (max) between transfers
|   |        |            |__EndpointMaxPacketSize
|   |        |__Attributes(EndpointType)
|   |__EndpointAddress(I=In,O=Out)
|__Endpoint info tag

The interval is nonzero for all periodic (interrupt or isochronous) endpoints. For high speed endpoints the transfer interval may be measured in microseconds rather than milliseconds.

For high speed periodic endpoints, the EndpointMaxPacketSize reflects the per-microframe data transfer size. For “high bandwidth” endpoints, that can reflect two or three packets (for up to 3KBytes every 125 usec) per endpoint.

With the Linux-USB stack, periodic bandwidth reservations use the transfer intervals and sizes provided by URBs, which can be less than those found in endpoint descriptor.

Usage examples¶

If a user or script is interested only in Topology info, for example, use something like grep ^T: /sys/kernel/debug/usb/devices for only the Topology lines. A command like grep -i ^[tdp]: /sys/kernel/debug/usb/devices can be used to list only the lines that begin with the characters in square brackets, where the valid characters are TDPCIE. With a slightly more able script, it can display any selected lines (for example, only T, D, and P lines) and change their output format. (The procusb Perl script is the beginning of this idea. It will list only selected lines [selected from TBDPSCIE] or “All” lines from /sys/kernel/debug/usb/devices.)

The Topology lines can be used to generate a graphic/pictorial of the USB devices on a system’s root hub. (See more below on how to do this.)

The Interface lines can be used to determine what driver is being used for each device, and which altsetting it activated.

The Configuration lines could be used to list maximum power (in milliamps) that a system’s USB devices are using. For example, grep ^C: /sys/kernel/debug/usb/devices.

Here’s an example, from a system which has a UHCI root hub, an external hub connected to the root hub, and a mouse and a serial converter connected to the external hub.

T:  Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=12   MxCh= 2
B:  Alloc= 28/900 us ( 3%), #Int=  2, #Iso=  0
D:  Ver= 1.00 Cls=09(hub  ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
P:  Vendor=0000 ProdID=0000 Rev= 0.00
S:  Product=USB UHCI Root Hub
S:  SerialNumber=dce0
C:* #Ifs= 1 Cfg#= 1 Atr=40 MxPwr=  0mA
I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
E:  Ad=81(I) Atr=03(Int.) MxPS=   8 Ivl=255ms

T:  Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  2 Spd=12   MxCh= 4
D:  Ver= 1.00 Cls=09(hub  ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
P:  Vendor=0451 ProdID=1446 Rev= 1.00
C:* #Ifs= 1 Cfg#= 1 Atr=e0 MxPwr=100mA
I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
E:  Ad=81(I) Atr=03(Int.) MxPS=   1 Ivl=255ms

T:  Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#=  3 Spd=1.5  MxCh= 0
D:  Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
P:  Vendor=04b4 ProdID=0001 Rev= 0.00
C:* #Ifs= 1 Cfg#= 1 Atr=80 MxPwr=100mA
I:  If#= 0 Alt= 0 #EPs= 1 Cls=03(HID  ) Sub=01 Prot=02 Driver=mouse
E:  Ad=81(I) Atr=03(Int.) MxPS=   3 Ivl= 10ms

T:  Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#=  4 Spd=12   MxCh= 0
D:  Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
P:  Vendor=0565 ProdID=0001 Rev= 1.08
S:  Manufacturer=Peracom Networks, Inc.
S:  Product=Peracom USB to Serial Converter
C:* #Ifs= 1 Cfg#= 1 Atr=a0 MxPwr=100mA
I:  If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial
E:  Ad=81(I) Atr=02(Bulk) MxPS=  64 Ivl= 16ms
E:  Ad=01(O) Atr=02(Bulk) MxPS=  16 Ivl= 16ms
E:  Ad=82(I) Atr=03(Int.) MxPS=   8 Ivl=  8ms

Selecting only the T: and I: lines from this (for example, by using procusb ti), we have

T:  Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=12   MxCh= 2
T:  Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  2 Spd=12   MxCh= 4
I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
T:  Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#=  3 Spd=1.5  MxCh= 0
I:  If#= 0 Alt= 0 #EPs= 1 Cls=03(HID  ) Sub=01 Prot=02 Driver=mouse
T:  Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#=  4 Spd=12   MxCh= 0
I:  If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial

Physically this looks like (or could be converted to):

                    +------------------+
                    |  PC/root_hub (12)|   Dev# = 1
                    +------------------+   (nn) is Mbps.
  Level 0           |  CN.0   |  CN.1  |   [CN = connector/port #]
                    +------------------+
                        /
                       /
          +-----------------------+
Level 1   | Dev#2: 4-port hub (12)|
          +-----------------------+
          |CN.0 |CN.1 |CN.2 |CN.3 |
          +-----------------------+
              \           \____________________
               \_____                          \
                     \                          \
             +--------------------+      +--------------------+
Level 2      | Dev# 3: mouse (1.5)|      | Dev# 4: serial (12)|
             +--------------------+      +--------------------+

Or, in a more tree-like structure (ports [Connectors] without connections could be omitted):

PC:  Dev# 1, root hub, 2 ports, 12 Mbps
|_ CN.0:  Dev# 2, hub, 4 ports, 12 Mbps
     |_ CN.0:  Dev #3, mouse, 1.5 Mbps
     |_ CN.1:
     |_ CN.2:  Dev #4, serial, 12 Mbps
     |_ CN.3:
|_ CN.1:

The Linux-USB Host Side API¶

Introduction to USB on Linux¶

USB Host-Side API Model¶

USB-Standard Types¶

Host-Side Data Types and Macros¶

USB Core APIs¶

Host Controller APIs¶

The USB character device nodes¶

What files are in “devtmpfs”?¶

/dev/bus/usb/BBB/DDD¶

Life Cycle of User Mode Drivers¶

The ioctl() Requests¶

Management/Status Requests¶

Synchronous I/O Support¶

Asynchronous I/O Support¶

The USB devices¶

/sys/kernel/debug/usb/devices¶

/sys/kernel/debug/usb/devices output format¶

Topology info¶

Bandwidth info¶

Device descriptor info & Product ID info¶

String descriptor info¶

Configuration descriptor info¶

Interface descriptor info (can be multiple per Config)¶

Endpoint descriptor info (can be multiple per Interface)¶

Usage examples¶

The Linux Kernel

Navigation

Related Topics

1.5	Mbit/s for low speed USB
12	Mbit/s for full speed USB
480	Mbit/s for high speed USB (added for USB 2.0); also used for Wireless USB, which has no fixed speed
5000	Mbit/s for SuperSpeed USB (added for USB 3.0)