Low Level Serial API

This document is meant as a brief overview of some aspects of the new serial driver. It is not complete, any questions you have should be directed to <rmk@arm.linux.org.uk>

The reference implementation is contained within amba-pl011.c.

Low Level Serial Hardware Driver

The low level serial hardware driver is responsible for supplying port information (defined by uart_port) and a set of control methods (defined by uart_ops) to the core serial driver. The low level driver is also responsible for handling interrupts for the port, and providing any console support.

Console Support

The serial core provides a few helper functions. This includes identifing the correct port structure (via uart_get_console) and decoding command line arguments (uart_parse_options).

There is also a helper function (uart_console_write) which performs a character by character write, translating newlines to CRLF sequences. Driver writers are recommended to use this function rather than implementing their own version.

Locking

It is the responsibility of the low level hardware driver to perform the necessary locking using port->lock. There are some exceptions (which are described in the uart_ops listing below.)

There are two locks. A per-port spinlock, and an overall semaphore.

From the core driver perspective, the port->lock locks the following data:

port->mctrl
port->icount
port->state->xmit.head (circ_buf->head)
port->state->xmit.tail (circ_buf->tail)

The low level driver is free to use this lock to provide any additional locking.

The port_sem semaphore is used to protect against ports being added/ removed or reconfigured at inappropriate times. Since v2.6.27, this semaphore has been the ‘mutex’ member of the tty_port struct, and commonly referred to as the port mutex.

uart_ops

The uart_ops structure is the main interface between serial_core and the hardware specific driver. It contains all the methods to control the hardware.

tx_empty(port)

This function tests whether the transmitter fifo and shifter for the port described by ‘port’ is empty. If it is empty, this function should return TIOCSER_TEMT, otherwise return 0. If the port does not support this operation, then it should return TIOCSER_TEMT.

Locking: none.

Interrupts: caller dependent.

This call must not sleep

set_mctrl(port, mctrl)

This function sets the modem control lines for port described by ‘port’ to the state described by mctrl. The relevant bits of mctrl are:

• TIOCM_RTS RTS signal.

• TIOCM_DTR DTR signal.

• TIOCM_OUT1 OUT1 signal.

• TIOCM_OUT2 OUT2 signal.

• TIOCM_LOOP Set the port into loopback mode.

If the appropriate bit is set, the signal should be driven active. If the bit is clear, the signal should be driven inactive.

Locking: port->lock taken.

Interrupts: locally disabled.

This call must not sleep

get_mctrl(port)

Returns the current state of modem control inputs. The state of the outputs should not be returned, since the core keeps track of their state. The state information should include:

• TIOCM_CAR state of DCD signal

• TIOCM_CTS state of CTS signal

• TIOCM_DSR state of DSR signal

• TIOCM_RI state of RI signal

The bit is set if the signal is currently driven active. If the port does not support CTS, DCD or DSR, the driver should indicate that the signal is permanently active. If RI is not available, the signal should not be indicated as active.

Locking: port->lock taken.

Interrupts: locally disabled.

This call must not sleep

stop_tx(port)

Stop transmitting characters. This might be due to the CTS line becoming inactive or the tty layer indicating we want to stop transmission due to an XOFF character.

The driver should stop transmitting characters as soon as possible.

Locking: port->lock taken.

Interrupts: locally disabled.

This call must not sleep

start_tx(port)

Start transmitting characters.

Locking: port->lock taken.

Interrupts: locally disabled.

This call must not sleep

throttle(port)

Notify the serial driver that input buffers for the line discipline are close to full, and it should somehow signal that no more characters should be sent to the serial port. This will be called only if hardware assisted flow control is enabled.

Locking: serialized with .unthrottle() and termios modification by the tty layer.

unthrottle(port)

Notify the serial driver that characters can now be sent to the serial port without fear of overrunning the input buffers of the line disciplines.

This will be called only if hardware assisted flow control is enabled.

Locking: serialized with .throttle() and termios modification by the tty layer.

send_xchar(port,ch)

Transmit a high priority character, even if the port is stopped. This is used to implement XON/XOFF flow control and tcflow(). If the serial driver does not implement this function, the tty core will append the character to the circular buffer and then call start_tx() / stop_tx() to flush the data out.

Do not transmit if ch == ‘0’ (__DISABLED_CHAR).

Locking: none.

Interrupts: caller dependent.

stop_rx(port)

Stop receiving characters; the port is in the process of being closed.

Locking: port->lock taken.

Interrupts: locally disabled.

This call must not sleep

enable_ms(port)

Enable the modem status interrupts.

This method may be called multiple times. Modem status interrupts should be disabled when the shutdown method is called.

Locking: port->lock taken.

Interrupts: locally disabled.

This call must not sleep

break_ctl(port,ctl)

Control the transmission of a break signal. If ctl is nonzero, the break signal should be transmitted. The signal should be terminated when another call is made with a zero ctl.

Locking: caller holds tty_port->mutex

startup(port)

Grab any interrupt resources and initialise any low level driver state. Enable the port for reception. It should not activate RTS nor DTR; this will be done via a separate call to set_mctrl.

This method will only be called when the port is initially opened.

Locking: port_sem taken.

Interrupts: globally disabled.

shutdown(port)

Disable the port, disable any break condition that may be in effect, and free any interrupt resources. It should not disable RTS nor DTR; this will have already been done via a separate call to set_mctrl.

Drivers must not access port->state once this call has completed.

This method will only be called when there are no more users of this port.

Locking: port_sem taken.

Interrupts: caller dependent.

flush_buffer(port)

Flush any write buffers, reset any DMA state and stop any ongoing DMA transfers.

This will be called whenever the port->state->xmit circular buffer is cleared.

Locking: port->lock taken.

Interrupts: locally disabled.

This call must not sleep

set_termios(port,termios,oldtermios)

Change the port parameters, including word length, parity, stop bits. Update read_status_mask and ignore_status_mask to indicate the types of events we are interested in receiving. Relevant termios->c_cflag bits are:

CSIZE

• word size

CSTOPB

• 2 stop bits

PARENB

• parity enable

PARODD

• odd parity (when PARENB is in force)

CREAD

• enable reception of characters (if not set, still receive characters from the port, but throw them away.

CRTSCTS

• if set, enable CTS status change reporting

CLOCAL

• if not set, enable modem status change reporting.

Relevant termios->c_iflag bits are:

INPCK

• enable frame and parity error events to be passed to the TTY layer.

BRKINT / PARMRK

• both of these enable break events to be passed to the TTY layer.

IGNPAR

• ignore parity and framing errors

IGNBRK

• ignore break errors, If IGNPAR is also set, ignore overrun errors as well.

The interaction of the iflag bits is as follows (parity error given as an example):

Parity error	INPCK	IGNPAR
n/a	0	n/a	character received, marked as TTY_NORMAL
None	1	n/a	character received, marked as TTY_NORMAL
Yes	1	0	character received, marked as TTY_PARITY
Yes	1	1	character discarded

Other flags may be used (eg, xon/xoff characters) if your hardware supports hardware “soft” flow control.

Locking: caller holds tty_port->mutex

Interrupts: caller dependent.

This call must not sleep

set_ldisc(port,termios)

Notifier for discipline change. See Documentation/driver-api/serial/tty.rst.

Locking: caller holds tty_port->mutex

pm(port,state,oldstate)

Perform any power management related activities on the specified port. State indicates the new state (defined by enum uart_pm_state), oldstate indicates the previous state.

This function should not be used to grab any resources.

This will be called when the port is initially opened and finally closed, except when the port is also the system console. This will occur even if CONFIG_PM is not set.

Locking: none.

Interrupts: caller dependent.

type(port)

Return a pointer to a string constant describing the specified port, or return NULL, in which case the string ‘unknown’ is substituted.

Locking: none.

Interrupts: caller dependent.

release_port(port)

Release any memory and IO region resources currently in use by the port.

Locking: none.

Interrupts: caller dependent.

request_port(port)

Request any memory and IO region resources required by the port. If any fail, no resources should be registered when this function returns, and it should return -EBUSY on failure.

Locking: none.

Interrupts: caller dependent.

config_port(port,type)

Perform any autoconfiguration steps required for the port. type contains a bit mask of the required configuration. UART_CONFIG_TYPE indicates that the port requires detection and identification. port->type should be set to the type found, or PORT_UNKNOWN if no port was detected.

UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal, which should be probed using standard kernel autoprobing techniques. This is not necessary on platforms where ports have interrupts internally hard wired (eg, system on a chip implementations).

Locking: none.

Interrupts: caller dependent.

verify_port(port,serinfo)

Verify the new serial port information contained within serinfo is suitable for this port type.

Locking: none.

Interrupts: caller dependent.

ioctl(port,cmd,arg)

Perform any port specific IOCTLs. IOCTL commands must be defined using the standard numbering system found in <asm/ioctl.h>

Locking: none.

Interrupts: caller dependent.

poll_init(port)

Called by kgdb to perform the minimal hardware initialization needed to support poll_put_char() and poll_get_char(). Unlike ->startup() this should not request interrupts.

Locking: tty_mutex and tty_port->mutex taken.

Interrupts: n/a.

poll_put_char(port,ch)

Called by kgdb to write a single character directly to the serial port. It can and should block until there is space in the TX FIFO.

Locking: none.

Interrupts: caller dependent.

This call must not sleep

poll_get_char(port)

Called by kgdb to read a single character directly from the serial port. If data is available, it should be returned; otherwise the function should return NO_POLL_CHAR immediately.

Locking: none.

Interrupts: caller dependent.

This call must not sleep

Other functions

uart_update_timeout(port,cflag,baud)

Update the FIFO drain timeout, port->timeout, according to the number of bits, parity, stop bits and baud rate.

Locking: caller is expected to take port->lock

Interrupts: n/a

uart_get_baud_rate(port,termios,old,min,max)

Return the numeric baud rate for the specified termios, taking account of the special 38400 baud “kludge”. The B0 baud rate is mapped to 9600 baud.

If the baud rate is not within min..max, then if old is non-NULL, the original baud rate will be tried. If that exceeds the min..max constraint, 9600 baud will be returned. termios will be updated to the baud rate in use.

Note: min..max must always allow 9600 baud to be selected.

Locking: caller dependent.

Interrupts: n/a

uart_get_divisor(port,baud)

Return the divisor (baud_base / baud) for the specified baud rate, appropriately rounded.

If 38400 baud and custom divisor is selected, return the custom divisor instead.

Locking: caller dependent.

Interrupts: n/a

uart_match_port(port1,port2)

This utility function can be used to determine whether two uart_port structures describe the same port.

Locking: n/a

Interrupts: n/a

uart_write_wakeup(port)

A driver is expected to call this function when the number of characters in the transmit buffer have dropped below a threshold.

Locking: port->lock should be held.

Interrupts: n/a

uart_register_driver(drv)

Register a uart driver with the core driver. We in turn register with the tty layer, and initialise the core driver per-port state.

drv->port should be NULL, and the per-port structures should be registered using uart_add_one_port after this call has succeeded.

Locking: none

Interrupts: enabled

uart_unregister_driver()

Remove all references to a driver from the core driver. The low level driver must have removed all its ports via the uart_remove_one_port() if it registered them with uart_add_one_port().

Locking: none

Interrupts: enabled

uart_suspend_port()

uart_resume_port()

uart_add_one_port()

uart_remove_one_port()

Other notes

It is intended some day to drop the ‘unused’ entries from uart_port, and allow low level drivers to register their own individual uart_port’s with the core. This will allow drivers to use uart_port as a pointer to a structure containing both the uart_port entry with their own extensions, thus:

struct my_port {
        struct uart_port        port;
        int                     my_stuff;
};

Modem control lines via GPIO

Some helpers are provided in order to set/get modem control lines via GPIO.

mctrl_gpio_init(port, idx):

This will get the {cts,rts,…}-gpios from device tree if they are present and request them, set direction etc, and return an allocated structure. devm_* functions are used, so there’s no need to call mctrl_gpio_free(). As this sets up the irq handling make sure to not handle changes to the gpio input lines in your driver, too.

mctrl_gpio_free(dev, gpios):

This will free the requested gpios in mctrl_gpio_init(). As devm_* functions are used, there’s generally no need to call this function.

mctrl_gpio_to_gpiod(gpios, gidx)

This returns the gpio_desc structure associated to the modem line index.

mctrl_gpio_set(gpios, mctrl):

This will sets the gpios according to the mctrl state.

mctrl_gpio_get(gpios, mctrl):

This will update mctrl with the gpios values.

mctrl_gpio_enable_ms(gpios):

Enables irqs and handling of changes to the ms lines.

mctrl_gpio_disable_ms(gpios):

Disables irqs and handling of changes to the ms lines.