Documentation/driver-api/serial/driver.rst

====================
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 identifying
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 struct 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
--------

.. kernel-doc:: include/linux/serial_core.h
   :identifiers: uart_ops

Other functions
---------------

.. kernel-doc:: drivers/tty/serial/serial_core.c
   :identifiers: uart_update_timeout uart_get_baud_rate uart_get_divisor
           uart_match_port uart_write_wakeup uart_register_driver
           uart_unregister_driver uart_suspend_port uart_resume_port
           uart_add_one_port uart_remove_one_port uart_console_write
           uart_parse_earlycon uart_parse_options uart_set_options
           uart_get_lsr_info uart_handle_dcd_change uart_handle_cts_change
           uart_try_toggle_sysrq uart_get_console

.. kernel-doc:: include/linux/serial_core.h
   :identifiers: uart_port_tx_limited uart_port_tx

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.

.. kernel-doc:: drivers/tty/serial/serial_mctrl_gpio.c
   :identifiers: mctrl_gpio_init mctrl_gpio_free mctrl_gpio_to_gpiod
           mctrl_gpio_set mctrl_gpio_get mctrl_gpio_enable_ms
           mctrl_gpio_disable_ms