====================
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/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.