Documentation/serial/driver - kernel/msm - Gitiles


 			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_write_console) 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 three locks.  A per-port spinlock, a per-port tmpbuf semaphore,
 and an overall semaphore.

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

 	port->mctrl
 	port->icount
 	info->xmit.head (circ->head)
 	info->xmit.tail (circ->tail)

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

 The core driver uses the info->tmpbuf_sem lock to prevent multi-threaded
 access to the info->tmpbuf bouncebuffer used for port writes.

 The port_sem semaphore is used to protect against ports being added/
 removed or reconfigured at inappropriate times.


 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_DCD	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

   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: none.
 	Interrupts: caller dependent.
 	This call must not sleep

   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->info 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->info->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: none.
 	Interrupts: caller dependent.
 	This call must not sleep

   pm(port,state,oldstate)
 	Perform any power management related activities on the specified
 	port.  State indicates the new state (defined by ACPI D0-D3),
 	oldstate indicates the previous state.  Essentially, D0 means
 	fully on, D3 means powered down.

 	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.

 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 divsor (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;
 	};

	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_write_console) 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 three locks. A per-port spinlock, a per-port tmpbuf semaphore,
	and an overall semaphore.

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

	port->mctrl
	port->icount
	info->xmit.head (circ->head)
	info->xmit.tail (circ->tail)

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

	The core driver uses the info->tmpbuf_sem lock to prevent multi-threaded
	access to the info->tmpbuf bouncebuffer used for port writes.

	The port_sem semaphore is used to protect against ports being added/
	removed or reconfigured at inappropriate times.


	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_DCD 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

	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: none.
	Interrupts: caller dependent.
	This call must not sleep

	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->info 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->info->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: none.
	Interrupts: caller dependent.
	This call must not sleep

	pm(port,state,oldstate)
	Perform any power management related activities on the specified
	port. State indicates the new state (defined by ACPI D0-D3),
	oldstate indicates the previous state. Essentially, D0 means
	fully on, D3 means powered down.

	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.

	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 divsor (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;
	};