Documentation / serial / driver

Custom Search

Based on kernel version 3.9. Page generated on 2013-05-02 23:14 EST.

	
				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_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
	
	  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.
	
		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: 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
		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.
	
	  set_wake(port,state)
		Enable/disable power management wakeup on serial activity.  Not
		currently implemented.
	
	  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 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;
		};

• [ serial ]
• 00-INDEX
• digiepca.txt
• driver
• moxa-smartio
• n_gsm.txt
• README.cycladesZ
• riscom8.txt
• rocket.txt
• serial-rs485.txt
• specialix.txt
• stallion.txt
• sx.txt
• tty.txt
•