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.
  
    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;
  	};