Adding upstream version 4.19.249.upstream/4.19.249upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 486 insertions, 0 deletions

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+
+			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.txt.
+
+	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.