v4.19.13 snapshot.
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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.