487 lines
15 KiB
Plaintext
487 lines
15 KiB
Plaintext
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Low Level Serial API
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--------------------
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This document is meant as a brief overview of some aspects of the new serial
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driver. It is not complete, any questions you have should be directed to
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<rmk@arm.linux.org.uk>
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The reference implementation is contained within amba-pl011.c.
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Low Level Serial Hardware Driver
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--------------------------------
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The low level serial hardware driver is responsible for supplying port
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information (defined by uart_port) and a set of control methods (defined
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by uart_ops) to the core serial driver. The low level driver is also
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responsible for handling interrupts for the port, and providing any
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console support.
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Console Support
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---------------
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The serial core provides a few helper functions. This includes identifing
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the correct port structure (via uart_get_console) and decoding command line
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arguments (uart_parse_options).
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There is also a helper function (uart_console_write) which performs a
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character by character write, translating newlines to CRLF sequences.
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Driver writers are recommended to use this function rather than implementing
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their own version.
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Locking
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-------
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It is the responsibility of the low level hardware driver to perform the
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necessary locking using port->lock. There are some exceptions (which
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are described in the uart_ops listing below.)
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There are two locks. A per-port spinlock, and an overall semaphore.
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From the core driver perspective, the port->lock locks the following
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data:
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port->mctrl
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port->icount
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port->state->xmit.head (circ_buf->head)
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port->state->xmit.tail (circ_buf->tail)
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The low level driver is free to use this lock to provide any additional
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locking.
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The port_sem semaphore is used to protect against ports being added/
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removed or reconfigured at inappropriate times. Since v2.6.27, this
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semaphore has been the 'mutex' member of the tty_port struct, and
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commonly referred to as the port mutex.
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uart_ops
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--------
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The uart_ops structure is the main interface between serial_core and the
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hardware specific driver. It contains all the methods to control the
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hardware.
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tx_empty(port)
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This function tests whether the transmitter fifo and shifter
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for the port described by 'port' is empty. If it is empty,
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this function should return TIOCSER_TEMT, otherwise return 0.
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If the port does not support this operation, then it should
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return TIOCSER_TEMT.
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Locking: none.
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Interrupts: caller dependent.
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This call must not sleep
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set_mctrl(port, mctrl)
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This function sets the modem control lines for port described
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by 'port' to the state described by mctrl. The relevant bits
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of mctrl are:
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- TIOCM_RTS RTS signal.
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- TIOCM_DTR DTR signal.
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- TIOCM_OUT1 OUT1 signal.
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- TIOCM_OUT2 OUT2 signal.
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- TIOCM_LOOP Set the port into loopback mode.
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If the appropriate bit is set, the signal should be driven
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active. If the bit is clear, the signal should be driven
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inactive.
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Locking: port->lock taken.
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Interrupts: locally disabled.
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This call must not sleep
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get_mctrl(port)
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Returns the current state of modem control inputs. The state
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of the outputs should not be returned, since the core keeps
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track of their state. The state information should include:
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- TIOCM_CAR state of DCD signal
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- TIOCM_CTS state of CTS signal
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- TIOCM_DSR state of DSR signal
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- TIOCM_RI state of RI signal
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The bit is set if the signal is currently driven active. If
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the port does not support CTS, DCD or DSR, the driver should
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indicate that the signal is permanently active. If RI is
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not available, the signal should not be indicated as active.
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Locking: port->lock taken.
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Interrupts: locally disabled.
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This call must not sleep
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stop_tx(port)
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Stop transmitting characters. This might be due to the CTS
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line becoming inactive or the tty layer indicating we want
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to stop transmission due to an XOFF character.
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The driver should stop transmitting characters as soon as
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possible.
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Locking: port->lock taken.
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Interrupts: locally disabled.
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This call must not sleep
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start_tx(port)
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Start transmitting characters.
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Locking: port->lock taken.
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Interrupts: locally disabled.
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This call must not sleep
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throttle(port)
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Notify the serial driver that input buffers for the line discipline are
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close to full, and it should somehow signal that no more characters
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should be sent to the serial port.
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This will be called only if hardware assisted flow control is enabled.
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Locking: serialized with .unthrottle() and termios modification by the
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tty layer.
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unthrottle(port)
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Notify the serial driver that characters can now be sent to the serial
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port without fear of overrunning the input buffers of the line
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disciplines.
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This will be called only if hardware assisted flow control is enabled.
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Locking: serialized with .throttle() and termios modification by the
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tty layer.
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send_xchar(port,ch)
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Transmit a high priority character, even if the port is stopped.
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This is used to implement XON/XOFF flow control and tcflow(). If
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the serial driver does not implement this function, the tty core
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will append the character to the circular buffer and then call
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start_tx() / stop_tx() to flush the data out.
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Do not transmit if ch == '\0' (__DISABLED_CHAR).
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Locking: none.
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Interrupts: caller dependent.
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stop_rx(port)
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Stop receiving characters; the port is in the process of
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being closed.
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Locking: port->lock taken.
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Interrupts: locally disabled.
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This call must not sleep
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enable_ms(port)
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Enable the modem status interrupts.
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This method may be called multiple times. Modem status
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interrupts should be disabled when the shutdown method is
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called.
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Locking: port->lock taken.
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Interrupts: locally disabled.
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This call must not sleep
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break_ctl(port,ctl)
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Control the transmission of a break signal. If ctl is
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nonzero, the break signal should be transmitted. The signal
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should be terminated when another call is made with a zero
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ctl.
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Locking: caller holds tty_port->mutex
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startup(port)
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Grab any interrupt resources and initialise any low level driver
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state. Enable the port for reception. It should not activate
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RTS nor DTR; this will be done via a separate call to set_mctrl.
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This method will only be called when the port is initially opened.
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Locking: port_sem taken.
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Interrupts: globally disabled.
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shutdown(port)
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Disable the port, disable any break condition that may be in
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effect, and free any interrupt resources. It should not disable
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RTS nor DTR; this will have already been done via a separate
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call to set_mctrl.
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Drivers must not access port->state once this call has completed.
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This method will only be called when there are no more users of
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this port.
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Locking: port_sem taken.
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Interrupts: caller dependent.
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flush_buffer(port)
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Flush any write buffers, reset any DMA state and stop any
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ongoing DMA transfers.
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This will be called whenever the port->state->xmit circular
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buffer is cleared.
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Locking: port->lock taken.
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Interrupts: locally disabled.
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This call must not sleep
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set_termios(port,termios,oldtermios)
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Change the port parameters, including word length, parity, stop
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bits. Update read_status_mask and ignore_status_mask to indicate
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the types of events we are interested in receiving. Relevant
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termios->c_cflag bits are:
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CSIZE - word size
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CSTOPB - 2 stop bits
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PARENB - parity enable
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PARODD - odd parity (when PARENB is in force)
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CREAD - enable reception of characters (if not set,
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still receive characters from the port, but
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throw them away.
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CRTSCTS - if set, enable CTS status change reporting
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CLOCAL - if not set, enable modem status change
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reporting.
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Relevant termios->c_iflag bits are:
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INPCK - enable frame and parity error events to be
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passed to the TTY layer.
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BRKINT
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PARMRK - both of these enable break events to be
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passed to the TTY layer.
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IGNPAR - ignore parity and framing errors
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IGNBRK - ignore break errors, If IGNPAR is also
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set, ignore overrun errors as well.
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The interaction of the iflag bits is as follows (parity error
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given as an example):
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Parity error INPCK IGNPAR
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n/a 0 n/a character received, marked as
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TTY_NORMAL
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None 1 n/a character received, marked as
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TTY_NORMAL
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Yes 1 0 character received, marked as
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TTY_PARITY
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Yes 1 1 character discarded
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Other flags may be used (eg, xon/xoff characters) if your
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hardware supports hardware "soft" flow control.
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Locking: caller holds tty_port->mutex
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Interrupts: caller dependent.
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This call must not sleep
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set_ldisc(port,termios)
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Notifier for discipline change. See Documentation/serial/tty.txt.
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Locking: caller holds tty_port->mutex
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pm(port,state,oldstate)
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Perform any power management related activities on the specified
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port. State indicates the new state (defined by
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enum uart_pm_state), oldstate indicates the previous state.
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This function should not be used to grab any resources.
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This will be called when the port is initially opened and finally
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closed, except when the port is also the system console. This
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will occur even if CONFIG_PM is not set.
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Locking: none.
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Interrupts: caller dependent.
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type(port)
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Return a pointer to a string constant describing the specified
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port, or return NULL, in which case the string 'unknown' is
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substituted.
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Locking: none.
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Interrupts: caller dependent.
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release_port(port)
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Release any memory and IO region resources currently in use by
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the port.
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Locking: none.
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Interrupts: caller dependent.
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request_port(port)
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Request any memory and IO region resources required by the port.
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If any fail, no resources should be registered when this function
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returns, and it should return -EBUSY on failure.
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Locking: none.
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Interrupts: caller dependent.
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config_port(port,type)
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Perform any autoconfiguration steps required for the port. `type`
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contains a bit mask of the required configuration. UART_CONFIG_TYPE
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indicates that the port requires detection and identification.
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port->type should be set to the type found, or PORT_UNKNOWN if
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no port was detected.
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UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
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which should be probed using standard kernel autoprobing techniques.
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This is not necessary on platforms where ports have interrupts
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internally hard wired (eg, system on a chip implementations).
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Locking: none.
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Interrupts: caller dependent.
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verify_port(port,serinfo)
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Verify the new serial port information contained within serinfo is
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suitable for this port type.
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Locking: none.
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Interrupts: caller dependent.
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ioctl(port,cmd,arg)
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Perform any port specific IOCTLs. IOCTL commands must be defined
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using the standard numbering system found in <asm/ioctl.h>
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Locking: none.
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Interrupts: caller dependent.
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poll_init(port)
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Called by kgdb to perform the minimal hardware initialization needed
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to support poll_put_char() and poll_get_char(). Unlike ->startup()
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this should not request interrupts.
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Locking: tty_mutex and tty_port->mutex taken.
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Interrupts: n/a.
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poll_put_char(port,ch)
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Called by kgdb to write a single character directly to the serial
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port. It can and should block until there is space in the TX FIFO.
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Locking: none.
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Interrupts: caller dependent.
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This call must not sleep
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poll_get_char(port)
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Called by kgdb to read a single character directly from the serial
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port. If data is available, it should be returned; otherwise
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the function should return NO_POLL_CHAR immediately.
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Locking: none.
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Interrupts: caller dependent.
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This call must not sleep
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Other functions
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---------------
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uart_update_timeout(port,cflag,baud)
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Update the FIFO drain timeout, port->timeout, according to the
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number of bits, parity, stop bits and baud rate.
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Locking: caller is expected to take port->lock
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Interrupts: n/a
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uart_get_baud_rate(port,termios,old,min,max)
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Return the numeric baud rate for the specified termios, taking
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account of the special 38400 baud "kludge". The B0 baud rate
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is mapped to 9600 baud.
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If the baud rate is not within min..max, then if old is non-NULL,
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the original baud rate will be tried. If that exceeds the
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min..max constraint, 9600 baud will be returned. termios will
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be updated to the baud rate in use.
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Note: min..max must always allow 9600 baud to be selected.
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Locking: caller dependent.
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Interrupts: n/a
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uart_get_divisor(port,baud)
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Return the divisor (baud_base / baud) for the specified baud
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rate, appropriately rounded.
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If 38400 baud and custom divisor is selected, return the
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custom divisor instead.
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Locking: caller dependent.
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Interrupts: n/a
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uart_match_port(port1,port2)
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This utility function can be used to determine whether two
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uart_port structures describe the same port.
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Locking: n/a
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Interrupts: n/a
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uart_write_wakeup(port)
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A driver is expected to call this function when the number of
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characters in the transmit buffer have dropped below a threshold.
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Locking: port->lock should be held.
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Interrupts: n/a
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uart_register_driver(drv)
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Register a uart driver with the core driver. We in turn register
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with the tty layer, and initialise the core driver per-port state.
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drv->port should be NULL, and the per-port structures should be
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registered using uart_add_one_port after this call has succeeded.
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Locking: none
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Interrupts: enabled
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uart_unregister_driver()
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Remove all references to a driver from the core driver. The low
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level driver must have removed all its ports via the
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uart_remove_one_port() if it registered them with uart_add_one_port().
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Locking: none
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Interrupts: enabled
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uart_suspend_port()
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uart_resume_port()
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uart_add_one_port()
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uart_remove_one_port()
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Other notes
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-----------
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It is intended some day to drop the 'unused' entries from uart_port, and
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allow low level drivers to register their own individual uart_port's with
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the core. This will allow drivers to use uart_port as a pointer to a
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structure containing both the uart_port entry with their own extensions,
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thus:
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struct my_port {
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struct uart_port port;
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int my_stuff;
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};
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Modem control lines via GPIO
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----------------------------
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Some helpers are provided in order to set/get modem control lines via GPIO.
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mctrl_gpio_init(port, idx):
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This will get the {cts,rts,...}-gpios from device tree if they are
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present and request them, set direction etc, and return an
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allocated structure. devm_* functions are used, so there's no need
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to call mctrl_gpio_free().
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As this sets up the irq handling make sure to not handle changes to the
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gpio input lines in your driver, too.
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mctrl_gpio_free(dev, gpios):
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This will free the requested gpios in mctrl_gpio_init().
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As devm_* functions are used, there's generally no need to call
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this function.
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mctrl_gpio_to_gpiod(gpios, gidx)
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This returns the gpio_desc structure associated to the modem line
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index.
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mctrl_gpio_set(gpios, mctrl):
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This will sets the gpios according to the mctrl state.
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mctrl_gpio_get(gpios, mctrl):
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This will update mctrl with the gpios values.
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mctrl_gpio_enable_ms(gpios):
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Enables irqs and handling of changes to the ms lines.
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mctrl_gpio_disable_ms(gpios):
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Disables irqs and handling of changes to the ms lines.
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