b129a8ccd5
The start_tx and stop_tx methods were passed a flag to indicate whether the start/stop was from the tty start/stop callbacks, and some drivers used this flag to decide whether to ask the UART to immediately stop transmission (where the UART supports such a feature.) There are other cases when we wish this to occur - when CTS is lowered, or if we change from soft to hard flow control and CTS is inactive. In these cases, this flag was false, and we would allow the transmitter to drain before stopping. There is really only one case where we want to let the transmitter drain before disabling, and that's when we run out of characters to send. Hence, re-jig the start_tx and stop_tx methods to eliminate this flag, and introduce new functions for the special "disable and allow transmitter to drain" case. Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
324 lines
9.6 KiB
Plaintext
324 lines
9.6 KiB
Plaintext
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Low Level Serial API
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--------------------
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$Id: driver,v 1.10 2002/07/22 15:27:30 rmk Exp $
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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 serial_amba.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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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 three locks. A per-port spinlock, a per-port tmpbuf semaphore,
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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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info->xmit.head (circ->head)
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info->xmit.tail (circ->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 core driver uses the info->tmpbuf_sem lock to prevent multi-threaded
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access to the info->tmpbuf bouncebuffer used for port writes.
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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.
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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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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_DCD 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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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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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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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: none.
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Interrupts: caller dependent.
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This call must not sleep
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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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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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Locking: port_sem taken.
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Interrupts: caller dependent.
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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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None n/a n/a character received
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Yes n/a 0 character discarded
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Yes 0 1 character received, marked as
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TTY_NORMAL
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Yes 1 1 character received, marked as
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TTY_PARITY
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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: none.
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Interrupts: caller dependent.
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This call must not sleep
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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 ACPI D0-D3),
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oldstate indicates the previous state. Essentially, D0 means
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fully on, D3 means powered down.
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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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Other functions
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---------------
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uart_update_timeout(port,cflag,quot)
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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 quotient.
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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)
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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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Locking: caller dependent.
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Interrupts: n/a
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uart_get_divisor(port,termios,oldtermios)
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Return the divsor (baud_base / baud) for the selected baud rate
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specified by termios. If the baud rate is out of range, try
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the original baud rate specified by oldtermios (if non-NULL).
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If that fails, try 9600 baud.
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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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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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