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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

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_DCD     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

  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.

  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 ACPI D0-D3),

        oldstate indicates the previous state.  Essentially, D0 means

        fully on, D3 means powered down.

        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 

        Locking: none.

        Interrupts: caller dependent.

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;

        };