linux/Documentation/serial/tty.txt
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   1
   2                        The Lockronomicon
   3
   4Your guide to the ancient and twisted locking policies of the tty layer and
   5the warped logic behind them. Beware all ye who read on.
   6
   7FIXME: still need to work out the full set of BKL assumptions and document
   8them so they can eventually be killed off.
   9
  10
  11Line Discipline
  12---------------
  13
  14Line disciplines are registered with tty_register_ldisc() passing the
  15discipline number and the ldisc structure. At the point of registration the 
  16discipline must be ready to use and it is possible it will get used before
  17the call returns success. If the call returns an error then it won't get
  18called. Do not re-use ldisc numbers as they are part of the userspace ABI
  19and writing over an existing ldisc will cause demons to eat your computer.
  20After the return the ldisc data has been copied so you may free your own 
  21copy of the structure. You must not re-register over the top of the line
  22discipline even with the same data or your computer again will be eaten by
  23demons.
  24
  25In order to remove a line discipline call tty_unregister_ldisc().
  26In ancient times this always worked. In modern times the function will
  27return -EBUSY if the ldisc is currently in use. Since the ldisc referencing
  28code manages the module counts this should not usually be a concern.
  29
  30Heed this warning: the reference count field of the registered copies of the
  31tty_ldisc structure in the ldisc table counts the number of lines using this
  32discipline. The reference count of the tty_ldisc structure within a tty 
  33counts the number of active users of the ldisc at this instant. In effect it
  34counts the number of threads of execution within an ldisc method (plus those
  35about to enter and exit although this detail matters not).
  36
  37Line Discipline Methods
  38-----------------------
  39
  40TTY side interfaces:
  41
  42open()          -       Called when the line discipline is attached to
  43                        the terminal. No other call into the line
  44                        discipline for this tty will occur until it
  45                        completes successfully. Can sleep.
  46
  47close()         -       This is called on a terminal when the line
  48                        discipline is being unplugged. At the point of
  49                        execution no further users will enter the
  50                        ldisc code for this tty. Can sleep.
  51
  52hangup()        -       Called when the tty line is hung up.
  53                        The line discipline should cease I/O to the tty.
  54                        No further calls into the ldisc code will occur.
  55                        Can sleep.
  56
  57write()         -       A process is writing data through the line
  58                        discipline.  Multiple write calls are serialized
  59                        by the tty layer for the ldisc.  May sleep. 
  60
  61flush_buffer()  -       (optional) May be called at any point between
  62                        open and close, and instructs the line discipline
  63                        to empty its input buffer.
  64
  65chars_in_buffer() -     (optional) Report the number of bytes in the input
  66                        buffer.
  67
  68set_termios()   -       (optional) Called on termios structure changes.
  69                        The caller passes the old termios data and the
  70                        current data is in the tty. Called under the
  71                        termios semaphore so allowed to sleep. Serialized
  72                        against itself only.
  73
  74read()          -       Move data from the line discipline to the user.
  75                        Multiple read calls may occur in parallel and the
  76                        ldisc must deal with serialization issues. May 
  77                        sleep.
  78
  79poll()          -       Check the status for the poll/select calls. Multiple
  80                        poll calls may occur in parallel. May sleep.
  81
  82ioctl()         -       Called when an ioctl is handed to the tty layer
  83                        that might be for the ldisc. Multiple ioctl calls
  84                        may occur in parallel. May sleep. 
  85
  86Driver Side Interfaces:
  87
  88receive_buf()   -       Hand buffers of bytes from the driver to the ldisc
  89                        for processing. Semantics currently rather
  90                        mysterious 8(
  91
  92write_wakeup()  -       May be called at any point between open and close.
  93                        The TTY_DO_WRITE_WAKEUP flag indicates if a call
  94                        is needed but always races versus calls. Thus the
  95                        ldisc must be careful about setting order and to
  96                        handle unexpected calls. Must not sleep.
  97
  98                        The driver is forbidden from calling this directly
  99                        from the ->write call from the ldisc as the ldisc
 100                        is permitted to call the driver write method from
 101                        this function. In such a situation defer it.
 102
 103
 104Driver Access
 105
 106Line discipline methods can call the following methods of the underlying
 107hardware driver through the function pointers within the tty->driver
 108structure:
 109
 110write()                 Write a block of characters to the tty device.
 111                        Returns the number of characters accepted. The
 112                        character buffer passed to this method is already
 113                        in kernel space.
 114
 115put_char()              Queues a character for writing to the tty device.
 116                        If there is no room in the queue, the character is
 117                        ignored.
 118
 119flush_chars()           (Optional) If defined, must be called after
 120                        queueing characters with put_char() in order to
 121                        start transmission.
 122
 123write_room()            Returns the numbers of characters the tty driver
 124                        will accept for queueing to be written.
 125
 126ioctl()                 Invoke device specific ioctl.
 127                        Expects data pointers to refer to userspace.
 128                        Returns ENOIOCTLCMD for unrecognized ioctl numbers.
 129
 130set_termios()           Notify the tty driver that the device's termios
 131                        settings have changed. New settings are in
 132                        tty->termios. Previous settings should be passed in
 133                        the "old" argument.
 134
 135                        The API is defined such that the driver should return
 136                        the actual modes selected. This means that the
 137                        driver function is responsible for modifying any
 138                        bits in the request it cannot fulfill to indicate
 139                        the actual modes being used. A device with no
 140                        hardware capability for change (eg a USB dongle or
 141                        virtual port) can provide NULL for this method.
 142
 143throttle()              Notify the tty driver that input buffers for the
 144                        line discipline are close to full, and it should
 145                        somehow signal that no more characters should be
 146                        sent to the tty.
 147
 148unthrottle()            Notify the tty driver that characters can now be
 149                        sent to the tty without fear of overrunning the
 150                        input buffers of the line disciplines.
 151
 152stop()                  Ask the tty driver to stop outputting characters
 153                        to the tty device.
 154
 155start()                 Ask the tty driver to resume sending characters
 156                        to the tty device.
 157
 158hangup()                Ask the tty driver to hang up the tty device.
 159
 160break_ctl()             (Optional) Ask the tty driver to turn on or off
 161                        BREAK status on the RS-232 port.  If state is -1,
 162                        then the BREAK status should be turned on; if
 163                        state is 0, then BREAK should be turned off.
 164                        If this routine is not implemented, use ioctls
 165                        TIOCSBRK / TIOCCBRK instead.
 166
 167wait_until_sent()       Waits until the device has written out all of the
 168                        characters in its transmitter FIFO.
 169
 170send_xchar()            Send a high-priority XON/XOFF character to the device.
 171
 172
 173Flags
 174
 175Line discipline methods have access to tty->flags field containing the
 176following interesting flags:
 177
 178TTY_THROTTLED           Driver input is throttled. The ldisc should call
 179                        tty->driver->unthrottle() in order to resume
 180                        reception when it is ready to process more data.
 181
 182TTY_DO_WRITE_WAKEUP     If set, causes the driver to call the ldisc's
 183                        write_wakeup() method in order to resume
 184                        transmission when it can accept more data
 185                        to transmit.
 186
 187TTY_IO_ERROR            If set, causes all subsequent userspace read/write
 188                        calls on the tty to fail, returning -EIO.
 189
 190TTY_OTHER_CLOSED        Device is a pty and the other side has closed.
 191
 192TTY_NO_WRITE_SPLIT      Prevent driver from splitting up writes into
 193                        smaller chunks.
 194
 195
 196Locking
 197
 198Callers to the line discipline functions from the tty layer are required to
 199take line discipline locks. The same is true of calls from the driver side
 200but not yet enforced.
 201
 202Three calls are now provided
 203
 204        ldisc = tty_ldisc_ref(tty);
 205
 206takes a handle to the line discipline in the tty and returns it. If no ldisc
 207is currently attached or the ldisc is being closed and re-opened at this
 208point then NULL is returned. While this handle is held the ldisc will not
 209change or go away.
 210
 211        tty_ldisc_deref(ldisc)
 212
 213Returns the ldisc reference and allows the ldisc to be closed. Returning the
 214reference takes away your right to call the ldisc functions until you take
 215a new reference.
 216
 217        ldisc = tty_ldisc_ref_wait(tty);
 218
 219Performs the same function as tty_ldisc_ref except that it will wait for an
 220ldisc change to complete and then return a reference to the new ldisc. 
 221
 222While these functions are slightly slower than the old code they should have
 223minimal impact as most receive logic uses the flip buffers and they only
 224need to take a reference when they push bits up through the driver.
 225
 226A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc 
 227functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
 228fail in this situation if used within these functions. Ldisc and driver
 229code calling its own functions must be careful in this case. 
 230
 231
 232Driver Interface
 233----------------
 234
 235open()          -       Called when a device is opened. May sleep
 236
 237close()         -       Called when a device is closed. At the point of
 238                        return from this call the driver must make no 
 239                        further ldisc calls of any kind. May sleep
 240
 241write()         -       Called to write bytes to the device. May not
 242                        sleep. May occur in parallel in special cases. 
 243                        Because this includes panic paths drivers generally
 244                        shouldn't try and do clever locking here.
 245
 246put_char()      -       Stuff a single character onto the queue. The
 247                        driver is guaranteed following up calls to
 248                        flush_chars.
 249
 250flush_chars()   -       Ask the kernel to write put_char queue
 251
 252write_room()    -       Return the number of characters tht can be stuffed
 253                        into the port buffers without overflow (or less).
 254                        The ldisc is responsible for being intelligent
 255                        about multi-threading of write_room/write calls
 256
 257ioctl()         -       Called when an ioctl may be for the driver
 258
 259set_termios()   -       Called on termios change, serialized against
 260                        itself by a semaphore. May sleep.
 261
 262set_ldisc()     -       Notifier for discipline change. At the point this 
 263                        is done the discipline is not yet usable. Can now
 264                        sleep (I think)
 265
 266throttle()      -       Called by the ldisc to ask the driver to do flow
 267                        control.  Serialization including with unthrottle
 268                        is the job of the ldisc layer.
 269
 270unthrottle()    -       Called by the ldisc to ask the driver to stop flow
 271                        control.
 272
 273stop()          -       Ldisc notifier to the driver to stop output. As with
 274                        throttle the serializations with start() are down
 275                        to the ldisc layer.
 276
 277start()         -       Ldisc notifier to the driver to start output.
 278
 279hangup()        -       Ask the tty driver to cause a hangup initiated
 280                        from the host side. [Can sleep ??]
 281
 282break_ctl()     -       Send RS232 break. Can sleep. Can get called in
 283                        parallel, driver must serialize (for now), and
 284                        with write calls.
 285
 286wait_until_sent() -     Wait for characters to exit the hardware queue
 287                        of the driver. Can sleep
 288
 289send_xchar()      -     Send XON/XOFF and if possible jump the queue with
 290                        it in order to get fast flow control responses.
 291                        Cannot sleep ??
 292
 293