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. Returning an error will
  46                        prevent the ldisc from being attached. Can sleep.
  47
  48close()         -       This is called on a terminal when the line
  49                        discipline is being unplugged. At the point of
  50                        execution no further users will enter the
  51                        ldisc code for this tty. Can sleep.
  52
  53hangup()        -       Called when the tty line is hung up.
  54                        The line discipline should cease I/O to the tty.
  55                        No further calls into the ldisc code will occur.
  56                        The return value is ignored. Can sleep.
  57
  58write()         -       A process is writing data through the line
  59                        discipline.  Multiple write calls are serialized
  60                        by the tty layer for the ldisc.  May sleep. 
  61
  62flush_buffer()  -       (optional) May be called at any point between
  63                        open and close, and instructs the line discipline
  64                        to empty its input buffer.
  65
  66chars_in_buffer() -     (optional) Report the number of bytes in the input
  67                        buffer.
  68
  69set_termios()   -       (optional) Called on termios structure changes.
  70                        The caller passes the old termios data and the
  71                        current data is in the tty. Called under the
  72                        termios semaphore so allowed to sleep. Serialized
  73                        against itself only.
  74
  75read()          -       Move data from the line discipline to the user.
  76                        Multiple read calls may occur in parallel and the
  77                        ldisc must deal with serialization issues. May 
  78                        sleep.
  79
  80poll()          -       Check the status for the poll/select calls. Multiple
  81                        poll calls may occur in parallel. May sleep.
  82
  83ioctl()         -       Called when an ioctl is handed to the tty layer
  84                        that might be for the ldisc. Multiple ioctl calls
  85                        may occur in parallel. May sleep. 
  86
  87compat_ioctl()  -       Called when a 32 bit ioctl is handed to the tty layer
  88                        that might be for the ldisc. Multiple ioctl calls
  89                        may occur in parallel. May sleep.
  90
  91Driver Side Interfaces:
  92
  93receive_buf()   -       Hand buffers of bytes from the driver to the ldisc
  94                        for processing. Semantics currently rather
  95                        mysterious 8(
  96
  97write_wakeup()  -       May be called at any point between open and close.
  98                        The TTY_DO_WRITE_WAKEUP flag indicates if a call
  99                        is needed but always races versus calls. Thus the
 100                        ldisc must be careful about setting order and to
 101                        handle unexpected calls. Must not sleep.
 102
 103                        The driver is forbidden from calling this directly
 104                        from the ->write call from the ldisc as the ldisc
 105                        is permitted to call the driver write method from
 106                        this function. In such a situation defer it.
 107
 108dcd_change()    -       Report to the tty line the current DCD pin status
 109                        changes and the relative timestamp. The timestamp
 110                        cannot be NULL.
 111
 112
 113Driver Access
 114
 115Line discipline methods can call the following methods of the underlying
 116hardware driver through the function pointers within the tty->driver
 117structure:
 118
 119write()                 Write a block of characters to the tty device.
 120                        Returns the number of characters accepted. The
 121                        character buffer passed to this method is already
 122                        in kernel space.
 123
 124put_char()              Queues a character for writing to the tty device.
 125                        If there is no room in the queue, the character is
 126                        ignored.
 127
 128flush_chars()           (Optional) If defined, must be called after
 129                        queueing characters with put_char() in order to
 130                        start transmission.
 131
 132write_room()            Returns the numbers of characters the tty driver
 133                        will accept for queueing to be written.
 134
 135ioctl()                 Invoke device specific ioctl.
 136                        Expects data pointers to refer to userspace.
 137                        Returns ENOIOCTLCMD for unrecognized ioctl numbers.
 138
 139set_termios()           Notify the tty driver that the device's termios
 140                        settings have changed. New settings are in
 141                        tty->termios. Previous settings should be passed in
 142                        the "old" argument.
 143
 144                        The API is defined such that the driver should return
 145                        the actual modes selected. This means that the
 146                        driver function is responsible for modifying any
 147                        bits in the request it cannot fulfill to indicate
 148                        the actual modes being used. A device with no
 149                        hardware capability for change (eg a USB dongle or
 150                        virtual port) can provide NULL for this method.
 151
 152throttle()              Notify the tty driver that input buffers for the
 153                        line discipline are close to full, and it should
 154                        somehow signal that no more characters should be
 155                        sent to the tty.
 156
 157unthrottle()            Notify the tty driver that characters can now be
 158                        sent to the tty without fear of overrunning the
 159                        input buffers of the line disciplines.
 160
 161stop()                  Ask the tty driver to stop outputting characters
 162                        to the tty device.
 163
 164start()                 Ask the tty driver to resume sending characters
 165                        to the tty device.
 166
 167hangup()                Ask the tty driver to hang up the tty device.
 168
 169break_ctl()             (Optional) Ask the tty driver to turn on or off
 170                        BREAK status on the RS-232 port.  If state is -1,
 171                        then the BREAK status should be turned on; if
 172                        state is 0, then BREAK should be turned off.
 173                        If this routine is not implemented, use ioctls
 174                        TIOCSBRK / TIOCCBRK instead.
 175
 176wait_until_sent()       Waits until the device has written out all of the
 177                        characters in its transmitter FIFO.
 178
 179send_xchar()            Send a high-priority XON/XOFF character to the device.
 180
 181
 182Flags
 183
 184Line discipline methods have access to tty->flags field containing the
 185following interesting flags:
 186
 187TTY_THROTTLED           Driver input is throttled. The ldisc should call
 188                        tty->driver->unthrottle() in order to resume
 189                        reception when it is ready to process more data.
 190
 191TTY_DO_WRITE_WAKEUP     If set, causes the driver to call the ldisc's
 192                        write_wakeup() method in order to resume
 193                        transmission when it can accept more data
 194                        to transmit.
 195
 196TTY_IO_ERROR            If set, causes all subsequent userspace read/write
 197                        calls on the tty to fail, returning -EIO.
 198
 199TTY_OTHER_CLOSED        Device is a pty and the other side has closed.
 200
 201TTY_NO_WRITE_SPLIT      Prevent driver from splitting up writes into
 202                        smaller chunks.
 203
 204
 205Locking
 206
 207Callers to the line discipline functions from the tty layer are required to
 208take line discipline locks. The same is true of calls from the driver side
 209but not yet enforced.
 210
 211Three calls are now provided
 212
 213        ldisc = tty_ldisc_ref(tty);
 214
 215takes a handle to the line discipline in the tty and returns it. If no ldisc
 216is currently attached or the ldisc is being closed and re-opened at this
 217point then NULL is returned. While this handle is held the ldisc will not
 218change or go away.
 219
 220        tty_ldisc_deref(ldisc)
 221
 222Returns the ldisc reference and allows the ldisc to be closed. Returning the
 223reference takes away your right to call the ldisc functions until you take
 224a new reference.
 225
 226        ldisc = tty_ldisc_ref_wait(tty);
 227
 228Performs the same function as tty_ldisc_ref except that it will wait for an
 229ldisc change to complete and then return a reference to the new ldisc. 
 230
 231While these functions are slightly slower than the old code they should have
 232minimal impact as most receive logic uses the flip buffers and they only
 233need to take a reference when they push bits up through the driver.
 234
 235A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc 
 236functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
 237fail in this situation if used within these functions. Ldisc and driver
 238code calling its own functions must be careful in this case. 
 239
 240
 241Driver Interface
 242----------------
 243
 244open()          -       Called when a device is opened. May sleep
 245
 246close()         -       Called when a device is closed. At the point of
 247                        return from this call the driver must make no 
 248                        further ldisc calls of any kind. May sleep
 249
 250write()         -       Called to write bytes to the device. May not
 251                        sleep. May occur in parallel in special cases. 
 252                        Because this includes panic paths drivers generally
 253                        shouldn't try and do clever locking here.
 254
 255put_char()      -       Stuff a single character onto the queue. The
 256                        driver is guaranteed following up calls to
 257                        flush_chars.
 258
 259flush_chars()   -       Ask the kernel to write put_char queue
 260
 261write_room()    -       Return the number of characters tht can be stuffed
 262                        into the port buffers without overflow (or less).
 263                        The ldisc is responsible for being intelligent
 264                        about multi-threading of write_room/write calls
 265
 266ioctl()         -       Called when an ioctl may be for the driver
 267
 268set_termios()   -       Called on termios change, serialized against
 269                        itself by a semaphore. May sleep.
 270
 271set_ldisc()     -       Notifier for discipline change. At the point this 
 272                        is done the discipline is not yet usable. Can now
 273                        sleep (I think)
 274
 275throttle()      -       Called by the ldisc to ask the driver to do flow
 276                        control.  Serialization including with unthrottle
 277                        is the job of the ldisc layer.
 278
 279unthrottle()    -       Called by the ldisc to ask the driver to stop flow
 280                        control.
 281
 282stop()          -       Ldisc notifier to the driver to stop output. As with
 283                        throttle the serializations with start() are down
 284                        to the ldisc layer.
 285
 286start()         -       Ldisc notifier to the driver to start output.
 287
 288hangup()        -       Ask the tty driver to cause a hangup initiated
 289                        from the host side. [Can sleep ??]
 290
 291break_ctl()     -       Send RS232 break. Can sleep. Can get called in
 292                        parallel, driver must serialize (for now), and
 293                        with write calls.
 294
 295wait_until_sent() -     Wait for characters to exit the hardware queue
 296                        of the driver. Can sleep
 297
 298send_xchar()      -     Send XON/XOFF and if possible jump the queue with
 299                        it in order to get fast flow control responses.
 300                        Cannot sleep ??
 301
 302
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