Documentation/serial/tty.txt

   1
   2                         The Lockronomicon
   3
   4 Your guide to the ancient and twisted locking policies of the tty layer and
   5 the warped logic behind them. Beware all ye who read on.
   6
   7
   8 Line Discipline
   9 ---------------
  10
  11 Line disciplines are registered with tty_register_ldisc() passing the
  12 discipline number and the ldisc structure. At the point of registration the
  13 discipline must be ready to use and it is possible it will get used before
  14 the call returns success. If the call returns an error then it won't get
  15 called. Do not re-use ldisc numbers as they are part of the userspace ABI
  16 and writing over an existing ldisc will cause demons to eat your computer.
  17 After the return the ldisc data has been copied so you may free your own
  18 copy of the structure. You must not re-register over the top of the line
  19 discipline even with the same data or your computer again will be eaten by
  20 demons.
  21
  22 In order to remove a line discipline call tty_unregister_ldisc().
  23 In ancient times this always worked. In modern times the function will
  24 return -EBUSY if the ldisc is currently in use. Since the ldisc referencing
  25 code manages the module counts this should not usually be a concern.
  26
  27 Heed this warning: the reference count field of the registered copies of the
  28 tty_ldisc structure in the ldisc table counts the number of lines using this
  29 discipline. The reference count of the tty_ldisc structure within a tty
  30 counts the number of active users of the ldisc at this instant. In effect it
  31 counts the number of threads of execution within an ldisc method (plus those
  32 about to enter and exit although this detail matters not).
  33
  34 Line Discipline Methods
  35 -----------------------
  36
  37 TTY side interfaces:
  38
  39 open()          -       Called when the line discipline is attached to
  40                         the terminal. No other call into the line
  41                         discipline for this tty will occur until it
  42                         completes successfully. Returning an error will
  43                         prevent the ldisc from being attached. Can sleep.
  44
  45 close()         -       This is called on a terminal when the line
  46                         discipline is being unplugged. At the point of
  47                         execution no further users will enter the
  48                         ldisc code for this tty. Can sleep.
  49
  50 hangup()        -       Called when the tty line is hung up.
  51                         The line discipline should cease I/O to the tty.
  52                         No further calls into the ldisc code will occur.
  53                         The return value is ignored. Can sleep.
  54
  55 write()         -       A process is writing data through the line
  56                         discipline.  Multiple write calls are serialized
  57                         by the tty layer for the ldisc.  May sleep.
  58
  59 flush_buffer()  -       (optional) May be called at any point between
  60                         open and close, and instructs the line discipline
  61                         to empty its input buffer.
  62
  63 chars_in_buffer() -     (optional) Report the number of bytes in the input
  64                         buffer.
  65
  66 set_termios()   -       (optional) Called on termios structure changes.
  67                         The caller passes the old termios data and the
  68                         current data is in the tty. Called under the
  69                         termios semaphore so allowed to sleep. Serialized
  70                         against itself only.
  71
  72 read()          -       Move data from the line discipline to the user.
  73                         Multiple read calls may occur in parallel and the
  74                         ldisc must deal with serialization issues. May
  75                         sleep.
  76
  77 poll()          -       Check the status for the poll/select calls. Multiple
  78                         poll calls may occur in parallel. May sleep.
  79
  80 ioctl()         -       Called when an ioctl is handed to the tty layer
  81                         that might be for the ldisc. Multiple ioctl calls
  82                         may occur in parallel. May sleep.
  83
  84 compat_ioctl()  -       Called when a 32 bit ioctl is handed to the tty layer
  85                         that might be for the ldisc. Multiple ioctl calls
  86                         may occur in parallel. May sleep.
  87
  88 Driver Side Interfaces:
  89
  90 receive_buf()   -       Hand buffers of bytes from the driver to the ldisc
  91                         for processing. Semantics currently rather
  92                         mysterious 8(
  93
  94 write_wakeup()  -       May be called at any point between open and close.
  95                         The TTY_DO_WRITE_WAKEUP flag indicates if a call
  96                         is needed but always races versus calls. Thus the
  97                         ldisc must be careful about setting order and to
  98                         handle unexpected calls. Must not sleep.
  99
 100                         The driver is forbidden from calling this directly
 101                         from the ->write call from the ldisc as the ldisc
 102                         is permitted to call the driver write method from
 103                         this function. In such a situation defer it.
 104
 105 dcd_change()    -       Report to the tty line the current DCD pin status
 106                         changes and the relative timestamp. The timestamp
 107                         cannot be NULL.
 108
 109
 110 Driver Access
 111
 112 Line discipline methods can call the following methods of the underlying
 113 hardware driver through the function pointers within the tty->driver
 114 structure:
 115
 116 write()                 Write a block of characters to the tty device.
 117                         Returns the number of characters accepted. The
 118                         character buffer passed to this method is already
 119                         in kernel space.
 120
 121 put_char()              Queues a character for writing to the tty device.
 122                         If there is no room in the queue, the character is
 123                         ignored.
 124
 125 flush_chars()           (Optional) If defined, must be called after
 126                         queueing characters with put_char() in order to
 127                         start transmission.
 128
 129 write_room()            Returns the numbers of characters the tty driver
 130                         will accept for queueing to be written.
 131
 132 ioctl()                 Invoke device specific ioctl.
 133                         Expects data pointers to refer to userspace.
 134                         Returns ENOIOCTLCMD for unrecognized ioctl numbers.
 135
 136 set_termios()           Notify the tty driver that the device's termios
 137                         settings have changed. New settings are in
 138                         tty->termios. Previous settings should be passed in
 139                         the "old" argument.
 140
 141                         The API is defined such that the driver should return
 142                         the actual modes selected. This means that the
 143                         driver function is responsible for modifying any
 144                         bits in the request it cannot fulfill to indicate
 145                         the actual modes being used. A device with no
 146                         hardware capability for change (e.g. a USB dongle or
 147                         virtual port) can provide NULL for this method.
 148
 149 throttle()              Notify the tty driver that input buffers for the
 150                         line discipline are close to full, and it should
 151                         somehow signal that no more characters should be
 152                         sent to the tty.
 153
 154 unthrottle()            Notify the tty driver that characters can now be
 155                         sent to the tty without fear of overrunning the
 156                         input buffers of the line disciplines.
 157
 158 stop()                  Ask the tty driver to stop outputting characters
 159                         to the tty device.
 160
 161 start()                 Ask the tty driver to resume sending characters
 162                         to the tty device.
 163
 164 hangup()                Ask the tty driver to hang up the tty device.
 165
 166 break_ctl()             (Optional) Ask the tty driver to turn on or off
 167                         BREAK status on the RS-232 port.  If state is -1,
 168                         then the BREAK status should be turned on; if
 169                         state is 0, then BREAK should be turned off.
 170                         If this routine is not implemented, use ioctls
 171                         TIOCSBRK / TIOCCBRK instead.
 172
 173 wait_until_sent()       Waits until the device has written out all of the
 174                         characters in its transmitter FIFO.
 175
 176 send_xchar()            Send a high-priority XON/XOFF character to the device.
 177
 178
 179 Flags
 180
 181 Line discipline methods have access to tty->flags field containing the
 182 following interesting flags:
 183
 184 TTY_THROTTLED           Driver input is throttled. The ldisc should call
 185                         tty->driver->unthrottle() in order to resume
 186                         reception when it is ready to process more data.
 187
 188 TTY_DO_WRITE_WAKEUP     If set, causes the driver to call the ldisc's
 189                         write_wakeup() method in order to resume
 190                         transmission when it can accept more data
 191                         to transmit.
 192
 193 TTY_IO_ERROR            If set, causes all subsequent userspace read/write
 194                         calls on the tty to fail, returning -EIO.
 195
 196 TTY_OTHER_CLOSED        Device is a pty and the other side has closed.
 197
 198 TTY_OTHER_DONE          Device is a pty and the other side has closed and
 199                         all pending input processing has been completed.
 200
 201 TTY_NO_WRITE_SPLIT      Prevent driver from splitting up writes into
 202                         smaller chunks.
 203
 204
 205 Locking
 206
 207 Callers to the line discipline functions from the tty layer are required to
 208 take line discipline locks. The same is true of calls from the driver side
 209 but not yet enforced.
 210
 211 Three calls are now provided
 212
 213         ldisc = tty_ldisc_ref(tty);
 214
 215 takes a handle to the line discipline in the tty and returns it. If no ldisc
 216 is currently attached or the ldisc is being closed and re-opened at this
 217 point then NULL is returned. While this handle is held the ldisc will not
 218 change or go away.
 219
 220         tty_ldisc_deref(ldisc)
 221
 222 Returns the ldisc reference and allows the ldisc to be closed. Returning the
 223 reference takes away your right to call the ldisc functions until you take
 224 a new reference.
 225
 226         ldisc = tty_ldisc_ref_wait(tty);
 227
 228 Performs the same function as tty_ldisc_ref except that it will wait for an
 229 ldisc change to complete and then return a reference to the new ldisc.
 230
 231 While these functions are slightly slower than the old code they should have
 232 minimal impact as most receive logic uses the flip buffers and they only
 233 need to take a reference when they push bits up through the driver.
 234
 235 A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc
 236 functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
 237 fail in this situation if used within these functions. Ldisc and driver
 238 code calling its own functions must be careful in this case.
 239
 240
 241 Driver Interface
 242 ----------------
 243
 244 open()          -       Called when a device is opened. May sleep
 245
 246 close()         -       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
 250 write()         -       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
 255 put_char()      -       Stuff a single character onto the queue. The
 256                         driver is guaranteed following up calls to
 257                         flush_chars.
 258
 259 flush_chars()   -       Ask the kernel to write put_char queue
 260
 261 write_room()    -       Return the number of characters that 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
 266 ioctl()         -       Called when an ioctl may be for the driver
 267
 268 set_termios()   -       Called on termios change, serialized against
 269                         itself by a semaphore. May sleep.
 270
 271 set_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
 275 throttle()      -       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
 279 unthrottle()    -       Called by the ldisc to ask the driver to stop flow
 280                         control.
 281
 282 stop()          -       Ldisc notifier to the driver to stop output. As with
 283                         throttle the serializations with start() are down
 284                         to the ldisc layer.
 285
 286 start()         -       Ldisc notifier to the driver to start output.
 287
 288 hangup()        -       Ask the tty driver to cause a hangup initiated
 289                         from the host side. [Can sleep ??]
 290
 291 break_ctl()     -       Send RS232 break. Can sleep. Can get called in
 292                         parallel, driver must serialize (for now), and
 293                         with write calls.
 294
 295 wait_until_sent() -     Wait for characters to exit the hardware queue
 296                         of the driver. Can sleep
 297
 298 send_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