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. Should initialize any
  43                         state needed by the ldisc, and set receive_room
  44                         in the tty_struct to the maximum amount of data
  45                         the line discipline is willing to accept from the
  46                         driver with a single call to receive_buf().
  47                         Returning an error will prevent the ldisc from
  48                         being attached. Can sleep.
  49
  50 close()         -       This is called on a terminal when the line
  51                         discipline is being unplugged. At the point of
  52                         execution no further users will enter the
  53                         ldisc code for this tty. Can sleep.
  54
  55 hangup()        -       Called when the tty line is hung up.
  56                         The line discipline should cease I/O to the tty.
  57                         No further calls into the ldisc code will occur.
  58                         The return value is ignored. Can sleep.
  59
  60 read()          -       (optional) A process requests reading data from
  61                         the line. Multiple read calls may occur in parallel
  62                         and the ldisc must deal with serialization issues.
  63                         If not defined, the process will receive an EIO
  64                         error. May sleep.
  65
  66 write()         -       (optional) A process requests writing data to the
  67                         line. Multiple write calls are serialized by the
  68                         tty layer for the ldisc. If not defined, the
  69                         process will receive an EIO error. May sleep.
  70
  71 flush_buffer()  -       (optional) May be called at any point between
  72                         open and close, and instructs the line discipline
  73                         to empty its input buffer.
  74
  75 chars_in_buffer() -     (optional) Report the number of bytes in the input
  76                         buffer.
  77
  78 set_termios()   -       (optional) Called on termios structure changes.
  79                         The caller passes the old termios data and the
  80                         current data is in the tty. Called under the
  81                         termios semaphore so allowed to sleep. Serialized
  82                         against itself only.
  83
  84 poll()          -       (optional) Check the status for the poll/select
  85                         calls. Multiple poll calls may occur in parallel.
  86                         May sleep.
  87
  88 ioctl()         -       (optional) Called when an ioctl is handed to the
  89                         tty layer that might be for the ldisc. Multiple
  90                         ioctl calls may occur in parallel. May sleep.
  91
  92 compat_ioctl()  -       (optional) Called when a 32 bit ioctl is handed
  93                         to the tty layer that might be for the ldisc.
  94                         Multiple ioctl calls may occur in parallel.
  95                         May sleep.
  96
  97 Driver Side Interfaces:
  98
  99 receive_buf()   -       (optional) Called by the low-level driver to hand
 100                         a buffer of received bytes to the ldisc for
 101                         processing. The number of bytes is guaranteed not
 102                         to exceed the current value of tty->receive_room.
 103                         All bytes must be processed.
 104
 105 receive_buf2()  -       (optional) Called by the low-level driver to hand
 106                         a buffer of received bytes to the ldisc for
 107                         processing. Returns the number of bytes processed.
 108
 109                         If both receive_buf() and receive_buf2() are
 110                         defined, receive_buf2() should be preferred.
 111
 112 write_wakeup()  -       May be called at any point between open and close.
 113                         The TTY_DO_WRITE_WAKEUP flag indicates if a call
 114                         is needed but always races versus calls. Thus the
 115                         ldisc must be careful about setting order and to
 116                         handle unexpected calls. Must not sleep.
 117
 118                         The driver is forbidden from calling this directly
 119                         from the ->write call from the ldisc as the ldisc
 120                         is permitted to call the driver write method from
 121                         this function. In such a situation defer it.
 122
 123 dcd_change()    -       Report to the tty line the current DCD pin status
 124                         changes and the relative timestamp. The timestamp
 125                         cannot be NULL.
 126
 127
 128 Driver Access
 129
 130 Line discipline methods can call the following methods of the underlying
 131 hardware driver through the function pointers within the tty->driver
 132 structure:
 133
 134 write()                 Write a block of characters to the tty device.
 135                         Returns the number of characters accepted. The
 136                         character buffer passed to this method is already
 137                         in kernel space.
 138
 139 put_char()              Queues a character for writing to the tty device.
 140                         If there is no room in the queue, the character is
 141                         ignored.
 142
 143 flush_chars()           (Optional) If defined, must be called after
 144                         queueing characters with put_char() in order to
 145                         start transmission.
 146
 147 write_room()            Returns the numbers of characters the tty driver
 148                         will accept for queueing to be written.
 149
 150 ioctl()                 Invoke device specific ioctl.
 151                         Expects data pointers to refer to userspace.
 152                         Returns ENOIOCTLCMD for unrecognized ioctl numbers.
 153
 154 set_termios()           Notify the tty driver that the device's termios
 155                         settings have changed. New settings are in
 156                         tty->termios. Previous settings should be passed in
 157                         the "old" argument.
 158
 159                         The API is defined such that the driver should return
 160                         the actual modes selected. This means that the
 161                         driver function is responsible for modifying any
 162                         bits in the request it cannot fulfill to indicate
 163                         the actual modes being used. A device with no
 164                         hardware capability for change (e.g. a USB dongle or
 165                         virtual port) can provide NULL for this method.
 166
 167 throttle()              Notify the tty driver that input buffers for the
 168                         line discipline are close to full, and it should
 169                         somehow signal that no more characters should be
 170                         sent to the tty.
 171
 172 unthrottle()            Notify the tty driver that characters can now be
 173                         sent to the tty without fear of overrunning the
 174                         input buffers of the line disciplines.
 175
 176 stop()                  Ask the tty driver to stop outputting characters
 177                         to the tty device.
 178
 179 start()                 Ask the tty driver to resume sending characters
 180                         to the tty device.
 181
 182 hangup()                Ask the tty driver to hang up the tty device.
 183
 184 break_ctl()             (Optional) Ask the tty driver to turn on or off
 185                         BREAK status on the RS-232 port.  If state is -1,
 186                         then the BREAK status should be turned on; if
 187                         state is 0, then BREAK should be turned off.
 188                         If this routine is not implemented, use ioctls
 189                         TIOCSBRK / TIOCCBRK instead.
 190
 191 wait_until_sent()       Waits until the device has written out all of the
 192                         characters in its transmitter FIFO.
 193
 194 send_xchar()            Send a high-priority XON/XOFF character to the device.
 195
 196
 197 Flags
 198
 199 Line discipline methods have access to tty->flags field containing the
 200 following interesting flags:
 201
 202 TTY_THROTTLED           Driver input is throttled. The ldisc should call
 203                         tty->driver->unthrottle() in order to resume
 204                         reception when it is ready to process more data.
 205
 206 TTY_DO_WRITE_WAKEUP     If set, causes the driver to call the ldisc's
 207                         write_wakeup() method in order to resume
 208                         transmission when it can accept more data
 209                         to transmit.
 210
 211 TTY_IO_ERROR            If set, causes all subsequent userspace read/write
 212                         calls on the tty to fail, returning -EIO.
 213
 214 TTY_OTHER_CLOSED        Device is a pty and the other side has closed.
 215
 216 TTY_OTHER_DONE          Device is a pty and the other side has closed and
 217                         all pending input processing has been completed.
 218
 219 TTY_NO_WRITE_SPLIT      Prevent driver from splitting up writes into
 220                         smaller chunks.
 221
 222
 223 Locking
 224
 225 Callers to the line discipline functions from the tty layer are required to
 226 take line discipline locks. The same is true of calls from the driver side
 227 but not yet enforced.
 228
 229 Three calls are now provided
 230
 231         ldisc = tty_ldisc_ref(tty);
 232
 233 takes a handle to the line discipline in the tty and returns it. If no ldisc
 234 is currently attached or the ldisc is being closed and re-opened at this
 235 point then NULL is returned. While this handle is held the ldisc will not
 236 change or go away.
 237
 238         tty_ldisc_deref(ldisc)
 239
 240 Returns the ldisc reference and allows the ldisc to be closed. Returning the
 241 reference takes away your right to call the ldisc functions until you take
 242 a new reference.
 243
 244         ldisc = tty_ldisc_ref_wait(tty);
 245
 246 Performs the same function as tty_ldisc_ref except that it will wait for an
 247 ldisc change to complete and then return a reference to the new ldisc.
 248
 249 While these functions are slightly slower than the old code they should have
 250 minimal impact as most receive logic uses the flip buffers and they only
 251 need to take a reference when they push bits up through the driver.
 252
 253 A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc
 254 functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
 255 fail in this situation if used within these functions. Ldisc and driver
 256 code calling its own functions must be careful in this case.
 257
 258
 259 Driver Interface
 260 ----------------
 261
 262 open()          -       Called when a device is opened. May sleep
 263
 264 close()         -       Called when a device is closed. At the point of
 265                         return from this call the driver must make no
 266                         further ldisc calls of any kind. May sleep
 267
 268 write()         -       Called to write bytes to the device. May not
 269                         sleep. May occur in parallel in special cases.
 270                         Because this includes panic paths drivers generally
 271                         shouldn't try and do clever locking here.
 272
 273 put_char()      -       Stuff a single character onto the queue. The
 274                         driver is guaranteed following up calls to
 275                         flush_chars.
 276
 277 flush_chars()   -       Ask the kernel to write put_char queue
 278
 279 write_room()    -       Return the number of characters that can be stuffed
 280                         into the port buffers without overflow (or less).
 281                         The ldisc is responsible for being intelligent
 282                         about multi-threading of write_room/write calls
 283
 284 ioctl()         -       Called when an ioctl may be for the driver
 285
 286 set_termios()   -       Called on termios change, serialized against
 287                         itself by a semaphore. May sleep.
 288
 289 set_ldisc()     -       Notifier for discipline change. At the point this
 290                         is done the discipline is not yet usable. Can now
 291                         sleep (I think)
 292
 293 throttle()      -       Called by the ldisc to ask the driver to do flow
 294                         control.  Serialization including with unthrottle
 295                         is the job of the ldisc layer.
 296
 297 unthrottle()    -       Called by the ldisc to ask the driver to stop flow
 298                         control.
 299
 300 stop()          -       Ldisc notifier to the driver to stop output. As with
 301                         throttle the serializations with start() are down
 302                         to the ldisc layer.
 303
 304 start()         -       Ldisc notifier to the driver to start output.
 305
 306 hangup()        -       Ask the tty driver to cause a hangup initiated
 307                         from the host side. [Can sleep ??]
 308
 309 break_ctl()     -       Send RS232 break. Can sleep. Can get called in
 310                         parallel, driver must serialize (for now), and
 311                         with write calls.
 312
 313 wait_until_sent() -     Wait for characters to exit the hardware queue
 314                         of the driver. Can sleep
 315
 316 send_xchar()      -     Send XON/XOFF and if possible jump the queue with
 317                         it in order to get fast flow control responses.
 318                         Cannot sleep ??
 319