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