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 FIXME: still need to work out the full set of BKL assumptions and document
   8 them so they can eventually be killed off.
   9
  10
  11 Line Discipline
  12 ---------------
  13
  14 Line disciplines are registered with tty_register_ldisc() passing the
  15 discipline number and the ldisc structure. At the point of registration the
  16 discipline must be ready to use and it is possible it will get used before
  17 the call returns success. If the call returns an error then it won't get
  18 called. Do not re-use ldisc numbers as they are part of the userspace ABI
  19 and writing over an existing ldisc will cause demons to eat your computer.
  20 After the return the ldisc data has been copied so you may free your own
  21 copy of the structure. You must not re-register over the top of the line
  22 discipline even with the same data or your computer again will be eaten by
  23 demons.
  24
  25 In order to remove a line discipline call tty_unregister_ldisc().
  26 In ancient times this always worked. In modern times the function will
  27 return -EBUSY if the ldisc is currently in use. Since the ldisc referencing
  28 code manages the module counts this should not usually be a concern.
  29
  30 Heed this warning: the reference count field of the registered copies of the
  31 tty_ldisc structure in the ldisc table counts the number of lines using this
  32 discipline. The reference count of the tty_ldisc structure within a tty
  33 counts the number of active users of the ldisc at this instant. In effect it
  34 counts the number of threads of execution within an ldisc method (plus those
  35 about to enter and exit although this detail matters not).
  36
  37 Line Discipline Methods
  38 -----------------------
  39
  40 TTY side interfaces:
  41
  42 open()          -       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
  48 close()         -       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
  53 hangup()        -       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
  58 write()         -       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
  62 flush_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
  66 chars_in_buffer() -     (optional) Report the number of bytes in the input
  67                         buffer.
  68
  69 set_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
  75 read()          -       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
  80 poll()          -       Check the status for the poll/select calls. Multiple
  81                         poll calls may occur in parallel. May sleep.
  82
  83 ioctl()         -       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
  87 compat_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
  91 Driver Side Interfaces:
  92
  93 receive_buf()   -       Hand buffers of bytes from the driver to the ldisc
  94                         for processing. Semantics currently rather
  95                         mysterious 8(
  96
  97 write_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
 108
 109 Driver Access
 110
 111 Line discipline methods can call the following methods of the underlying
 112 hardware driver through the function pointers within the tty->driver
 113 structure:
 114
 115 write()                 Write a block of characters to the tty device.
 116                         Returns the number of characters accepted. The
 117                         character buffer passed to this method is already
 118                         in kernel space.
 119
 120 put_char()              Queues a character for writing to the tty device.
 121                         If there is no room in the queue, the character is
 122                         ignored.
 123
 124 flush_chars()           (Optional) If defined, must be called after
 125                         queueing characters with put_char() in order to
 126                         start transmission.
 127
 128 write_room()            Returns the numbers of characters the tty driver
 129                         will accept for queueing to be written.
 130
 131 ioctl()                 Invoke device specific ioctl.
 132                         Expects data pointers to refer to userspace.
 133                         Returns ENOIOCTLCMD for unrecognized ioctl numbers.
 134
 135 set_termios()           Notify the tty driver that the device's termios
 136                         settings have changed. New settings are in
 137                         tty->termios. Previous settings should be passed in
 138                         the "old" argument.
 139
 140                         The API is defined such that the driver should return
 141                         the actual modes selected. This means that the
 142                         driver function is responsible for modifying any
 143                         bits in the request it cannot fulfill to indicate
 144                         the actual modes being used. A device with no
 145                         hardware capability for change (eg a USB dongle or
 146                         virtual port) can provide NULL for this method.
 147
 148 throttle()              Notify the tty driver that input buffers for the
 149                         line discipline are close to full, and it should
 150                         somehow signal that no more characters should be
 151                         sent to the tty.
 152
 153 unthrottle()            Notify the tty driver that characters can now be
 154                         sent to the tty without fear of overrunning the
 155                         input buffers of the line disciplines.
 156
 157 stop()                  Ask the tty driver to stop outputting characters
 158                         to the tty device.
 159
 160 start()                 Ask the tty driver to resume sending characters
 161                         to the tty device.
 162
 163 hangup()                Ask the tty driver to hang up the tty device.
 164
 165 break_ctl()             (Optional) Ask the tty driver to turn on or off
 166                         BREAK status on the RS-232 port.  If state is -1,
 167                         then the BREAK status should be turned on; if
 168                         state is 0, then BREAK should be turned off.
 169                         If this routine is not implemented, use ioctls
 170                         TIOCSBRK / TIOCCBRK instead.
 171
 172 wait_until_sent()       Waits until the device has written out all of the
 173                         characters in its transmitter FIFO.
 174
 175 send_xchar()            Send a high-priority XON/XOFF character to the device.
 176
 177
 178 Flags
 179
 180 Line discipline methods have access to tty->flags field containing the
 181 following interesting flags:
 182
 183 TTY_THROTTLED           Driver input is throttled. The ldisc should call
 184                         tty->driver->unthrottle() in order to resume
 185                         reception when it is ready to process more data.
 186
 187 TTY_DO_WRITE_WAKEUP     If set, causes the driver to call the ldisc's
 188                         write_wakeup() method in order to resume
 189                         transmission when it can accept more data
 190                         to transmit.
 191
 192 TTY_IO_ERROR            If set, causes all subsequent userspace read/write
 193                         calls on the tty to fail, returning -EIO.
 194
 195 TTY_OTHER_CLOSED        Device is a pty and the other side has closed.
 196
 197 TTY_NO_WRITE_SPLIT      Prevent driver from splitting up writes into
 198                         smaller chunks.
 199
 200
 201 Locking
 202
 203 Callers to the line discipline functions from the tty layer are required to
 204 take line discipline locks. The same is true of calls from the driver side
 205 but not yet enforced.
 206
 207 Three calls are now provided
 208
 209         ldisc = tty_ldisc_ref(tty);
 210
 211 takes a handle to the line discipline in the tty and returns it. If no ldisc
 212 is currently attached or the ldisc is being closed and re-opened at this
 213 point then NULL is returned. While this handle is held the ldisc will not
 214 change or go away.
 215
 216         tty_ldisc_deref(ldisc)
 217
 218 Returns the ldisc reference and allows the ldisc to be closed. Returning the
 219 reference takes away your right to call the ldisc functions until you take
 220 a new reference.
 221
 222         ldisc = tty_ldisc_ref_wait(tty);
 223
 224 Performs the same function as tty_ldisc_ref except that it will wait for an
 225 ldisc change to complete and then return a reference to the new ldisc.
 226
 227 While these functions are slightly slower than the old code they should have
 228 minimal impact as most receive logic uses the flip buffers and they only
 229 need to take a reference when they push bits up through the driver.
 230
 231 A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc
 232 functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
 233 fail in this situation if used within these functions. Ldisc and driver
 234 code calling its own functions must be careful in this case.
 235
 236
 237 Driver Interface
 238 ----------------
 239
 240 open()          -       Called when a device is opened. May sleep
 241
 242 close()         -       Called when a device is closed. At the point of
 243                         return from this call the driver must make no
 244                         further ldisc calls of any kind. May sleep
 245
 246 write()         -       Called to write bytes to the device. May not
 247                         sleep. May occur in parallel in special cases.
 248                         Because this includes panic paths drivers generally
 249                         shouldn't try and do clever locking here.
 250
 251 put_char()      -       Stuff a single character onto the queue. The
 252                         driver is guaranteed following up calls to
 253                         flush_chars.
 254
 255 flush_chars()   -       Ask the kernel to write put_char queue
 256
 257 write_room()    -       Return the number of characters tht can be stuffed
 258                         into the port buffers without overflow (or less).
 259                         The ldisc is responsible for being intelligent
 260                         about multi-threading of write_room/write calls
 261
 262 ioctl()         -       Called when an ioctl may be for the driver
 263
 264 set_termios()   -       Called on termios change, serialized against
 265                         itself by a semaphore. May sleep.
 266
 267 set_ldisc()     -       Notifier for discipline change. At the point this
 268                         is done the discipline is not yet usable. Can now
 269                         sleep (I think)
 270
 271 throttle()      -       Called by the ldisc to ask the driver to do flow
 272                         control.  Serialization including with unthrottle
 273                         is the job of the ldisc layer.
 274
 275 unthrottle()    -       Called by the ldisc to ask the driver to stop flow
 276                         control.
 277
 278 stop()          -       Ldisc notifier to the driver to stop output. As with
 279                         throttle the serializations with start() are down
 280                         to the ldisc layer.
 281
 282 start()         -       Ldisc notifier to the driver to start output.
 283
 284 hangup()        -       Ask the tty driver to cause a hangup initiated
 285                         from the host side. [Can sleep ??]
 286
 287 break_ctl()     -       Send RS232 break. Can sleep. Can get called in
 288                         parallel, driver must serialize (for now), and
 289                         with write calls.
 290
 291 wait_until_sent() -     Wait for characters to exit the hardware queue
 292                         of the driver. Can sleep
 293
 294 send_xchar()      -     Send XON/XOFF and if possible jump the queue with
 295                         it in order to get fast flow control responses.
 296                         Cannot sleep ??
 297