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2 The Lockronomicon
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.
8 Line Discipline
9 ---------------
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.
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.
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).
34 Line Discipline Methods
35 -----------------------
37 TTY side interfaces:
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.
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.
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.
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.
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.
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.
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.
81 poll() - (optional) Check the status for the poll/select
82 calls. Multiple poll calls may occur in parallel.
83 May sleep.
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.
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.
94 Driver Side Interfaces:
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.
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.
106 If both receive_buf() and receive_buf2() are
107 defined, receive_buf2() should be preferred.
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.
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.
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.
125 Driver Access
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:
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.
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.
140 flush_chars() (Optional) If defined, must be called after
141 queueing characters with put_char() in order to
142 start transmission.
144 write_room() Returns the numbers of characters the tty driver
145 will accept for queueing to be written.
147 ioctl() Invoke device specific ioctl.
148 Expects data pointers to refer to userspace.
149 Returns ENOIOCTLCMD for unrecognized ioctl numbers.
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.
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.
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.
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.
173 stop() Ask the tty driver to stop outputting characters
174 to the tty device.
176 start() Ask the tty driver to resume sending characters
177 to the tty device.
179 hangup() Ask the tty driver to hang up the tty device.
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.
188 wait_until_sent() Waits until the device has written out all of the
189 characters in its transmitter FIFO.
191 send_xchar() Send a high-priority XON/XOFF character to the device.
194 Flags
196 Line discipline methods have access to tty->flags field containing the
197 following interesting flags:
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.
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.
208 TTY_IO_ERROR If set, causes all subsequent userspace read/write
209 calls on the tty to fail, returning -EIO.
211 TTY_OTHER_CLOSED Device is a pty and the other side has closed.
213 TTY_NO_WRITE_SPLIT Prevent driver from splitting up writes into
214 smaller chunks.
217 Locking
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.
223 Three calls are now provided
225 ldisc = tty_ldisc_ref(tty);
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.
232 tty_ldisc_deref(ldisc)
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.
238 ldisc = tty_ldisc_ref_wait(tty);
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.
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.
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.
253 Driver Interface
254 ----------------
256 open() - Called when a device is opened. May sleep
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
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.
267 put_char() - Stuff a single character onto the queue. The
268 driver is guaranteed following up calls to
269 flush_chars.
271 flush_chars() - Ask the kernel to write put_char queue
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
278 ioctl() - Called when an ioctl may be for the driver
280 set_termios() - Called on termios change, serialized against
281 itself by a semaphore. May sleep.
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)
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.
291 unthrottle() - Called by the ldisc to ask the driver to stop flow
292 control.
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.
298 start() - Ldisc notifier to the driver to start output.
300 hangup() - Ask the tty driver to cause a hangup initiated
301 from the host side. [Can sleep ??]
303 break_ctl() - Send RS232 break. Can sleep. Can get called in
304 parallel, driver must serialize (for now), and
305 with write calls.
307 wait_until_sent() - Wait for characters to exit the hardware queue
308 of the driver. Can sleep
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 ??