Root/Documentation/serial/tty.txt

1
2            The Lockronomicon
3
4Your guide to the ancient and twisted locking policies of the tty layer and
5the warped logic behind them. Beware all ye who read on.
6
7FIXME: still need to work out the full set of BKL assumptions and document
8them so they can eventually be killed off.
9
10
11Line Discipline
12---------------
13
14Line disciplines are registered with tty_register_ldisc() passing the
15discipline number and the ldisc structure. At the point of registration the
16discipline must be ready to use and it is possible it will get used before
17the call returns success. If the call returns an error then it won't get
18called. Do not re-use ldisc numbers as they are part of the userspace ABI
19and writing over an existing ldisc will cause demons to eat your computer.
20After the return the ldisc data has been copied so you may free your own
21copy of the structure. You must not re-register over the top of the line
22discipline even with the same data or your computer again will be eaten by
23demons.
24
25In order to remove a line discipline call tty_unregister_ldisc().
26In ancient times this always worked. In modern times the function will
27return -EBUSY if the ldisc is currently in use. Since the ldisc referencing
28code manages the module counts this should not usually be a concern.
29
30Heed this warning: the reference count field of the registered copies of the
31tty_ldisc structure in the ldisc table counts the number of lines using this
32discipline. The reference count of the tty_ldisc structure within a tty
33counts the number of active users of the ldisc at this instant. In effect it
34counts the number of threads of execution within an ldisc method (plus those
35about to enter and exit although this detail matters not).
36
37Line Discipline Methods
38-----------------------
39
40TTY side interfaces:
41
42open() - 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
48close() - 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
53hangup() - 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
58write() - 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
62flush_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
66chars_in_buffer() - (optional) Report the number of bytes in the input
67            buffer.
68
69set_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
75read() - 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
80poll() - Check the status for the poll/select calls. Multiple
81            poll calls may occur in parallel. May sleep.
82
83ioctl() - 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
87compat_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
91Driver Side Interfaces:
92
93receive_buf() - Hand buffers of bytes from the driver to the ldisc
94            for processing. Semantics currently rather
95            mysterious 8(
96
97write_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
108dcd_change() - Report to the tty line the current DCD pin status
109            changes and the relative timestamp. The timestamp
110            can be NULL.
111
112
113Driver Access
114
115Line discipline methods can call the following methods of the underlying
116hardware driver through the function pointers within the tty->driver
117structure:
118
119write() Write a block of characters to the tty device.
120            Returns the number of characters accepted. The
121            character buffer passed to this method is already
122            in kernel space.
123
124put_char() Queues a character for writing to the tty device.
125            If there is no room in the queue, the character is
126            ignored.
127
128flush_chars() (Optional) If defined, must be called after
129            queueing characters with put_char() in order to
130            start transmission.
131
132write_room() Returns the numbers of characters the tty driver
133            will accept for queueing to be written.
134
135ioctl() Invoke device specific ioctl.
136            Expects data pointers to refer to userspace.
137            Returns ENOIOCTLCMD for unrecognized ioctl numbers.
138
139set_termios() Notify the tty driver that the device's termios
140            settings have changed. New settings are in
141            tty->termios. Previous settings should be passed in
142            the "old" argument.
143
144            The API is defined such that the driver should return
145            the actual modes selected. This means that the
146            driver function is responsible for modifying any
147            bits in the request it cannot fulfill to indicate
148            the actual modes being used. A device with no
149            hardware capability for change (eg a USB dongle or
150            virtual port) can provide NULL for this method.
151
152throttle() Notify the tty driver that input buffers for the
153            line discipline are close to full, and it should
154            somehow signal that no more characters should be
155            sent to the tty.
156
157unthrottle() Notify the tty driver that characters can now be
158            sent to the tty without fear of overrunning the
159            input buffers of the line disciplines.
160
161stop() Ask the tty driver to stop outputting characters
162            to the tty device.
163
164start() Ask the tty driver to resume sending characters
165            to the tty device.
166
167hangup() Ask the tty driver to hang up the tty device.
168
169break_ctl() (Optional) Ask the tty driver to turn on or off
170            BREAK status on the RS-232 port. If state is -1,
171            then the BREAK status should be turned on; if
172            state is 0, then BREAK should be turned off.
173            If this routine is not implemented, use ioctls
174            TIOCSBRK / TIOCCBRK instead.
175
176wait_until_sent() Waits until the device has written out all of the
177            characters in its transmitter FIFO.
178
179send_xchar() Send a high-priority XON/XOFF character to the device.
180
181
182Flags
183
184Line discipline methods have access to tty->flags field containing the
185following interesting flags:
186
187TTY_THROTTLED Driver input is throttled. The ldisc should call
188            tty->driver->unthrottle() in order to resume
189            reception when it is ready to process more data.
190
191TTY_DO_WRITE_WAKEUP If set, causes the driver to call the ldisc's
192            write_wakeup() method in order to resume
193            transmission when it can accept more data
194            to transmit.
195
196TTY_IO_ERROR If set, causes all subsequent userspace read/write
197            calls on the tty to fail, returning -EIO.
198
199TTY_OTHER_CLOSED Device is a pty and the other side has closed.
200
201TTY_NO_WRITE_SPLIT Prevent driver from splitting up writes into
202            smaller chunks.
203
204
205Locking
206
207Callers to the line discipline functions from the tty layer are required to
208take line discipline locks. The same is true of calls from the driver side
209but not yet enforced.
210
211Three calls are now provided
212
213    ldisc = tty_ldisc_ref(tty);
214
215takes a handle to the line discipline in the tty and returns it. If no ldisc
216is currently attached or the ldisc is being closed and re-opened at this
217point then NULL is returned. While this handle is held the ldisc will not
218change or go away.
219
220    tty_ldisc_deref(ldisc)
221
222Returns the ldisc reference and allows the ldisc to be closed. Returning the
223reference takes away your right to call the ldisc functions until you take
224a new reference.
225
226    ldisc = tty_ldisc_ref_wait(tty);
227
228Performs the same function as tty_ldisc_ref except that it will wait for an
229ldisc change to complete and then return a reference to the new ldisc.
230
231While these functions are slightly slower than the old code they should have
232minimal impact as most receive logic uses the flip buffers and they only
233need to take a reference when they push bits up through the driver.
234
235A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc
236functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
237fail in this situation if used within these functions. Ldisc and driver
238code calling its own functions must be careful in this case.
239
240
241Driver Interface
242----------------
243
244open() - Called when a device is opened. May sleep
245
246close() - 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
250write() - 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
255put_char() - Stuff a single character onto the queue. The
256            driver is guaranteed following up calls to
257            flush_chars.
258
259flush_chars() - Ask the kernel to write put_char queue
260
261write_room() - Return the number of characters tht 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
266ioctl() - Called when an ioctl may be for the driver
267
268set_termios() - Called on termios change, serialized against
269            itself by a semaphore. May sleep.
270
271set_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
275throttle() - 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
279unthrottle() - Called by the ldisc to ask the driver to stop flow
280            control.
281
282stop() - Ldisc notifier to the driver to stop output. As with
283            throttle the serializations with start() are down
284            to the ldisc layer.
285
286start() - Ldisc notifier to the driver to start output.
287
288hangup() - Ask the tty driver to cause a hangup initiated
289            from the host side. [Can sleep ??]
290
291break_ctl() - Send RS232 break. Can sleep. Can get called in
292            parallel, driver must serialize (for now), and
293            with write calls.
294
295wait_until_sent() - Wait for characters to exit the hardware queue
296            of the driver. Can sleep
297
298send_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
302

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