Commit | Line | Data |
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1da177e4 LT |
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 | ||
1da177e4 LT |
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 | ||
bfb07599 | 22 | In order to remove a line discipline call tty_unregister_ldisc(). |
1da177e4 LT |
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 | ||
1f59c140 TS |
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 | |
7e11a0fb TS |
42 | completes successfully. Returning an error will |
43 | prevent the ldisc from being attached. Can sleep. | |
1f59c140 | 44 | |
1da177e4 LT |
45 | close() - This is called on a terminal when the line |
46 | discipline is being unplugged. At the point of | |
47 | execution no further users will enter the | |
48 | ldisc code for this tty. Can sleep. | |
49 | ||
1f59c140 TS |
50 | hangup() - Called when the tty line is hung up. |
51 | The line discipline should cease I/O to the tty. | |
52 | No further calls into the ldisc code will occur. | |
7e11a0fb | 53 | The return value is ignored. Can sleep. |
1da177e4 LT |
54 | |
55 | write() - A process is writing data through the line | |
56 | discipline. Multiple write calls are serialized | |
57 | by the tty layer for the ldisc. May sleep. | |
58 | ||
1f59c140 TS |
59 | flush_buffer() - (optional) May be called at any point between |
60 | open and close, and instructs the line discipline | |
61 | to empty its input buffer. | |
1da177e4 | 62 | |
1f59c140 TS |
63 | chars_in_buffer() - (optional) Report the number of bytes in the input |
64 | buffer. | |
1da177e4 | 65 | |
1f59c140 TS |
66 | set_termios() - (optional) Called on termios structure changes. |
67 | The caller passes the old termios data and the | |
68 | current data is in the tty. Called under the | |
69 | termios semaphore so allowed to sleep. Serialized | |
70 | against itself only. | |
1da177e4 LT |
71 | |
72 | read() - Move data from the line discipline to the user. | |
73 | Multiple read calls may occur in parallel and the | |
74 | ldisc must deal with serialization issues. May | |
75 | sleep. | |
76 | ||
77 | poll() - Check the status for the poll/select calls. Multiple | |
78 | poll calls may occur in parallel. May sleep. | |
79 | ||
80 | ioctl() - Called when an ioctl is handed to the tty layer | |
81 | that might be for the ldisc. Multiple ioctl calls | |
82 | may occur in parallel. May sleep. | |
83 | ||
7e11a0fb TS |
84 | compat_ioctl() - Called when a 32 bit ioctl is handed to the tty layer |
85 | that might be for the ldisc. Multiple ioctl calls | |
86 | may occur in parallel. May sleep. | |
87 | ||
1da177e4 LT |
88 | Driver Side Interfaces: |
89 | ||
90 | receive_buf() - Hand buffers of bytes from the driver to the ldisc | |
91 | for processing. Semantics currently rather | |
92 | mysterious 8( | |
93 | ||
1da177e4 LT |
94 | write_wakeup() - May be called at any point between open and close. |
95 | The TTY_DO_WRITE_WAKEUP flag indicates if a call | |
96 | is needed but always races versus calls. Thus the | |
97 | ldisc must be careful about setting order and to | |
98 | handle unexpected calls. Must not sleep. | |
99 | ||
100 | The driver is forbidden from calling this directly | |
101 | from the ->write call from the ldisc as the ldisc | |
102 | is permitted to call the driver write method from | |
103 | this function. In such a situation defer it. | |
104 | ||
b3e63afe RG |
105 | dcd_change() - Report to the tty line the current DCD pin status |
106 | changes and the relative timestamp. The timestamp | |
12f9b1f9 | 107 | cannot be NULL. |
b3e63afe | 108 | |
1da177e4 | 109 | |
1f59c140 TS |
110 | Driver Access |
111 | ||
112 | Line discipline methods can call the following methods of the underlying | |
113 | hardware driver through the function pointers within the tty->driver | |
114 | structure: | |
115 | ||
116 | write() Write a block of characters to the tty device. | |
6309ed7c A |
117 | Returns the number of characters accepted. The |
118 | character buffer passed to this method is already | |
119 | in kernel space. | |
1f59c140 TS |
120 | |
121 | put_char() Queues a character for writing to the tty device. | |
122 | If there is no room in the queue, the character is | |
123 | ignored. | |
124 | ||
125 | flush_chars() (Optional) If defined, must be called after | |
126 | queueing characters with put_char() in order to | |
127 | start transmission. | |
128 | ||
129 | write_room() Returns the numbers of characters the tty driver | |
130 | will accept for queueing to be written. | |
131 | ||
132 | ioctl() Invoke device specific ioctl. | |
133 | Expects data pointers to refer to userspace. | |
134 | Returns ENOIOCTLCMD for unrecognized ioctl numbers. | |
135 | ||
136 | set_termios() Notify the tty driver that the device's termios | |
137 | settings have changed. New settings are in | |
138 | tty->termios. Previous settings should be passed in | |
139 | the "old" argument. | |
140 | ||
3ac40b9b AC |
141 | The API is defined such that the driver should return |
142 | the actual modes selected. This means that the | |
143 | driver function is responsible for modifying any | |
144 | bits in the request it cannot fulfill to indicate | |
145 | the actual modes being used. A device with no | |
83ee73c1 | 146 | hardware capability for change (e.g. a USB dongle or |
3ac40b9b AC |
147 | virtual port) can provide NULL for this method. |
148 | ||
1f59c140 TS |
149 | throttle() Notify the tty driver that input buffers for the |
150 | line discipline are close to full, and it should | |
151 | somehow signal that no more characters should be | |
152 | sent to the tty. | |
153 | ||
154 | unthrottle() Notify the tty driver that characters can now be | |
155 | sent to the tty without fear of overrunning the | |
156 | input buffers of the line disciplines. | |
157 | ||
158 | stop() Ask the tty driver to stop outputting characters | |
159 | to the tty device. | |
160 | ||
161 | start() Ask the tty driver to resume sending characters | |
162 | to the tty device. | |
163 | ||
164 | hangup() Ask the tty driver to hang up the tty device. | |
165 | ||
166 | break_ctl() (Optional) Ask the tty driver to turn on or off | |
167 | BREAK status on the RS-232 port. If state is -1, | |
168 | then the BREAK status should be turned on; if | |
169 | state is 0, then BREAK should be turned off. | |
170 | If this routine is not implemented, use ioctls | |
171 | TIOCSBRK / TIOCCBRK instead. | |
172 | ||
173 | wait_until_sent() Waits until the device has written out all of the | |
174 | characters in its transmitter FIFO. | |
175 | ||
176 | send_xchar() Send a high-priority XON/XOFF character to the device. | |
177 | ||
178 | ||
179 | Flags | |
180 | ||
181 | Line discipline methods have access to tty->flags field containing the | |
182 | following interesting flags: | |
183 | ||
184 | TTY_THROTTLED Driver input is throttled. The ldisc should call | |
185 | tty->driver->unthrottle() in order to resume | |
186 | reception when it is ready to process more data. | |
187 | ||
188 | TTY_DO_WRITE_WAKEUP If set, causes the driver to call the ldisc's | |
189 | write_wakeup() method in order to resume | |
190 | transmission when it can accept more data | |
191 | to transmit. | |
192 | ||
193 | TTY_IO_ERROR If set, causes all subsequent userspace read/write | |
194 | calls on the tty to fail, returning -EIO. | |
195 | ||
196 | TTY_OTHER_CLOSED Device is a pty and the other side has closed. | |
197 | ||
1a48632f PH |
198 | TTY_OTHER_DONE Device is a pty and the other side has closed and |
199 | all pending input processing has been completed. | |
200 | ||
1f59c140 TS |
201 | TTY_NO_WRITE_SPLIT Prevent driver from splitting up writes into |
202 | smaller chunks. | |
203 | ||
204 | ||
1da177e4 LT |
205 | Locking |
206 | ||
207 | Callers to the line discipline functions from the tty layer are required to | |
208 | take line discipline locks. The same is true of calls from the driver side | |
209 | but not yet enforced. | |
210 | ||
211 | Three calls are now provided | |
212 | ||
213 | ldisc = tty_ldisc_ref(tty); | |
214 | ||
215 | takes a handle to the line discipline in the tty and returns it. If no ldisc | |
216 | is currently attached or the ldisc is being closed and re-opened at this | |
217 | point then NULL is returned. While this handle is held the ldisc will not | |
218 | change or go away. | |
219 | ||
220 | tty_ldisc_deref(ldisc) | |
221 | ||
222 | Returns the ldisc reference and allows the ldisc to be closed. Returning the | |
223 | reference takes away your right to call the ldisc functions until you take | |
224 | a new reference. | |
225 | ||
226 | ldisc = tty_ldisc_ref_wait(tty); | |
227 | ||
228 | Performs the same function as tty_ldisc_ref except that it will wait for an | |
229 | ldisc change to complete and then return a reference to the new ldisc. | |
230 | ||
231 | While these functions are slightly slower than the old code they should have | |
232 | minimal impact as most receive logic uses the flip buffers and they only | |
233 | need to take a reference when they push bits up through the driver. | |
234 | ||
235 | A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc | |
236 | functions are called with the ldisc unavailable. Thus tty_ldisc_ref will | |
237 | fail in this situation if used within these functions. Ldisc and driver | |
238 | code calling its own functions must be careful in this case. | |
239 | ||
240 | ||
241 | Driver Interface | |
242 | ---------------- | |
243 | ||
244 | open() - Called when a device is opened. May sleep | |
245 | ||
246 | close() - 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 | ||
250 | write() - 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 | ||
255 | put_char() - Stuff a single character onto the queue. The | |
256 | driver is guaranteed following up calls to | |
257 | flush_chars. | |
258 | ||
259 | flush_chars() - Ask the kernel to write put_char queue | |
260 | ||
83ee73c1 | 261 | write_room() - Return the number of characters that can be stuffed |
1da177e4 LT |
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 | ||
266 | ioctl() - Called when an ioctl may be for the driver | |
267 | ||
268 | set_termios() - Called on termios change, serialized against | |
269 | itself by a semaphore. May sleep. | |
270 | ||
271 | set_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 | ||
275 | throttle() - 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 | ||
279 | unthrottle() - Called by the ldisc to ask the driver to stop flow | |
280 | control. | |
281 | ||
282 | stop() - Ldisc notifier to the driver to stop output. As with | |
283 | throttle the serializations with start() are down | |
284 | to the ldisc layer. | |
285 | ||
286 | start() - Ldisc notifier to the driver to start output. | |
287 | ||
288 | hangup() - Ask the tty driver to cause a hangup initiated | |
289 | from the host side. [Can sleep ??] | |
290 | ||
291 | break_ctl() - Send RS232 break. Can sleep. Can get called in | |
292 | parallel, driver must serialize (for now), and | |
293 | with write calls. | |
294 | ||
295 | wait_until_sent() - Wait for characters to exit the hardware queue | |
296 | of the driver. Can sleep | |
297 | ||
298 | send_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 |