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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 | ||
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 | ||
bfb07599 | 25 | In order to remove a line discipline call tty_unregister_ldisc(). |
1da177e4 LT |
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 | ||
1f59c140 TS |
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 | |
7e11a0fb TS |
45 | completes successfully. Returning an error will |
46 | prevent the ldisc from being attached. Can sleep. | |
1f59c140 | 47 | |
1da177e4 LT |
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 | ||
1f59c140 TS |
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. | |
7e11a0fb | 56 | The return value is ignored. Can sleep. |
1da177e4 LT |
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 | ||
1f59c140 TS |
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. | |
1da177e4 | 65 | |
1f59c140 TS |
66 | chars_in_buffer() - (optional) Report the number of bytes in the input |
67 | buffer. | |
1da177e4 | 68 | |
1f59c140 TS |
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. | |
1da177e4 LT |
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 | ||
7e11a0fb TS |
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 | ||
1da177e4 LT |
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 | ||
1da177e4 LT |
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 | ||
1f59c140 TS |
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. | |
6309ed7c AC |
116 | Returns the number of characters accepted. The |
117 | character buffer passed to this method is already | |
118 | in kernel space. | |
1f59c140 TS |
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 | ||
3ac40b9b AC |
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 | ||
1f59c140 TS |
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 | ||
1da177e4 LT |
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 |