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 | ||
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 | |
45 | completes successfully. Can sleep. | |
46 | ||
1da177e4 LT |
47 | close() - This is called on a terminal when the line |
48 | discipline is being unplugged. At the point of | |
49 | execution no further users will enter the | |
50 | ldisc code for this tty. Can sleep. | |
51 | ||
1f59c140 TS |
52 | hangup() - Called when the tty line is hung up. |
53 | The line discipline should cease I/O to the tty. | |
54 | No further calls into the ldisc code will occur. | |
55 | Can sleep. | |
1da177e4 LT |
56 | |
57 | write() - A process is writing data through the line | |
58 | discipline. Multiple write calls are serialized | |
59 | by the tty layer for the ldisc. May sleep. | |
60 | ||
1f59c140 TS |
61 | flush_buffer() - (optional) May be called at any point between |
62 | open and close, and instructs the line discipline | |
63 | to empty its input buffer. | |
1da177e4 | 64 | |
1f59c140 TS |
65 | chars_in_buffer() - (optional) Report the number of bytes in the input |
66 | buffer. | |
1da177e4 | 67 | |
1f59c140 TS |
68 | set_termios() - (optional) Called on termios structure changes. |
69 | The caller passes the old termios data and the | |
70 | current data is in the tty. Called under the | |
71 | termios semaphore so allowed to sleep. Serialized | |
72 | against itself only. | |
1da177e4 LT |
73 | |
74 | read() - Move data from the line discipline to the user. | |
75 | Multiple read calls may occur in parallel and the | |
76 | ldisc must deal with serialization issues. May | |
77 | sleep. | |
78 | ||
79 | poll() - Check the status for the poll/select calls. Multiple | |
80 | poll calls may occur in parallel. May sleep. | |
81 | ||
82 | ioctl() - Called when an ioctl is handed to the tty layer | |
83 | that might be for the ldisc. Multiple ioctl calls | |
84 | may occur in parallel. May sleep. | |
85 | ||
86 | Driver Side Interfaces: | |
87 | ||
88 | receive_buf() - Hand buffers of bytes from the driver to the ldisc | |
89 | for processing. Semantics currently rather | |
90 | mysterious 8( | |
91 | ||
1da177e4 LT |
92 | write_wakeup() - May be called at any point between open and close. |
93 | The TTY_DO_WRITE_WAKEUP flag indicates if a call | |
94 | is needed but always races versus calls. Thus the | |
95 | ldisc must be careful about setting order and to | |
96 | handle unexpected calls. Must not sleep. | |
97 | ||
98 | The driver is forbidden from calling this directly | |
99 | from the ->write call from the ldisc as the ldisc | |
100 | is permitted to call the driver write method from | |
101 | this function. In such a situation defer it. | |
102 | ||
103 | ||
1f59c140 TS |
104 | Driver Access |
105 | ||
106 | Line discipline methods can call the following methods of the underlying | |
107 | hardware driver through the function pointers within the tty->driver | |
108 | structure: | |
109 | ||
110 | write() Write a block of characters to the tty device. | |
111 | Returns the number of characters accepted. | |
112 | ||
113 | put_char() Queues a character for writing to the tty device. | |
114 | If there is no room in the queue, the character is | |
115 | ignored. | |
116 | ||
117 | flush_chars() (Optional) If defined, must be called after | |
118 | queueing characters with put_char() in order to | |
119 | start transmission. | |
120 | ||
121 | write_room() Returns the numbers of characters the tty driver | |
122 | will accept for queueing to be written. | |
123 | ||
124 | ioctl() Invoke device specific ioctl. | |
125 | Expects data pointers to refer to userspace. | |
126 | Returns ENOIOCTLCMD for unrecognized ioctl numbers. | |
127 | ||
128 | set_termios() Notify the tty driver that the device's termios | |
129 | settings have changed. New settings are in | |
130 | tty->termios. Previous settings should be passed in | |
131 | the "old" argument. | |
132 | ||
133 | throttle() Notify the tty driver that input buffers for the | |
134 | line discipline are close to full, and it should | |
135 | somehow signal that no more characters should be | |
136 | sent to the tty. | |
137 | ||
138 | unthrottle() Notify the tty driver that characters can now be | |
139 | sent to the tty without fear of overrunning the | |
140 | input buffers of the line disciplines. | |
141 | ||
142 | stop() Ask the tty driver to stop outputting characters | |
143 | to the tty device. | |
144 | ||
145 | start() Ask the tty driver to resume sending characters | |
146 | to the tty device. | |
147 | ||
148 | hangup() Ask the tty driver to hang up the tty device. | |
149 | ||
150 | break_ctl() (Optional) Ask the tty driver to turn on or off | |
151 | BREAK status on the RS-232 port. If state is -1, | |
152 | then the BREAK status should be turned on; if | |
153 | state is 0, then BREAK should be turned off. | |
154 | If this routine is not implemented, use ioctls | |
155 | TIOCSBRK / TIOCCBRK instead. | |
156 | ||
157 | wait_until_sent() Waits until the device has written out all of the | |
158 | characters in its transmitter FIFO. | |
159 | ||
160 | send_xchar() Send a high-priority XON/XOFF character to the device. | |
161 | ||
162 | ||
163 | Flags | |
164 | ||
165 | Line discipline methods have access to tty->flags field containing the | |
166 | following interesting flags: | |
167 | ||
168 | TTY_THROTTLED Driver input is throttled. The ldisc should call | |
169 | tty->driver->unthrottle() in order to resume | |
170 | reception when it is ready to process more data. | |
171 | ||
172 | TTY_DO_WRITE_WAKEUP If set, causes the driver to call the ldisc's | |
173 | write_wakeup() method in order to resume | |
174 | transmission when it can accept more data | |
175 | to transmit. | |
176 | ||
177 | TTY_IO_ERROR If set, causes all subsequent userspace read/write | |
178 | calls on the tty to fail, returning -EIO. | |
179 | ||
180 | TTY_OTHER_CLOSED Device is a pty and the other side has closed. | |
181 | ||
182 | TTY_NO_WRITE_SPLIT Prevent driver from splitting up writes into | |
183 | smaller chunks. | |
184 | ||
185 | ||
1da177e4 LT |
186 | Locking |
187 | ||
188 | Callers to the line discipline functions from the tty layer are required to | |
189 | take line discipline locks. The same is true of calls from the driver side | |
190 | but not yet enforced. | |
191 | ||
192 | Three calls are now provided | |
193 | ||
194 | ldisc = tty_ldisc_ref(tty); | |
195 | ||
196 | takes a handle to the line discipline in the tty and returns it. If no ldisc | |
197 | is currently attached or the ldisc is being closed and re-opened at this | |
198 | point then NULL is returned. While this handle is held the ldisc will not | |
199 | change or go away. | |
200 | ||
201 | tty_ldisc_deref(ldisc) | |
202 | ||
203 | Returns the ldisc reference and allows the ldisc to be closed. Returning the | |
204 | reference takes away your right to call the ldisc functions until you take | |
205 | a new reference. | |
206 | ||
207 | ldisc = tty_ldisc_ref_wait(tty); | |
208 | ||
209 | Performs the same function as tty_ldisc_ref except that it will wait for an | |
210 | ldisc change to complete and then return a reference to the new ldisc. | |
211 | ||
212 | While these functions are slightly slower than the old code they should have | |
213 | minimal impact as most receive logic uses the flip buffers and they only | |
214 | need to take a reference when they push bits up through the driver. | |
215 | ||
216 | A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc | |
217 | functions are called with the ldisc unavailable. Thus tty_ldisc_ref will | |
218 | fail in this situation if used within these functions. Ldisc and driver | |
219 | code calling its own functions must be careful in this case. | |
220 | ||
221 | ||
222 | Driver Interface | |
223 | ---------------- | |
224 | ||
225 | open() - Called when a device is opened. May sleep | |
226 | ||
227 | close() - Called when a device is closed. At the point of | |
228 | return from this call the driver must make no | |
229 | further ldisc calls of any kind. May sleep | |
230 | ||
231 | write() - Called to write bytes to the device. May not | |
232 | sleep. May occur in parallel in special cases. | |
233 | Because this includes panic paths drivers generally | |
234 | shouldn't try and do clever locking here. | |
235 | ||
236 | put_char() - Stuff a single character onto the queue. The | |
237 | driver is guaranteed following up calls to | |
238 | flush_chars. | |
239 | ||
240 | flush_chars() - Ask the kernel to write put_char queue | |
241 | ||
242 | write_room() - Return the number of characters tht can be stuffed | |
243 | into the port buffers without overflow (or less). | |
244 | The ldisc is responsible for being intelligent | |
245 | about multi-threading of write_room/write calls | |
246 | ||
247 | ioctl() - Called when an ioctl may be for the driver | |
248 | ||
249 | set_termios() - Called on termios change, serialized against | |
250 | itself by a semaphore. May sleep. | |
251 | ||
252 | set_ldisc() - Notifier for discipline change. At the point this | |
253 | is done the discipline is not yet usable. Can now | |
254 | sleep (I think) | |
255 | ||
256 | throttle() - Called by the ldisc to ask the driver to do flow | |
257 | control. Serialization including with unthrottle | |
258 | is the job of the ldisc layer. | |
259 | ||
260 | unthrottle() - Called by the ldisc to ask the driver to stop flow | |
261 | control. | |
262 | ||
263 | stop() - Ldisc notifier to the driver to stop output. As with | |
264 | throttle the serializations with start() are down | |
265 | to the ldisc layer. | |
266 | ||
267 | start() - Ldisc notifier to the driver to start output. | |
268 | ||
269 | hangup() - Ask the tty driver to cause a hangup initiated | |
270 | from the host side. [Can sleep ??] | |
271 | ||
272 | break_ctl() - Send RS232 break. Can sleep. Can get called in | |
273 | parallel, driver must serialize (for now), and | |
274 | with write calls. | |
275 | ||
276 | wait_until_sent() - Wait for characters to exit the hardware queue | |
277 | of the driver. Can sleep | |
278 | ||
279 | send_xchar() - Send XON/XOFF and if possible jump the queue with | |
280 | it in order to get fast flow control responses. | |
281 | Cannot sleep ?? | |
282 |