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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 | |
25 | In order to remove a line discipline call tty_unregister_ldisc(). |
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 | |
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. Returning an error will |
46 | prevent the ldisc from being attached. Can sleep. |
47 | |
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 | |
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. |
56 | The return value is ignored. Can sleep. |
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 | |
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. |
65 | |
66 | chars_in_buffer() - (optional) Report the number of bytes in the input |
67 | buffer. |
68 | |
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. |
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 | |
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 | |
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 | |
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 | dcd_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 | |
113 | Driver Access |
114 | |
115 | Line discipline methods can call the following methods of the underlying |
116 | hardware driver through the function pointers within the tty->driver |
117 | structure: |
118 | |
119 | write() 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 | |
124 | put_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 | |
128 | flush_chars() (Optional) If defined, must be called after |
129 | queueing characters with put_char() in order to |
130 | start transmission. |
131 | |
132 | write_room() Returns the numbers of characters the tty driver |
133 | will accept for queueing to be written. |
134 | |
135 | ioctl() Invoke device specific ioctl. |
136 | Expects data pointers to refer to userspace. |
137 | Returns ENOIOCTLCMD for unrecognized ioctl numbers. |
138 | |
139 | set_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 | |
152 | throttle() 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 | |
157 | unthrottle() 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 | |
161 | stop() Ask the tty driver to stop outputting characters |
162 | to the tty device. |
163 | |
164 | start() Ask the tty driver to resume sending characters |
165 | to the tty device. |
166 | |
167 | hangup() Ask the tty driver to hang up the tty device. |
168 | |
169 | break_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 | |
176 | wait_until_sent() Waits until the device has written out all of the |
177 | characters in its transmitter FIFO. |
178 | |
179 | send_xchar() Send a high-priority XON/XOFF character to the device. |
180 | |
181 | |
182 | Flags |
183 | |
184 | Line discipline methods have access to tty->flags field containing the |
185 | following interesting flags: |
186 | |
187 | TTY_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 | |
191 | TTY_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 | |
196 | TTY_IO_ERROR If set, causes all subsequent userspace read/write |
197 | calls on the tty to fail, returning -EIO. |
198 | |
199 | TTY_OTHER_CLOSED Device is a pty and the other side has closed. |
200 | |
201 | TTY_NO_WRITE_SPLIT Prevent driver from splitting up writes into |
202 | smaller chunks. |
203 | |
204 | |
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 | |
261 | write_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 | |
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 | |
302 |
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