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Linus Torvalds1da177e2005-04-16 15:20:36 -07001
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
Alexey Dobriyanbfb07592005-06-23 00:10:32 -070025In order to remove a line discipline call tty_unregister_ldisc().
Linus Torvalds1da177e2005-04-16 15:20:36 -070026In 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
42close() - This is called on a terminal when the line
43 discipline is being unplugged. At the point of
44 execution no further users will enter the
45 ldisc code for this tty. Can sleep.
46
47open() - Called when the line discipline is attached to
48 the terminal. No other call into the line
49 discipline for this tty will occur until it
50 completes successfully. Can sleep.
51
52write() - A process is writing data through the line
53 discipline. Multiple write calls are serialized
54 by the tty layer for the ldisc. May sleep.
55
56flush_buffer() - May be called at any point between open and close.
57
58chars_in_buffer() - Report the number of bytes in the buffer.
59
60set_termios() - Called on termios structure changes. The caller
61 passes the old termios data and the current data
62 is in the tty. Called under the termios semaphore so
63 allowed to sleep. Serialized against itself only.
64
65read() - Move data from the line discipline to the user.
66 Multiple read calls may occur in parallel and the
67 ldisc must deal with serialization issues. May
68 sleep.
69
70poll() - Check the status for the poll/select calls. Multiple
71 poll calls may occur in parallel. May sleep.
72
73ioctl() - Called when an ioctl is handed to the tty layer
74 that might be for the ldisc. Multiple ioctl calls
75 may occur in parallel. May sleep.
76
77Driver Side Interfaces:
78
79receive_buf() - Hand buffers of bytes from the driver to the ldisc
80 for processing. Semantics currently rather
81 mysterious 8(
82
Linus Torvalds1da177e2005-04-16 15:20:36 -070083write_wakeup() - May be called at any point between open and close.
84 The TTY_DO_WRITE_WAKEUP flag indicates if a call
85 is needed but always races versus calls. Thus the
86 ldisc must be careful about setting order and to
87 handle unexpected calls. Must not sleep.
88
89 The driver is forbidden from calling this directly
90 from the ->write call from the ldisc as the ldisc
91 is permitted to call the driver write method from
92 this function. In such a situation defer it.
93
94
95Locking
96
97Callers to the line discipline functions from the tty layer are required to
98take line discipline locks. The same is true of calls from the driver side
99but not yet enforced.
100
101Three calls are now provided
102
103 ldisc = tty_ldisc_ref(tty);
104
105takes a handle to the line discipline in the tty and returns it. If no ldisc
106is currently attached or the ldisc is being closed and re-opened at this
107point then NULL is returned. While this handle is held the ldisc will not
108change or go away.
109
110 tty_ldisc_deref(ldisc)
111
112Returns the ldisc reference and allows the ldisc to be closed. Returning the
113reference takes away your right to call the ldisc functions until you take
114a new reference.
115
116 ldisc = tty_ldisc_ref_wait(tty);
117
118Performs the same function as tty_ldisc_ref except that it will wait for an
119ldisc change to complete and then return a reference to the new ldisc.
120
121While these functions are slightly slower than the old code they should have
122minimal impact as most receive logic uses the flip buffers and they only
123need to take a reference when they push bits up through the driver.
124
125A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc
126functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
127fail in this situation if used within these functions. Ldisc and driver
128code calling its own functions must be careful in this case.
129
130
131Driver Interface
132----------------
133
134open() - Called when a device is opened. May sleep
135
136close() - Called when a device is closed. At the point of
137 return from this call the driver must make no
138 further ldisc calls of any kind. May sleep
139
140write() - Called to write bytes to the device. May not
141 sleep. May occur in parallel in special cases.
142 Because this includes panic paths drivers generally
143 shouldn't try and do clever locking here.
144
145put_char() - Stuff a single character onto the queue. The
146 driver is guaranteed following up calls to
147 flush_chars.
148
149flush_chars() - Ask the kernel to write put_char queue
150
151write_room() - Return the number of characters tht can be stuffed
152 into the port buffers without overflow (or less).
153 The ldisc is responsible for being intelligent
154 about multi-threading of write_room/write calls
155
156ioctl() - Called when an ioctl may be for the driver
157
158set_termios() - Called on termios change, serialized against
159 itself by a semaphore. May sleep.
160
161set_ldisc() - Notifier for discipline change. At the point this
162 is done the discipline is not yet usable. Can now
163 sleep (I think)
164
165throttle() - Called by the ldisc to ask the driver to do flow
166 control. Serialization including with unthrottle
167 is the job of the ldisc layer.
168
169unthrottle() - Called by the ldisc to ask the driver to stop flow
170 control.
171
172stop() - Ldisc notifier to the driver to stop output. As with
173 throttle the serializations with start() are down
174 to the ldisc layer.
175
176start() - Ldisc notifier to the driver to start output.
177
178hangup() - Ask the tty driver to cause a hangup initiated
179 from the host side. [Can sleep ??]
180
181break_ctl() - Send RS232 break. Can sleep. Can get called in
182 parallel, driver must serialize (for now), and
183 with write calls.
184
185wait_until_sent() - Wait for characters to exit the hardware queue
186 of the driver. Can sleep
187
188send_xchar() - Send XON/XOFF and if possible jump the queue with
189 it in order to get fast flow control responses.
190 Cannot sleep ??
191