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1
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
25In order to remove a line discipline call tty_unregister_ldisc().
26In 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
83receive_room() - Can be called by the driver layer at any time when
84 the ldisc is opened. The ldisc must be able to
85 handle the reported amount of data at that instant.
86 Synchronization between active receive_buf and
87 receive_room calls is down to the driver not the
88 ldisc. Must not sleep.
89
90write_wakeup() - May be called at any point between open and close.
91 The TTY_DO_WRITE_WAKEUP flag indicates if a call
92 is needed but always races versus calls. Thus the
93 ldisc must be careful about setting order and to
94 handle unexpected calls. Must not sleep.
95
96 The driver is forbidden from calling this directly
97 from the ->write call from the ldisc as the ldisc
98 is permitted to call the driver write method from
99 this function. In such a situation defer it.
100
101
102Locking
103
104Callers to the line discipline functions from the tty layer are required to
105take line discipline locks. The same is true of calls from the driver side
106but not yet enforced.
107
108Three calls are now provided
109
110 ldisc = tty_ldisc_ref(tty);
111
112takes a handle to the line discipline in the tty and returns it. If no ldisc
113is currently attached or the ldisc is being closed and re-opened at this
114point then NULL is returned. While this handle is held the ldisc will not
115change or go away.
116
117 tty_ldisc_deref(ldisc)
118
119Returns the ldisc reference and allows the ldisc to be closed. Returning the
120reference takes away your right to call the ldisc functions until you take
121a new reference.
122
123 ldisc = tty_ldisc_ref_wait(tty);
124
125Performs the same function as tty_ldisc_ref except that it will wait for an
126ldisc change to complete and then return a reference to the new ldisc.
127
128While these functions are slightly slower than the old code they should have
129minimal impact as most receive logic uses the flip buffers and they only
130need to take a reference when they push bits up through the driver.
131
132A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc
133functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
134fail in this situation if used within these functions. Ldisc and driver
135code calling its own functions must be careful in this case.
136
137
138Driver Interface
139----------------
140
141open() - Called when a device is opened. May sleep
142
143close() - Called when a device is closed. At the point of
144 return from this call the driver must make no
145 further ldisc calls of any kind. May sleep
146
147write() - Called to write bytes to the device. May not
148 sleep. May occur in parallel in special cases.
149 Because this includes panic paths drivers generally
150 shouldn't try and do clever locking here.
151
152put_char() - Stuff a single character onto the queue. The
153 driver is guaranteed following up calls to
154 flush_chars.
155
156flush_chars() - Ask the kernel to write put_char queue
157
158write_room() - Return the number of characters tht can be stuffed
159 into the port buffers without overflow (or less).
160 The ldisc is responsible for being intelligent
161 about multi-threading of write_room/write calls
162
163ioctl() - Called when an ioctl may be for the driver
164
165set_termios() - Called on termios change, serialized against
166 itself by a semaphore. May sleep.
167
168set_ldisc() - Notifier for discipline change. At the point this
169 is done the discipline is not yet usable. Can now
170 sleep (I think)
171
172throttle() - Called by the ldisc to ask the driver to do flow
173 control. Serialization including with unthrottle
174 is the job of the ldisc layer.
175
176unthrottle() - Called by the ldisc to ask the driver to stop flow
177 control.
178
179stop() - Ldisc notifier to the driver to stop output. As with
180 throttle the serializations with start() are down
181 to the ldisc layer.
182
183start() - Ldisc notifier to the driver to start output.
184
185hangup() - Ask the tty driver to cause a hangup initiated
186 from the host side. [Can sleep ??]
187
188break_ctl() - Send RS232 break. Can sleep. Can get called in
189 parallel, driver must serialize (for now), and
190 with write calls.
191
192wait_until_sent() - Wait for characters to exit the hardware queue
193 of the driver. Can sleep
194
195send_xchar() - Send XON/XOFF and if possible jump the queue with
196 it in order to get fast flow control responses.
197 Cannot sleep ??
198