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.
7 FIXME: still need to work out the full set of BKL assumptions and document
8 them so they can eventually be killed off.
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
25 In order to remove a line discipline call tty_register_ldisc passing NULL.
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.
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).
37 Line Discipline Methods
38 -----------------------
42 close() - 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.
47 open() - 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.
52 write() - A process is writing data from user space
53 through the line discipline. Multiple write calls
54 are serialized by the tty layer for the ldisc. May
57 flush_buffer() - May be called at any point between open and close.
59 chars_in_buffer() - Report the number of bytes in the buffer.
61 set_termios() - Called on termios structure changes. The caller
62 passes the old termios data and the current data
63 is in the tty. Called under the termios semaphore so
64 allowed to sleep. Serialized against itself only.
66 read() - Move data from the line discipline to the user.
67 Multiple read calls may occur in parallel and the
68 ldisc must deal with serialization issues. May
71 poll() - Check the status for the poll/select calls. Multiple
72 poll calls may occur in parallel. May sleep.
74 ioctl() - Called when an ioctl is handed to the tty layer
75 that might be for the ldisc. Multiple ioctl calls
76 may occur in parallel. May sleep.
78 Driver Side Interfaces:
80 receive_buf() - Hand buffers of bytes from the driver to the ldisc
81 for processing. Semantics currently rather
84 receive_room() - Can be called by the driver layer at any time when
85 the ldisc is opened. The ldisc must be able to
86 handle the reported amount of data at that instant.
87 Synchronization between active receive_buf and
88 receive_room calls is down to the driver not the
89 ldisc. Must not sleep.
91 write_wakeup() - May be called at any point between open and close.
92 The TTY_DO_WRITE_WAKEUP flag indicates if a call
93 is needed but always races versus calls. Thus the
94 ldisc must be careful about setting order and to
95 handle unexpected calls. Must not sleep.
100 Callers to the line discipline functions from the tty layer are required to
101 take line discipline locks. The same is true of calls from the driver side
102 but not yet enforced.
104 Three calls are now provided
106 ldisc = tty_ldisc_ref(tty);
108 takes a handle to the line discipline in the tty and returns it. If no ldisc
109 is currently attached or the ldisc is being closed and re-opened at this
110 point then NULL is returned. While this handle is held the ldisc will not
113 tty_ldisc_deref(ldisc)
115 Returns the ldisc reference and allows the ldisc to be closed. Returning the
116 reference takes away your right to call the ldisc functions until you take
119 ldisc = tty_ldisc_ref_wait(tty);
121 Performs the same function as tty_ldisc_ref except that it will wait for an
122 ldisc change to complete and then return a reference to the new ldisc.
124 While these functions are slightly slower than the old code they should have
125 minimal impact as most receive logic uses the flip buffers and they only
126 need to take a reference when they push bits up through the driver.
128 A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc
129 functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
130 fail in this situation if used within these functions. Ldisc and driver
131 code calling its own functions must be careful in this case.
137 open() - Called when a device is opened. May sleep
139 close() - Called when a device is closed. At the point of
140 return from this call the driver must make no
141 further ldisc calls of any kind. May sleep
143 write() - Called to write bytes to the device. May not
144 sleep. May occur in parallel in special cases.
145 Because this includes panic paths drivers generally
146 shouldn't try and do clever locking here.
148 put_char() - Stuff a single character onto the queue. The
149 driver is guaranteed following up calls to
152 flush_chars() - Ask the kernel to write put_char queue
154 write_room() - Return the number of characters tht can be stuffed
155 into the port buffers without overflow (or less).
156 The ldisc is responsible for being intelligent
157 about multi-threading of write_room/write calls
159 ioctl() - Called when an ioctl may be for the driver
161 set_termios() - Called on termios change, serialized against
162 itself by a semaphore. May sleep.
164 set_ldisc() - Notifier for discipline change. At the point this
165 is done the discipline is not yet usable. Can now
168 throttle() - Called by the ldisc to ask the driver to do flow
169 control. Serialization including with unthrottle
170 is the job of the ldisc layer.
172 unthrottle() - Called by the ldisc to ask the driver to stop flow
175 stop() - Ldisc notifier to the driver to stop output. As with
176 throttle the serializations with start() are down
179 start() - Ldisc notifier to the driver to start output.
181 hangup() - Ask the tty driver to cause a hangup initiated
182 from the host side. [Can sleep ??]
184 break_ctl() - Send RS232 break. Can sleep. Can get called in
185 parallel, driver must serialize (for now), and
188 wait_until_sent() - Wait for characters to exit the hardware queue
189 of the driver. Can sleep
191 send_xchar() - Send XON/XOFF and if possible jump the queue with
192 it in order to get fast flow control responses.