ia64/linux-2.6.18-xen.hg

view Documentation/tty.txt @ 912:dd42cdb0ab89

[IA64] Build blktap2 driver by default in x86 builds.

add CONFIG_XEN_BLKDEV_TAP2=y to buildconfigs/linux-defconfig_xen_ia64.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
author Isaku Yamahata <yamahata@valinux.co.jp>
date Mon Jun 29 12:09:16 2009 +0900 (2009-06-29)
parents 831230e53067
children
line source
2 The Lockronomicon
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.
11 Line Discipline
12 ---------------
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.
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.
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 -----------------------
40 TTY side interfaces:
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 through the line
53 discipline. Multiple write calls are serialized
54 by the tty layer for the ldisc. May sleep.
56 flush_buffer() - May be called at any point between open and close.
58 chars_in_buffer() - Report the number of bytes in the buffer.
60 set_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.
65 read() - 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.
70 poll() - Check the status for the poll/select calls. Multiple
71 poll calls may occur in parallel. May sleep.
73 ioctl() - 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.
77 Driver Side Interfaces:
79 receive_buf() - Hand buffers of bytes from the driver to the ldisc
80 for processing. Semantics currently rather
81 mysterious 8(
83 write_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.
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.
95 Locking
97 Callers to the line discipline functions from the tty layer are required to
98 take line discipline locks. The same is true of calls from the driver side
99 but not yet enforced.
101 Three calls are now provided
103 ldisc = tty_ldisc_ref(tty);
105 takes a handle to the line discipline in the tty and returns it. If no ldisc
106 is currently attached or the ldisc is being closed and re-opened at this
107 point then NULL is returned. While this handle is held the ldisc will not
108 change or go away.
110 tty_ldisc_deref(ldisc)
112 Returns the ldisc reference and allows the ldisc to be closed. Returning the
113 reference takes away your right to call the ldisc functions until you take
114 a new reference.
116 ldisc = tty_ldisc_ref_wait(tty);
118 Performs the same function as tty_ldisc_ref except that it will wait for an
119 ldisc change to complete and then return a reference to the new ldisc.
121 While these functions are slightly slower than the old code they should have
122 minimal impact as most receive logic uses the flip buffers and they only
123 need to take a reference when they push bits up through the driver.
125 A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc
126 functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
127 fail in this situation if used within these functions. Ldisc and driver
128 code calling its own functions must be careful in this case.
131 Driver Interface
132 ----------------
134 open() - Called when a device is opened. May sleep
136 close() - 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
140 write() - 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.
145 put_char() - Stuff a single character onto the queue. The
146 driver is guaranteed following up calls to
147 flush_chars.
149 flush_chars() - Ask the kernel to write put_char queue
151 write_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
156 ioctl() - Called when an ioctl may be for the driver
158 set_termios() - Called on termios change, serialized against
159 itself by a semaphore. May sleep.
161 set_ldisc() - Notifier for discipline change. At the point this
162 is done the discipline is not yet usable. Can now
163 sleep (I think)
165 throttle() - 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.
169 unthrottle() - Called by the ldisc to ask the driver to stop flow
170 control.
172 stop() - Ldisc notifier to the driver to stop output. As with
173 throttle the serializations with start() are down
174 to the ldisc layer.
176 start() - Ldisc notifier to the driver to start output.
178 hangup() - Ask the tty driver to cause a hangup initiated
179 from the host side. [Can sleep ??]
181 break_ctl() - Send RS232 break. Can sleep. Can get called in
182 parallel, driver must serialize (for now), and
183 with write calls.
185 wait_until_sent() - Wait for characters to exit the hardware queue
186 of the driver. Can sleep
188 send_xchar() - Send XON/XOFF and if possible jump the queue with
189 it in order to get fast flow control responses.
190 Cannot sleep ??