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Based on kernel version 2.6.27. Page generated on 2008-10-13 09:54 EST.

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. Can sleep.
46	
47	close()		-	This is called on a terminal when the line
48				discipline is being unplugged. At the point of
49				execution no further users will enter the
50				ldisc code for this tty. Can sleep.
51	
52	hangup()	-	Called when the tty line is hung up.
53				The line discipline should cease I/O to the tty.
54				No further calls into the ldisc code will occur.
55				Can sleep.
56	
57	write()		-	A process is writing data through the line
58				discipline.  Multiple write calls are serialized
59				by the tty layer for the ldisc.  May sleep. 
60	
61	flush_buffer()	-	(optional) May be called at any point between
62				open and close, and instructs the line discipline
63				to empty its input buffer.
64	
65	chars_in_buffer() -	(optional) Report the number of bytes in the input
66				buffer.
67	
68	set_termios()	-	(optional) Called on termios structure changes.
69				The caller passes the old termios data and the
70				current data is in the tty. Called under the
71				termios semaphore so allowed to sleep. Serialized
72				against itself only.
73	
74	read()		-	Move data from the line discipline to the user.
75				Multiple read calls may occur in parallel and the
76				ldisc must deal with serialization issues. May 
77				sleep.
78	
79	poll()		-	Check the status for the poll/select calls. Multiple
80				poll calls may occur in parallel. May sleep.
81	
82	ioctl()		-	Called when an ioctl is handed to the tty layer
83				that might be for the ldisc. Multiple ioctl calls
84				may occur in parallel. May sleep. 
85	
86	Driver Side Interfaces:
87	
88	receive_buf()	-	Hand buffers of bytes from the driver to the ldisc
89				for processing. Semantics currently rather
90				mysterious 8(
91	
92	write_wakeup()	-	May be called at any point between open and close.
93				The TTY_DO_WRITE_WAKEUP flag indicates if a call
94				is needed but always races versus calls. Thus the
95				ldisc must be careful about setting order and to
96				handle unexpected calls. Must not sleep.
97	
98				The driver is forbidden from calling this directly
99				from the ->write call from the ldisc as the ldisc
100				is permitted to call the driver write method from
101				this function. In such a situation defer it.
102	
103	
104	Driver Access
105	
106	Line discipline methods can call the following methods of the underlying
107	hardware driver through the function pointers within the tty->driver
108	structure:
109	
110	write()			Write a block of characters to the tty device.
111				Returns the number of characters accepted. The
112				character buffer passed to this method is already
113				in kernel space.
114	
115	put_char()		Queues a character for writing to the tty device.
116				If there is no room in the queue, the character is
117				ignored.
118	
119	flush_chars()		(Optional) If defined, must be called after
120				queueing characters with put_char() in order to
121				start transmission.
122	
123	write_room()		Returns the numbers of characters the tty driver
124				will accept for queueing to be written.
125	
126	ioctl()			Invoke device specific ioctl.
127				Expects data pointers to refer to userspace.
128				Returns ENOIOCTLCMD for unrecognized ioctl numbers.
129	
130	set_termios()		Notify the tty driver that the device's termios
131				settings have changed. New settings are in
132				tty->termios. Previous settings should be passed in
133				the "old" argument.
134	
135				The API is defined such that the driver should return
136				the actual modes selected. This means that the
137				driver function is responsible for modifying any
138				bits in the request it cannot fulfill to indicate
139				the actual modes being used. A device with no
140				hardware capability for change (eg a USB dongle or
141				virtual port) can provide NULL for this method.
142	
143	throttle()		Notify the tty driver that input buffers for the
144				line discipline are close to full, and it should
145				somehow signal that no more characters should be
146				sent to the tty.
147	
148	unthrottle()		Notify the tty driver that characters can now be
149				sent to the tty without fear of overrunning the
150				input buffers of the line disciplines.
151	
152	stop()			Ask the tty driver to stop outputting characters
153				to the tty device.
154	
155	start()			Ask the tty driver to resume sending characters
156				to the tty device.
157	
158	hangup()		Ask the tty driver to hang up the tty device.
159	
160	break_ctl()		(Optional) Ask the tty driver to turn on or off
161				BREAK status on the RS-232 port.  If state is -1,
162				then the BREAK status should be turned on; if
163				state is 0, then BREAK should be turned off.
164				If this routine is not implemented, use ioctls
165				TIOCSBRK / TIOCCBRK instead.
166	
167	wait_until_sent()	Waits until the device has written out all of the
168				characters in its transmitter FIFO.
169	
170	send_xchar()		Send a high-priority XON/XOFF character to the device.
171	
172	
173	Flags
174	
175	Line discipline methods have access to tty->flags field containing the
176	following interesting flags:
177	
178	TTY_THROTTLED		Driver input is throttled. The ldisc should call
179				tty->driver->unthrottle() in order to resume
180				reception when it is ready to process more data.
181	
182	TTY_DO_WRITE_WAKEUP	If set, causes the driver to call the ldisc's
183				write_wakeup() method in order to resume
184				transmission when it can accept more data
185				to transmit.
186	
187	TTY_IO_ERROR		If set, causes all subsequent userspace read/write
188				calls on the tty to fail, returning -EIO.
189	
190	TTY_OTHER_CLOSED	Device is a pty and the other side has closed.
191	
192	TTY_NO_WRITE_SPLIT	Prevent driver from splitting up writes into
193				smaller chunks.
194	
195	
196	Locking
197	
198	Callers to the line discipline functions from the tty layer are required to
199	take line discipline locks. The same is true of calls from the driver side
200	but not yet enforced.
201	
202	Three calls are now provided
203	
204		ldisc = tty_ldisc_ref(tty);
205	
206	takes a handle to the line discipline in the tty and returns it. If no ldisc
207	is currently attached or the ldisc is being closed and re-opened at this
208	point then NULL is returned. While this handle is held the ldisc will not
209	change or go away.
210	
211		tty_ldisc_deref(ldisc)
212	
213	Returns the ldisc reference and allows the ldisc to be closed. Returning the
214	reference takes away your right to call the ldisc functions until you take
215	a new reference.
216	
217		ldisc = tty_ldisc_ref_wait(tty);
218	
219	Performs the same function as tty_ldisc_ref except that it will wait for an
220	ldisc change to complete and then return a reference to the new ldisc. 
221	
222	While these functions are slightly slower than the old code they should have
223	minimal impact as most receive logic uses the flip buffers and they only
224	need to take a reference when they push bits up through the driver.
225	
226	A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc 
227	functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
228	fail in this situation if used within these functions. Ldisc and driver
229	code calling its own functions must be careful in this case. 
230	
231	
232	Driver Interface
233	----------------
234	
235	open()		-	Called when a device is opened. May sleep
236	
237	close()		-	Called when a device is closed. At the point of
238				return from this call the driver must make no 
239				further ldisc calls of any kind. May sleep
240	
241	write()		-	Called to write bytes to the device. May not
242				sleep. May occur in parallel in special cases. 
243				Because this includes panic paths drivers generally
244				shouldn't try and do clever locking here.
245	
246	put_char()	-	Stuff a single character onto the queue. The
247				driver is guaranteed following up calls to
248				flush_chars.
249	
250	flush_chars()	-	Ask the kernel to write put_char queue
251	
252	write_room()	-	Return the number of characters tht can be stuffed
253				into the port buffers without overflow (or less).
254				The ldisc is responsible for being intelligent
255	 			about multi-threading of write_room/write calls
256	
257	ioctl()		-	Called when an ioctl may be for the driver
258	
259	set_termios()	-	Called on termios change, serialized against
260				itself by a semaphore. May sleep.
261	
262	set_ldisc()	-	Notifier for discipline change. At the point this 
263				is done the discipline is not yet usable. Can now
264				sleep (I think)
265	
266	throttle()	-	Called by the ldisc to ask the driver to do flow
267				control.  Serialization including with unthrottle
268				is the job of the ldisc layer.
269	
270	unthrottle()	-	Called by the ldisc to ask the driver to stop flow
271				control.
272	
273	stop()		-	Ldisc notifier to the driver to stop output. As with
274				throttle the serializations with start() are down
275				to the ldisc layer.
276	
277	start()		-	Ldisc notifier to the driver to start output.
278	
279	hangup()	-	Ask the tty driver to cause a hangup initiated
280				from the host side. [Can sleep ??]
281	
282	break_ctl()	-	Send RS232 break. Can sleep. Can get called in
283				parallel, driver must serialize (for now), and
284				with write calls.
285	
286	wait_until_sent() -	Wait for characters to exit the hardware queue
287				of the driver. Can sleep
288	
289	send_xchar()	  -	Send XON/XOFF and if possible jump the queue with
290				it in order to get fast flow control responses.
291				Cannot sleep ??
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