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Based on kernel version 4.1. Page generated on 2015-06-28 12:14 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. Returning an error will
46				prevent the ldisc from being attached. Can sleep.
47	
48	close()		-	This is called on a terminal when the line
49				discipline is being unplugged. At the point of
50				execution no further users will enter the
51				ldisc code for this tty. Can sleep.
52	
53	hangup()	-	Called when the tty line is hung up.
54				The line discipline should cease I/O to the tty.
55				No further calls into the ldisc code will occur.
56				The return value is ignored. Can sleep.
57	
58	write()		-	A process is writing data through the line
59				discipline.  Multiple write calls are serialized
60				by the tty layer for the ldisc.  May sleep. 
61	
62	flush_buffer()	-	(optional) May be called at any point between
63				open and close, and instructs the line discipline
64				to empty its input buffer.
65	
66	chars_in_buffer() -	(optional) Report the number of bytes in the input
67				buffer.
68	
69	set_termios()	-	(optional) Called on termios structure changes.
70				The caller passes the old termios data and the
71				current data is in the tty. Called under the
72				termios semaphore so allowed to sleep. Serialized
73				against itself only.
74	
75	read()		-	Move data from the line discipline to the user.
76				Multiple read calls may occur in parallel and the
77				ldisc must deal with serialization issues. May 
78				sleep.
79	
80	poll()		-	Check the status for the poll/select calls. Multiple
81				poll calls may occur in parallel. May sleep.
82	
83	ioctl()		-	Called when an ioctl is handed to the tty layer
84				that might be for the ldisc. Multiple ioctl calls
85				may occur in parallel. May sleep. 
86	
87	compat_ioctl()	-	Called when a 32 bit ioctl is handed to the tty layer
88				that might be for the ldisc. Multiple ioctl calls
89				may occur in parallel. May sleep.
90	
91	Driver Side Interfaces:
92	
93	receive_buf()	-	Hand buffers of bytes from the driver to the ldisc
94				for processing. Semantics currently rather
95				mysterious 8(
96	
97	write_wakeup()	-	May be called at any point between open and close.
98				The TTY_DO_WRITE_WAKEUP flag indicates if a call
99				is needed but always races versus calls. Thus the
100				ldisc must be careful about setting order and to
101				handle unexpected calls. Must not sleep.
102	
103				The driver is forbidden from calling this directly
104				from the ->write call from the ldisc as the ldisc
105				is permitted to call the driver write method from
106				this function. In such a situation defer it.
107	
108	dcd_change()	-	Report to the tty line the current DCD pin status
109				changes and the relative timestamp. The timestamp
110				cannot be NULL.
111	
112	
113	Driver Access
114	
115	Line discipline methods can call the following methods of the underlying
116	hardware driver through the function pointers within the tty->driver
117	structure:
118	
119	write()			Write a block of characters to the tty device.
120				Returns the number of characters accepted. The
121				character buffer passed to this method is already
122				in kernel space.
123	
124	put_char()		Queues a character for writing to the tty device.
125				If there is no room in the queue, the character is
126				ignored.
127	
128	flush_chars()		(Optional) If defined, must be called after
129				queueing characters with put_char() in order to
130				start transmission.
131	
132	write_room()		Returns the numbers of characters the tty driver
133				will accept for queueing to be written.
134	
135	ioctl()			Invoke device specific ioctl.
136				Expects data pointers to refer to userspace.
137				Returns ENOIOCTLCMD for unrecognized ioctl numbers.
138	
139	set_termios()		Notify the tty driver that the device's termios
140				settings have changed. New settings are in
141				tty->termios. Previous settings should be passed in
142				the "old" argument.
143	
144				The API is defined such that the driver should return
145				the actual modes selected. This means that the
146				driver function is responsible for modifying any
147				bits in the request it cannot fulfill to indicate
148				the actual modes being used. A device with no
149				hardware capability for change (e.g. a USB dongle or
150				virtual port) can provide NULL for this method.
151	
152	throttle()		Notify the tty driver that input buffers for the
153				line discipline are close to full, and it should
154				somehow signal that no more characters should be
155				sent to the tty.
156	
157	unthrottle()		Notify the tty driver that characters can now be
158				sent to the tty without fear of overrunning the
159				input buffers of the line disciplines.
160	
161	stop()			Ask the tty driver to stop outputting characters
162				to the tty device.
163	
164	start()			Ask the tty driver to resume sending characters
165				to the tty device.
166	
167	hangup()		Ask the tty driver to hang up the tty device.
168	
169	break_ctl()		(Optional) Ask the tty driver to turn on or off
170				BREAK status on the RS-232 port.  If state is -1,
171				then the BREAK status should be turned on; if
172				state is 0, then BREAK should be turned off.
173				If this routine is not implemented, use ioctls
174				TIOCSBRK / TIOCCBRK instead.
175	
176	wait_until_sent()	Waits until the device has written out all of the
177				characters in its transmitter FIFO.
178	
179	send_xchar()		Send a high-priority XON/XOFF character to the device.
180	
181	
182	Flags
183	
184	Line discipline methods have access to tty->flags field containing the
185	following interesting flags:
186	
187	TTY_THROTTLED		Driver input is throttled. The ldisc should call
188				tty->driver->unthrottle() in order to resume
189				reception when it is ready to process more data.
190	
191	TTY_DO_WRITE_WAKEUP	If set, causes the driver to call the ldisc's
192				write_wakeup() method in order to resume
193				transmission when it can accept more data
194				to transmit.
195	
196	TTY_IO_ERROR		If set, causes all subsequent userspace read/write
197				calls on the tty to fail, returning -EIO.
198	
199	TTY_OTHER_CLOSED	Device is a pty and the other side has closed.
200	
201	TTY_OTHER_DONE		Device is a pty and the other side has closed and
202				all pending input processing has been completed.
203	
204	TTY_NO_WRITE_SPLIT	Prevent driver from splitting up writes into
205				smaller chunks.
206	
207	
208	Locking
209	
210	Callers to the line discipline functions from the tty layer are required to
211	take line discipline locks. The same is true of calls from the driver side
212	but not yet enforced.
213	
214	Three calls are now provided
215	
216		ldisc = tty_ldisc_ref(tty);
217	
218	takes a handle to the line discipline in the tty and returns it. If no ldisc
219	is currently attached or the ldisc is being closed and re-opened at this
220	point then NULL is returned. While this handle is held the ldisc will not
221	change or go away.
222	
223		tty_ldisc_deref(ldisc)
224	
225	Returns the ldisc reference and allows the ldisc to be closed. Returning the
226	reference takes away your right to call the ldisc functions until you take
227	a new reference.
228	
229		ldisc = tty_ldisc_ref_wait(tty);
230	
231	Performs the same function as tty_ldisc_ref except that it will wait for an
232	ldisc change to complete and then return a reference to the new ldisc. 
233	
234	While these functions are slightly slower than the old code they should have
235	minimal impact as most receive logic uses the flip buffers and they only
236	need to take a reference when they push bits up through the driver.
237	
238	A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc 
239	functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
240	fail in this situation if used within these functions. Ldisc and driver
241	code calling its own functions must be careful in this case. 
242	
243	
244	Driver Interface
245	----------------
246	
247	open()		-	Called when a device is opened. May sleep
248	
249	close()		-	Called when a device is closed. At the point of
250				return from this call the driver must make no 
251				further ldisc calls of any kind. May sleep
252	
253	write()		-	Called to write bytes to the device. May not
254				sleep. May occur in parallel in special cases. 
255				Because this includes panic paths drivers generally
256				shouldn't try and do clever locking here.
257	
258	put_char()	-	Stuff a single character onto the queue. The
259				driver is guaranteed following up calls to
260				flush_chars.
261	
262	flush_chars()	-	Ask the kernel to write put_char queue
263	
264	write_room()	-	Return the number of characters that can be stuffed
265				into the port buffers without overflow (or less).
266				The ldisc is responsible for being intelligent
267	 			about multi-threading of write_room/write calls
268	
269	ioctl()		-	Called when an ioctl may be for the driver
270	
271	set_termios()	-	Called on termios change, serialized against
272				itself by a semaphore. May sleep.
273	
274	set_ldisc()	-	Notifier for discipline change. At the point this 
275				is done the discipline is not yet usable. Can now
276				sleep (I think)
277	
278	throttle()	-	Called by the ldisc to ask the driver to do flow
279				control.  Serialization including with unthrottle
280				is the job of the ldisc layer.
281	
282	unthrottle()	-	Called by the ldisc to ask the driver to stop flow
283				control.
284	
285	stop()		-	Ldisc notifier to the driver to stop output. As with
286				throttle the serializations with start() are down
287				to the ldisc layer.
288	
289	start()		-	Ldisc notifier to the driver to start output.
290	
291	hangup()	-	Ask the tty driver to cause a hangup initiated
292				from the host side. [Can sleep ??]
293	
294	break_ctl()	-	Send RS232 break. Can sleep. Can get called in
295				parallel, driver must serialize (for now), and
296				with write calls.
297	
298	wait_until_sent() -	Wait for characters to exit the hardware queue
299				of the driver. Can sleep
300	
301	send_xchar()	  -	Send XON/XOFF and if possible jump the queue with
302				it in order to get fast flow control responses.
303				Cannot sleep ??
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