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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:47 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	
8	Line Discipline
9	---------------
10	
11	Line disciplines are registered with tty_register_ldisc() passing the
12	discipline number and the ldisc structure. At the point of registration the 
13	discipline must be ready to use and it is possible it will get used before
14	the call returns success. If the call returns an error then it won't get
15	called. Do not re-use ldisc numbers as they are part of the userspace ABI
16	and writing over an existing ldisc will cause demons to eat your computer.
17	After the return the ldisc data has been copied so you may free your own 
18	copy of the structure. You must not re-register over the top of the line
19	discipline even with the same data or your computer again will be eaten by
20	demons.
21	
22	In order to remove a line discipline call tty_unregister_ldisc().
23	In ancient times this always worked. In modern times the function will
24	return -EBUSY if the ldisc is currently in use. Since the ldisc referencing
25	code manages the module counts this should not usually be a concern.
26	
27	Heed this warning: the reference count field of the registered copies of the
28	tty_ldisc structure in the ldisc table counts the number of lines using this
29	discipline. The reference count of the tty_ldisc structure within a tty 
30	counts the number of active users of the ldisc at this instant. In effect it
31	counts the number of threads of execution within an ldisc method (plus those
32	about to enter and exit although this detail matters not).
33	
34	Line Discipline Methods
35	-----------------------
36	
37	TTY side interfaces:
38	
39	open()		-	Called when the line discipline is attached to
40				the terminal. No other call into the line
41				discipline for this tty will occur until it
42				completes successfully. Should initialize any
43				state needed by the ldisc, and set receive_room
44				in the tty_struct to the maximum amount of data
45				the line discipline is willing to accept from the
46				driver with a single call to receive_buf().
47				Returning an error will prevent the ldisc from
48				being attached. Can sleep.
49	
50	close()		-	This is called on a terminal when the line
51				discipline is being unplugged. At the point of
52				execution no further users will enter the
53				ldisc code for this tty. Can sleep.
54	
55	hangup()	-	Called when the tty line is hung up.
56				The line discipline should cease I/O to the tty.
57				No further calls into the ldisc code will occur.
58				The return value is ignored. Can sleep.
59	
60	read()		-	(optional) A process requests reading data from
61				the line. Multiple read calls may occur in parallel
62				and the ldisc must deal with serialization issues.
63				If not defined, the process will receive an EIO
64				error. May sleep.
65	
66	write()		-	(optional) A process requests writing data to the
67				line. Multiple write calls are serialized by the
68				tty layer for the ldisc. If not defined, the
69				process will receive an EIO error. May sleep.
70	
71	flush_buffer()	-	(optional) May be called at any point between
72				open and close, and instructs the line discipline
73				to empty its input buffer.
74	
75	set_termios()	-	(optional) Called on termios structure changes.
76				The caller passes the old termios data and the
77				current data is in the tty. Called under the
78				termios semaphore so allowed to sleep. Serialized
79				against itself only.
80	
81	poll()		-	(optional) Check the status for the poll/select
82				calls. Multiple poll calls may occur in parallel.
83				May sleep.
84	
85	ioctl()		-	(optional) Called when an ioctl is handed to the
86				tty layer that might be for the ldisc. Multiple
87				ioctl calls may occur in parallel. May sleep.
88	
89	compat_ioctl()	-	(optional) Called when a 32 bit ioctl is handed
90				to the tty layer that might be for the ldisc.
91				Multiple ioctl calls may occur in parallel.
92				May sleep.
93	
94	Driver Side Interfaces:
95	
96	receive_buf()	-	(optional) Called by the low-level driver to hand
97				a buffer of received bytes to the ldisc for
98				processing. The number of bytes is guaranteed not
99				to exceed the current value of tty->receive_room.
100				All bytes must be processed.
101	
102	receive_buf2()	-	(optional) Called by the low-level driver to hand
103				a buffer of received bytes to the ldisc for
104				processing. Returns the number of bytes processed.
105	
106				If both receive_buf() and receive_buf2() are
107				defined, receive_buf2() should be preferred.
108	
109	write_wakeup()	-	May be called at any point between open and close.
110				The TTY_DO_WRITE_WAKEUP flag indicates if a call
111				is needed but always races versus calls. Thus the
112				ldisc must be careful about setting order and to
113				handle unexpected calls. Must not sleep.
114	
115				The driver is forbidden from calling this directly
116				from the ->write call from the ldisc as the ldisc
117				is permitted to call the driver write method from
118				this function. In such a situation defer it.
119	
120	dcd_change()	-	Report to the tty line the current DCD pin status
121				changes and the relative timestamp. The timestamp
122				cannot be NULL.
123	
124	
125	Driver Access
126	
127	Line discipline methods can call the following methods of the underlying
128	hardware driver through the function pointers within the tty->driver
129	structure:
130	
131	write()			Write a block of characters to the tty device.
132				Returns the number of characters accepted. The
133				character buffer passed to this method is already
134				in kernel space.
135	
136	put_char()		Queues a character for writing to the tty device.
137				If there is no room in the queue, the character is
138				ignored.
139	
140	flush_chars()		(Optional) If defined, must be called after
141				queueing characters with put_char() in order to
142				start transmission.
143	
144	write_room()		Returns the numbers of characters the tty driver
145				will accept for queueing to be written.
146	
147	ioctl()			Invoke device specific ioctl.
148				Expects data pointers to refer to userspace.
149				Returns ENOIOCTLCMD for unrecognized ioctl numbers.
150	
151	set_termios()		Notify the tty driver that the device's termios
152				settings have changed. New settings are in
153				tty->termios. Previous settings should be passed in
154				the "old" argument.
155	
156				The API is defined such that the driver should return
157				the actual modes selected. This means that the
158				driver function is responsible for modifying any
159				bits in the request it cannot fulfill to indicate
160				the actual modes being used. A device with no
161				hardware capability for change (e.g. a USB dongle or
162				virtual port) can provide NULL for this method.
163	
164	throttle()		Notify the tty driver that input buffers for the
165				line discipline are close to full, and it should
166				somehow signal that no more characters should be
167				sent to the tty.
168	
169	unthrottle()		Notify the tty driver that characters can now be
170				sent to the tty without fear of overrunning the
171				input buffers of the line disciplines.
172	
173	stop()			Ask the tty driver to stop outputting characters
174				to the tty device.
175	
176	start()			Ask the tty driver to resume sending characters
177				to the tty device.
178	
179	hangup()		Ask the tty driver to hang up the tty device.
180	
181	break_ctl()		(Optional) Ask the tty driver to turn on or off
182				BREAK status on the RS-232 port.  If state is -1,
183				then the BREAK status should be turned on; if
184				state is 0, then BREAK should be turned off.
185				If this routine is not implemented, use ioctls
186				TIOCSBRK / TIOCCBRK instead.
187	
188	wait_until_sent()	Waits until the device has written out all of the
189				characters in its transmitter FIFO.
190	
191	send_xchar()		Send a high-priority XON/XOFF character to the device.
192	
193	
194	Flags
195	
196	Line discipline methods have access to tty->flags field containing the
197	following interesting flags:
198	
199	TTY_THROTTLED		Driver input is throttled. The ldisc should call
200				tty->driver->unthrottle() in order to resume
201				reception when it is ready to process more data.
202	
203	TTY_DO_WRITE_WAKEUP	If set, causes the driver to call the ldisc's
204				write_wakeup() method in order to resume
205				transmission when it can accept more data
206				to transmit.
207	
208	TTY_IO_ERROR		If set, causes all subsequent userspace read/write
209				calls on the tty to fail, returning -EIO.
210	
211	TTY_OTHER_CLOSED	Device is a pty and the other side has closed.
212	
213	TTY_NO_WRITE_SPLIT	Prevent driver from splitting up writes into
214				smaller chunks.
215	
216	
217	Locking
218	
219	Callers to the line discipline functions from the tty layer are required to
220	take line discipline locks. The same is true of calls from the driver side
221	but not yet enforced.
222	
223	Three calls are now provided
224	
225		ldisc = tty_ldisc_ref(tty);
226	
227	takes a handle to the line discipline in the tty and returns it. If no ldisc
228	is currently attached or the ldisc is being closed and re-opened at this
229	point then NULL is returned. While this handle is held the ldisc will not
230	change or go away.
231	
232		tty_ldisc_deref(ldisc)
233	
234	Returns the ldisc reference and allows the ldisc to be closed. Returning the
235	reference takes away your right to call the ldisc functions until you take
236	a new reference.
237	
238		ldisc = tty_ldisc_ref_wait(tty);
239	
240	Performs the same function as tty_ldisc_ref except that it will wait for an
241	ldisc change to complete and then return a reference to the new ldisc. 
242	
243	While these functions are slightly slower than the old code they should have
244	minimal impact as most receive logic uses the flip buffers and they only
245	need to take a reference when they push bits up through the driver.
246	
247	A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc 
248	functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
249	fail in this situation if used within these functions. Ldisc and driver
250	code calling its own functions must be careful in this case. 
251	
252	
253	Driver Interface
254	----------------
255	
256	open()		-	Called when a device is opened. May sleep
257	
258	close()		-	Called when a device is closed. At the point of
259				return from this call the driver must make no 
260				further ldisc calls of any kind. May sleep
261	
262	write()		-	Called to write bytes to the device. May not
263				sleep. May occur in parallel in special cases. 
264				Because this includes panic paths drivers generally
265				shouldn't try and do clever locking here.
266	
267	put_char()	-	Stuff a single character onto the queue. The
268				driver is guaranteed following up calls to
269				flush_chars.
270	
271	flush_chars()	-	Ask the kernel to write put_char queue
272	
273	write_room()	-	Return the number of characters that can be stuffed
274				into the port buffers without overflow (or less).
275				The ldisc is responsible for being intelligent
276	 			about multi-threading of write_room/write calls
277	
278	ioctl()		-	Called when an ioctl may be for the driver
279	
280	set_termios()	-	Called on termios change, serialized against
281				itself by a semaphore. May sleep.
282	
283	set_ldisc()	-	Notifier for discipline change. At the point this 
284				is done the discipline is not yet usable. Can now
285				sleep (I think)
286	
287	throttle()	-	Called by the ldisc to ask the driver to do flow
288				control.  Serialization including with unthrottle
289				is the job of the ldisc layer.
290	
291	unthrottle()	-	Called by the ldisc to ask the driver to stop flow
292				control.
293	
294	stop()		-	Ldisc notifier to the driver to stop output. As with
295				throttle the serializations with start() are down
296				to the ldisc layer.
297	
298	start()		-	Ldisc notifier to the driver to start output.
299	
300	hangup()	-	Ask the tty driver to cause a hangup initiated
301				from the host side. [Can sleep ??]
302	
303	break_ctl()	-	Send RS232 break. Can sleep. Can get called in
304				parallel, driver must serialize (for now), and
305				with write calls.
306	
307	wait_until_sent() -	Wait for characters to exit the hardware queue
308				of the driver. Can sleep
309	
310	send_xchar()	  -	Send XON/XOFF and if possible jump the queue with
311				it in order to get fast flow control responses.
312				Cannot sleep ??
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