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Based on kernel version 4.3. Page generated on 2015-11-02 12:51 EST.

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.
8	Line Discipline
9	---------------
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.
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.
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).
34	Line Discipline Methods
35	-----------------------
37	TTY side interfaces:
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. Returning an error will
43				prevent the ldisc from being attached. Can sleep.
45	close()		-	This is called on a terminal when the line
46				discipline is being unplugged. At the point of
47				execution no further users will enter the
48				ldisc code for this tty. Can sleep.
50	hangup()	-	Called when the tty line is hung up.
51				The line discipline should cease I/O to the tty.
52				No further calls into the ldisc code will occur.
53				The return value is ignored. Can sleep.
55	write()		-	A process is writing data through the line
56				discipline.  Multiple write calls are serialized
57				by the tty layer for the ldisc.  May sleep. 
59	flush_buffer()	-	(optional) May be called at any point between
60				open and close, and instructs the line discipline
61				to empty its input buffer.
63	chars_in_buffer() -	(optional) Report the number of bytes in the input
64				buffer.
66	set_termios()	-	(optional) Called on termios structure changes.
67				The caller passes the old termios data and the
68				current data is in the tty. Called under the
69				termios semaphore so allowed to sleep. Serialized
70				against itself only.
72	read()		-	Move data from the line discipline to the user.
73				Multiple read calls may occur in parallel and the
74				ldisc must deal with serialization issues. May 
75				sleep.
77	poll()		-	Check the status for the poll/select calls. Multiple
78				poll calls may occur in parallel. May sleep.
80	ioctl()		-	Called when an ioctl is handed to the tty layer
81				that might be for the ldisc. Multiple ioctl calls
82				may occur in parallel. May sleep. 
84	compat_ioctl()	-	Called when a 32 bit ioctl is handed to the tty layer
85				that might be for the ldisc. Multiple ioctl calls
86				may occur in parallel. May sleep.
88	Driver Side Interfaces:
90	receive_buf()	-	Hand buffers of bytes from the driver to the ldisc
91				for processing. Semantics currently rather
92				mysterious 8(
94	write_wakeup()	-	May be called at any point between open and close.
95				The TTY_DO_WRITE_WAKEUP flag indicates if a call
96				is needed but always races versus calls. Thus the
97				ldisc must be careful about setting order and to
98				handle unexpected calls. Must not sleep.
100				The driver is forbidden from calling this directly
101				from the ->write call from the ldisc as the ldisc
102				is permitted to call the driver write method from
103				this function. In such a situation defer it.
105	dcd_change()	-	Report to the tty line the current DCD pin status
106				changes and the relative timestamp. The timestamp
107				cannot be NULL.
110	Driver Access
112	Line discipline methods can call the following methods of the underlying
113	hardware driver through the function pointers within the tty->driver
114	structure:
116	write()			Write a block of characters to the tty device.
117				Returns the number of characters accepted. The
118				character buffer passed to this method is already
119				in kernel space.
121	put_char()		Queues a character for writing to the tty device.
122				If there is no room in the queue, the character is
123				ignored.
125	flush_chars()		(Optional) If defined, must be called after
126				queueing characters with put_char() in order to
127				start transmission.
129	write_room()		Returns the numbers of characters the tty driver
130				will accept for queueing to be written.
132	ioctl()			Invoke device specific ioctl.
133				Expects data pointers to refer to userspace.
134				Returns ENOIOCTLCMD for unrecognized ioctl numbers.
136	set_termios()		Notify the tty driver that the device's termios
137				settings have changed. New settings are in
138				tty->termios. Previous settings should be passed in
139				the "old" argument.
141				The API is defined such that the driver should return
142				the actual modes selected. This means that the
143				driver function is responsible for modifying any
144				bits in the request it cannot fulfill to indicate
145				the actual modes being used. A device with no
146				hardware capability for change (e.g. a USB dongle or
147				virtual port) can provide NULL for this method.
149	throttle()		Notify the tty driver that input buffers for the
150				line discipline are close to full, and it should
151				somehow signal that no more characters should be
152				sent to the tty.
154	unthrottle()		Notify the tty driver that characters can now be
155				sent to the tty without fear of overrunning the
156				input buffers of the line disciplines.
158	stop()			Ask the tty driver to stop outputting characters
159				to the tty device.
161	start()			Ask the tty driver to resume sending characters
162				to the tty device.
164	hangup()		Ask the tty driver to hang up the tty device.
166	break_ctl()		(Optional) Ask the tty driver to turn on or off
167				BREAK status on the RS-232 port.  If state is -1,
168				then the BREAK status should be turned on; if
169				state is 0, then BREAK should be turned off.
170				If this routine is not implemented, use ioctls
171				TIOCSBRK / TIOCCBRK instead.
173	wait_until_sent()	Waits until the device has written out all of the
174				characters in its transmitter FIFO.
176	send_xchar()		Send a high-priority XON/XOFF character to the device.
179	Flags
181	Line discipline methods have access to tty->flags field containing the
182	following interesting flags:
184	TTY_THROTTLED		Driver input is throttled. The ldisc should call
185				tty->driver->unthrottle() in order to resume
186				reception when it is ready to process more data.
188	TTY_DO_WRITE_WAKEUP	If set, causes the driver to call the ldisc's
189				write_wakeup() method in order to resume
190				transmission when it can accept more data
191				to transmit.
193	TTY_IO_ERROR		If set, causes all subsequent userspace read/write
194				calls on the tty to fail, returning -EIO.
196	TTY_OTHER_CLOSED	Device is a pty and the other side has closed.
198	TTY_OTHER_DONE		Device is a pty and the other side has closed and
199				all pending input processing has been completed.
201	TTY_NO_WRITE_SPLIT	Prevent driver from splitting up writes into
202				smaller chunks.
205	Locking
207	Callers to the line discipline functions from the tty layer are required to
208	take line discipline locks. The same is true of calls from the driver side
209	but not yet enforced.
211	Three calls are now provided
213		ldisc = tty_ldisc_ref(tty);
215	takes a handle to the line discipline in the tty and returns it. If no ldisc
216	is currently attached or the ldisc is being closed and re-opened at this
217	point then NULL is returned. While this handle is held the ldisc will not
218	change or go away.
220		tty_ldisc_deref(ldisc)
222	Returns the ldisc reference and allows the ldisc to be closed. Returning the
223	reference takes away your right to call the ldisc functions until you take
224	a new reference.
226		ldisc = tty_ldisc_ref_wait(tty);
228	Performs the same function as tty_ldisc_ref except that it will wait for an
229	ldisc change to complete and then return a reference to the new ldisc. 
231	While these functions are slightly slower than the old code they should have
232	minimal impact as most receive logic uses the flip buffers and they only
233	need to take a reference when they push bits up through the driver.
235	A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc 
236	functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
237	fail in this situation if used within these functions. Ldisc and driver
238	code calling its own functions must be careful in this case. 
241	Driver Interface
242	----------------
244	open()		-	Called when a device is opened. May sleep
246	close()		-	Called when a device is closed. At the point of
247				return from this call the driver must make no 
248				further ldisc calls of any kind. May sleep
250	write()		-	Called to write bytes to the device. May not
251				sleep. May occur in parallel in special cases. 
252				Because this includes panic paths drivers generally
253				shouldn't try and do clever locking here.
255	put_char()	-	Stuff a single character onto the queue. The
256				driver is guaranteed following up calls to
257				flush_chars.
259	flush_chars()	-	Ask the kernel to write put_char queue
261	write_room()	-	Return the number of characters that can be stuffed
262				into the port buffers without overflow (or less).
263				The ldisc is responsible for being intelligent
264	 			about multi-threading of write_room/write calls
266	ioctl()		-	Called when an ioctl may be for the driver
268	set_termios()	-	Called on termios change, serialized against
269				itself by a semaphore. May sleep.
271	set_ldisc()	-	Notifier for discipline change. At the point this 
272				is done the discipline is not yet usable. Can now
273				sleep (I think)
275	throttle()	-	Called by the ldisc to ask the driver to do flow
276				control.  Serialization including with unthrottle
277				is the job of the ldisc layer.
279	unthrottle()	-	Called by the ldisc to ask the driver to stop flow
280				control.
282	stop()		-	Ldisc notifier to the driver to stop output. As with
283				throttle the serializations with start() are down
284				to the ldisc layer.
286	start()		-	Ldisc notifier to the driver to start output.
288	hangup()	-	Ask the tty driver to cause a hangup initiated
289				from the host side. [Can sleep ??]
291	break_ctl()	-	Send RS232 break. Can sleep. Can get called in
292				parallel, driver must serialize (for now), and
293				with write calls.
295	wait_until_sent() -	Wait for characters to exit the hardware queue
296				of the driver. Can sleep
298	send_xchar()	  -	Send XON/XOFF and if possible jump the queue with
299				it in order to get fast flow control responses.
300				Cannot sleep ??
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