Adding upstream version 4.19.249.upstream/4.19.249

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 313 insertions, 0 deletions

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+
+			The Lockronomicon
+
+Your guide to the ancient and twisted locking policies of the tty layer and
+the warped logic behind them. Beware all ye who read on.
+
+
+Line Discipline
+---------------
+
+Line disciplines are registered with tty_register_ldisc() passing the
+discipline number and the ldisc structure. At the point of registration the 
+discipline must be ready to use and it is possible it will get used before
+the call returns success. If the call returns an error then it won't get
+called. Do not re-use ldisc numbers as they are part of the userspace ABI
+and writing over an existing ldisc will cause demons to eat your computer.
+After the return the ldisc data has been copied so you may free your own 
+copy of the structure. You must not re-register over the top of the line
+discipline even with the same data or your computer again will be eaten by
+demons.
+
+In order to remove a line discipline call tty_unregister_ldisc().
+In ancient times this always worked. In modern times the function will
+return -EBUSY if the ldisc is currently in use. Since the ldisc referencing
+code manages the module counts this should not usually be a concern.
+
+Heed this warning: the reference count field of the registered copies of the
+tty_ldisc structure in the ldisc table counts the number of lines using this
+discipline. The reference count of the tty_ldisc structure within a tty 
+counts the number of active users of the ldisc at this instant. In effect it
+counts the number of threads of execution within an ldisc method (plus those
+about to enter and exit although this detail matters not).
+
+Line Discipline Methods
+-----------------------
+
+TTY side interfaces:
+
+open()		-	Called when the line discipline is attached to
+			the terminal. No other call into the line
+			discipline for this tty will occur until it
+			completes successfully. Should initialize any
+			state needed by the ldisc, and set receive_room
+			in the tty_struct to the maximum amount of data
+			the line discipline is willing to accept from the
+			driver with a single call to receive_buf().
+			Returning an error will prevent the ldisc from
+			being attached. Can sleep.
+
+close()		-	This is called on a terminal when the line
+			discipline is being unplugged. At the point of
+			execution no further users will enter the
+			ldisc code for this tty. Can sleep.
+
+hangup()	-	Called when the tty line is hung up.
+			The line discipline should cease I/O to the tty.
+			No further calls into the ldisc code will occur.
+			The return value is ignored. Can sleep.
+
+read()		-	(optional) A process requests reading data from
+			the line. Multiple read calls may occur in parallel
+			and the ldisc must deal with serialization issues.
+			If not defined, the process will receive an EIO
+			error. May sleep.
+
+write()		-	(optional) A process requests writing data to the
+			line. Multiple write calls are serialized by the
+			tty layer for the ldisc. If not defined, the
+			process will receive an EIO error. May sleep.
+
+flush_buffer()	-	(optional) May be called at any point between
+			open and close, and instructs the line discipline
+			to empty its input buffer.
+
+set_termios()	-	(optional) Called on termios structure changes.
+			The caller passes the old termios data and the
+			current data is in the tty. Called under the
+			termios semaphore so allowed to sleep. Serialized
+			against itself only.
+
+poll()		-	(optional) Check the status for the poll/select
+			calls. Multiple poll calls may occur in parallel.
+			May sleep.
+
+ioctl()		-	(optional) Called when an ioctl is handed to the
+			tty layer that might be for the ldisc. Multiple
+			ioctl calls may occur in parallel. May sleep.
+
+compat_ioctl()	-	(optional) Called when a 32 bit ioctl is handed
+			to the tty layer that might be for the ldisc.
+			Multiple ioctl calls may occur in parallel.
+			May sleep.
+
+Driver Side Interfaces:
+
+receive_buf()	-	(optional) Called by the low-level driver to hand
+			a buffer of received bytes to the ldisc for
+			processing. The number of bytes is guaranteed not
+			to exceed the current value of tty->receive_room.
+			All bytes must be processed.
+
+receive_buf2()	-	(optional) Called by the low-level driver to hand
+			a buffer of received bytes to the ldisc for
+			processing. Returns the number of bytes processed.
+
+			If both receive_buf() and receive_buf2() are
+			defined, receive_buf2() should be preferred.
+
+write_wakeup()	-	May be called at any point between open and close.
+			The TTY_DO_WRITE_WAKEUP flag indicates if a call
+			is needed but always races versus calls. Thus the
+			ldisc must be careful about setting order and to
+			handle unexpected calls. Must not sleep.
+
+			The driver is forbidden from calling this directly
+			from the ->write call from the ldisc as the ldisc
+			is permitted to call the driver write method from
+			this function. In such a situation defer it.
+
+dcd_change()	-	Report to the tty line the current DCD pin status
+			changes and the relative timestamp. The timestamp
+			cannot be NULL.
+
+
+Driver Access
+
+Line discipline methods can call the following methods of the underlying
+hardware driver through the function pointers within the tty->driver
+structure:
+
+write()			Write a block of characters to the tty device.
+			Returns the number of characters accepted. The
+			character buffer passed to this method is already
+			in kernel space.
+
+put_char()		Queues a character for writing to the tty device.
+			If there is no room in the queue, the character is
+			ignored.
+
+flush_chars()		(Optional) If defined, must be called after
+			queueing characters with put_char() in order to
+			start transmission.
+
+write_room()		Returns the numbers of characters the tty driver
+			will accept for queueing to be written.
+
+ioctl()			Invoke device specific ioctl.
+			Expects data pointers to refer to userspace.
+			Returns ENOIOCTLCMD for unrecognized ioctl numbers.
+
+set_termios()		Notify the tty driver that the device's termios
+			settings have changed. New settings are in
+			tty->termios. Previous settings should be passed in
+			the "old" argument.
+
+			The API is defined such that the driver should return
+			the actual modes selected. This means that the
+			driver function is responsible for modifying any
+			bits in the request it cannot fulfill to indicate
+			the actual modes being used. A device with no
+			hardware capability for change (e.g. a USB dongle or
+			virtual port) can provide NULL for this method.
+
+throttle()		Notify the tty driver that input buffers for the
+			line discipline are close to full, and it should
+			somehow signal that no more characters should be
+			sent to the tty.
+
+unthrottle()		Notify the tty driver that characters can now be
+			sent to the tty without fear of overrunning the
+			input buffers of the line disciplines.
+
+stop()			Ask the tty driver to stop outputting characters
+			to the tty device.
+
+start()			Ask the tty driver to resume sending characters
+			to the tty device.
+
+hangup()		Ask the tty driver to hang up the tty device.
+
+break_ctl()		(Optional) Ask the tty driver to turn on or off
+			BREAK status on the RS-232 port.  If state is -1,
+			then the BREAK status should be turned on; if
+			state is 0, then BREAK should be turned off.
+			If this routine is not implemented, use ioctls
+			TIOCSBRK / TIOCCBRK instead.
+
+wait_until_sent()	Waits until the device has written out all of the
+			characters in its transmitter FIFO.
+
+send_xchar()		Send a high-priority XON/XOFF character to the device.
+
+
+Flags
+
+Line discipline methods have access to tty->flags field containing the
+following interesting flags:
+
+TTY_THROTTLED		Driver input is throttled. The ldisc should call
+			tty->driver->unthrottle() in order to resume
+			reception when it is ready to process more data.
+
+TTY_DO_WRITE_WAKEUP	If set, causes the driver to call the ldisc's
+			write_wakeup() method in order to resume
+			transmission when it can accept more data
+			to transmit.
+
+TTY_IO_ERROR		If set, causes all subsequent userspace read/write
+			calls on the tty to fail, returning -EIO.
+
+TTY_OTHER_CLOSED	Device is a pty and the other side has closed.
+
+TTY_NO_WRITE_SPLIT	Prevent driver from splitting up writes into
+			smaller chunks.
+
+
+Locking
+
+Callers to the line discipline functions from the tty layer are required to
+take line discipline locks. The same is true of calls from the driver side
+but not yet enforced.
+
+Three calls are now provided
+
+	ldisc = tty_ldisc_ref(tty);
+
+takes a handle to the line discipline in the tty and returns it. If no ldisc
+is currently attached or the ldisc is being closed and re-opened at this
+point then NULL is returned. While this handle is held the ldisc will not
+change or go away.
+
+	tty_ldisc_deref(ldisc)
+
+Returns the ldisc reference and allows the ldisc to be closed. Returning the
+reference takes away your right to call the ldisc functions until you take
+a new reference.
+
+	ldisc = tty_ldisc_ref_wait(tty);
+
+Performs the same function as tty_ldisc_ref except that it will wait for an
+ldisc change to complete and then return a reference to the new ldisc. 
+
+While these functions are slightly slower than the old code they should have
+minimal impact as most receive logic uses the flip buffers and they only
+need to take a reference when they push bits up through the driver.
+
+A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc 
+functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
+fail in this situation if used within these functions. Ldisc and driver
+code calling its own functions must be careful in this case. 
+
+
+Driver Interface
+----------------
+
+open()		-	Called when a device is opened. May sleep
+
+close()		-	Called when a device is closed. At the point of
+			return from this call the driver must make no 
+			further ldisc calls of any kind. May sleep
+
+write()		-	Called to write bytes to the device. May not
+			sleep. May occur in parallel in special cases. 
+			Because this includes panic paths drivers generally
+			shouldn't try and do clever locking here.
+
+put_char()	-	Stuff a single character onto the queue. The
+			driver is guaranteed following up calls to
+			flush_chars.
+
+flush_chars()	-	Ask the kernel to write put_char queue
+
+write_room()	-	Return the number of characters that can be stuffed
+			into the port buffers without overflow (or less).
+			The ldisc is responsible for being intelligent
+ 			about multi-threading of write_room/write calls
+
+ioctl()		-	Called when an ioctl may be for the driver
+
+set_termios()	-	Called on termios change, serialized against
+			itself by a semaphore. May sleep.
+
+set_ldisc()	-	Notifier for discipline change. At the point this 
+			is done the discipline is not yet usable. Can now
+			sleep (I think)
+
+throttle()	-	Called by the ldisc to ask the driver to do flow
+			control.  Serialization including with unthrottle
+			is the job of the ldisc layer.
+
+unthrottle()	-	Called by the ldisc to ask the driver to stop flow
+			control.
+
+stop()		-	Ldisc notifier to the driver to stop output. As with
+			throttle the serializations with start() are down
+			to the ldisc layer.
+
+start()		-	Ldisc notifier to the driver to start output.
+
+hangup()	-	Ask the tty driver to cause a hangup initiated
+			from the host side. [Can sleep ??]
+
+break_ctl()	-	Send RS232 break. Can sleep. Can get called in
+			parallel, driver must serialize (for now), and
+			with write calls.
+
+wait_until_sent() -	Wait for characters to exit the hardware queue
+			of the driver. Can sleep
+
+send_xchar()	  -	Send XON/XOFF and if possible jump the queue with
+			it in order to get fast flow control responses.
+			Cannot sleep ??
+