tty: Implement lookahead to process XON/XOFF timely

When tty is not read from, XON/XOFF may get stuck into an
intermediate buffer. As those characters are there to do software
flow-control, it is not very useful. In the case where neither end
reads from ttys, the receiving ends might not be able receive the
XOFF characters and just keep sending more data to the opposite
direction. This problem is almost guaranteed to occur with DMA
which sends data in large chunks.

If TTY is slow to process characters, that is, eats less than given
amount in receive_buf, invoke lookahead for the rest of the chars
to process potential XON/XOFF characters.

We need to keep track of how many characters have been processed by the
lookahead to avoid processing the flow control char again on the normal
path. Bookkeeping occurs parallel on two layers (tty_buffer and n_tty)
to avoid passing the lookahead_count through the whole call chain.

When a flow-control char is processed, two things must occur:
  a) it must not be treated as normal char
  b) if not yet processed, flow-control actions need to be taken
The return value of n_tty_receive_char_flow_ctrl() tells caller a), and
b) is kept internal to n_tty_receive_char_flow_ctrl().

If characters were previous looked ahead, __receive_buf() makes two
calls to the appropriate n_tty_receive_buf_* function. First call is
made with lookahead_done=true for the characters that were subject to
lookahead earlier and then with lookahead=false for the new characters.
Either of the calls might be skipped when it has no characters to
handle.

Reported-by: Gilles Buloz <gilles.buloz@kontron.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220606153652.63554-2-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

3 files changed, 17 insertions, 0 deletions

diff --git a/include/linux/tty_buffer.h b/include/linux/tty_buffer.h
index 3b9d77604291..1796648c2907 100644
--- a/include/linux/tty_buffer.h
+++ b/include/linux/tty_buffer.h
@@ -15,6 +15,7 @@ struct tty_buffer {
 	int used;
 	int size;
 	int commit;
+	int lookahead;		/* Lazy update on recv, can become less than "read" */
 	int read;
 	int flags;
 	/* Data points here */
diff --git a/include/linux/tty_ldisc.h b/include/linux/tty_ldisc.h
index e85002b56752..33678e1936f6 100644
--- a/include/linux/tty_ldisc.h
+++ b/include/linux/tty_ldisc.h
@@ -186,6 +186,18 @@ int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
  *	indicate all data received is %TTY_NORMAL. If assigned, prefer this
  *	function for automatic flow control.
  *
+ * @lookahead_buf: [DRV] ``void ()(struct tty_struct *tty,
+ *			const unsigned char *cp, const char *fp, int count)
+ *
+ *	This function is called by the low-level tty driver for characters
+ *	not eaten by ->receive_buf() or ->receive_buf2(). It is useful for
+ *	processing high-priority characters such as software flow-control
+ *	characters that could otherwise get stuck into the intermediate
+ *	buffer until tty has room to receive them. Ldisc must be able to
+ *	handle later a ->receive_buf() or ->receive_buf2() call for the
+ *	same characters (e.g. by skipping the actions for high-priority
+ *	characters already handled by ->lookahead_buf()).
+ *
  * @owner: module containting this ldisc (for reference counting)
  *
  * This structure defines the interface between the tty line discipline
@@ -229,6 +241,8 @@ struct tty_ldisc_ops {
 	void	(*dcd_change)(struct tty_struct *tty, unsigned int status);
 	int	(*receive_buf2)(struct tty_struct *tty, const unsigned char *cp,
 				const char *fp, int count);
+	void	(*lookahead_buf)(struct tty_struct *tty, const unsigned char *cp,
+				 const unsigned char *fp, unsigned int count);
 
 	struct  module *owner;
 };
diff --git a/include/linux/tty_port.h b/include/linux/tty_port.h
index 58e9619116b7..fa3c3bdaa234 100644
--- a/include/linux/tty_port.h
+++ b/include/linux/tty_port.h
@@ -40,6 +40,8 @@ struct tty_port_operations {
 
 struct tty_port_client_operations {
 	int (*receive_buf)(struct tty_port *port, const unsigned char *, const unsigned char *, size_t);
+	void (*lookahead_buf)(struct tty_port *port, const unsigned char *cp,
+			      const unsigned char *fp, unsigned int count);
 	void (*write_wakeup)(struct tty_port *port);
 };