n_tty: Fix throttle for canon lines > 3967 chars

The tty driver will be mistakenly throttled if a line termination
has not been received, and the line exceeds 3967 chars. Thus, it is
possible for the driver to stop sending when it has not yet sent
the newline. This does not apply to the pty driver.

Don't throttle until at least one line termination has been
received.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_tty.c

@@ -247,6 +247,8 @@ static void n_tty_write_wakeup(struct tty_struct *tty)
 
 static void n_tty_check_throttle(struct tty_struct *tty)
 {
+	struct n_tty_data *ldata = tty->disc_data;
+
 	if (tty->driver->type == TTY_DRIVER_TYPE_PTY)
 		return;
 	/*
@@ -254,6 +256,9 @@ static void n_tty_check_throttle(struct tty_struct *tty)
 	 * mode.  We don't want to throttle the driver if we're in
 	 * canonical mode and don't have a newline yet!
 	 */
+	if (ldata->icanon && ldata->canon_head == ldata->read_tail)
+		return;
+
 	while (1) {
 		int throttled;
 		tty_set_flow_change(tty, TTY_THROTTLE_SAFE);