linux-kernel - [PATCH 3.2 108/126] isdn/gigaset: reset tty->receive_room when attaching ser

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Date:   Wed, 15 Feb 2017 22:41:34 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Tilman Schmidt" <tilman@...p.cc>,
        "David S. Miller" <davem@...emloft.net>
Subject: [PATCH 3.2 108/126] isdn/gigaset: reset tty->receive_room when
 attaching ser_gigaset

3.2.85-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Tilman Schmidt <tilman@...p.cc>

commit fd98e9419d8d622a4de91f76b306af6aa627aa9c upstream.

Commit 79901317ce80 ("n_tty: Don't flush buffer when closing ldisc"),
first merged in kernel release 3.10, caused the following regression
in the Gigaset M101 driver:

Before that commit, when closing the N_TTY line discipline in
preparation to switching to N_GIGASET_M101, receive_room would be
reset to a non-zero value by the call to n_tty_flush_buffer() in
n_tty's close method. With the removal of that call, receive_room
might be left at zero, blocking data reception on the serial line.

The present patch fixes that regression by setting receive_room
to an appropriate value in the ldisc open method.

Fixes: 79901317ce80 ("n_tty: Don't flush buffer when closing ldisc")
Signed-off-by: Tilman Schmidt <tilman@...p.cc>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/isdn/gigaset/ser-gigaset.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

--- a/drivers/isdn/gigaset/ser-gigaset.c
+++ b/drivers/isdn/gigaset/ser-gigaset.c
@@ -526,9 +526,18 @@ gigaset_tty_open(struct tty_struct *tty)
 	cs->hw.ser->tty = tty;
 	atomic_set(&cs->hw.ser->refcnt, 1);
 	init_completion(&cs->hw.ser->dead_cmp);
-
 	tty->disc_data = cs;
 
+	/* Set the amount of data we're willing to receive per call
+	 * from the hardware driver to half of the input buffer size
+	 * to leave some reserve.
+	 * Note: We don't do flow control towards the hardware driver.
+	 * If more data is received than will fit into the input buffer,
+	 * it will be dropped and an error will be logged. This should
+	 * never happen as the device is slow and the buffer size ample.
+	 */
+	tty->receive_room = RBUFSIZE/2;
+
 	/* OK.. Initialization of the datastructures and the HW is done.. Now
 	 * startup system and notify the LL that we are ready to run
 	 */