Re: [PATCH 1/2] isdn/gigaset: reset tty->receive_room when attaching ser_gigaset

[Peter Hurley <peter@hurleysoftware.com>]

On 07/13/2015 06:37 PM, Tilman Schmidt wrote:
> 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.

That commit didn't cause the problem; it was a bug all along.

For example, if the tty had first been hooked up to some other line
discipline which consumed most of tty->receive_room, _then_
switched to N_GIGASET_M101 line discipline, the same problem would
have occurred.

Non-flow controlling line disciplines _must_ set tty->receive_room
on line discipline open because they are declaring that every
input they can accept that much data.

Regards,
Peter Hurley

> 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@imap.cc>
> ---
>  drivers/isdn/gigaset/ser-gigaset.c |   11 ++++++++++-
>  1 files changed, 10 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/isdn/gigaset/ser-gigaset.c b/drivers/isdn/gigaset/ser-gigaset.c
> index 8c91fd5..3ac9c41 100644
> --- a/drivers/isdn/gigaset/ser-gigaset.c
> +++ b/drivers/isdn/gigaset/ser-gigaset.c
> @@ -524,9 +524,18 @@ 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
>  	 */
>