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



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