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    Subject[PATCH 4.9 73/86] serial: sh-sci: prevent lockup on full TTY buffers
    Date
    4.9-stable review patch.  If anyone has any objections, please let me know.

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

    From: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>

    commit 7842055bfce4bf0170d0f61df8b2add8399697be upstream.

    When the TTY buffers fill up to the configured maximum, a system lockup
    occurs:

    [ 598.820128] INFO: rcu_preempt detected stalls on CPUs/tasks:
    [ 598.825796] 0-...!: (1 GPs behind) idle=5a6/2/0 softirq=1974/1974 fqs=1
    [ 598.832577] (detected by 3, t=62517 jiffies, g=296, c=295, q=126)
    [ 598.838755] Task dump for CPU 0:
    [ 598.841977] swapper/0 R running task 0 0 0 0x00000022
    [ 598.849023] Call trace:
    [ 598.851476] __switch_to+0x98/0xb0
    [ 598.854870] (null)

    This can be prevented by doing a dummy read of the RX data register.

    This issue affects both HSCIF and SCIF ports. Reported for R-Car H3 ES2.0;
    reproduced and fixed on H3 ES1.1. Probably affects other R-Car platforms
    as well.

    Reported-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
    Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
    Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
    Cc: stable <stable@vger.kernel.org>
    Tested-by: Nguyen Viet Dung <dung.nguyen.aj@renesas.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/tty/serial/sh-sci.c | 2 ++
    1 file changed, 2 insertions(+)

    --- a/drivers/tty/serial/sh-sci.c
    +++ b/drivers/tty/serial/sh-sci.c
    @@ -935,6 +935,8 @@ static void sci_receive_chars(struct uar
    /* Tell the rest of the system the news. New characters! */
    tty_flip_buffer_push(tport);
    } else {
    + /* TTY buffers full; read from RX reg to prevent lockup */
    + serial_port_in(port, SCxRDR);
    serial_port_in(port, SCxSR); /* dummy read */
    sci_clear_SCxSR(port, SCxSR_RDxF_CLEAR(port));
    }

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