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    SubjectRe: How to avoid serial port buffer overruns?
    Hello,

    I took the time to try some of your suggestions and I managed to get rid
    of the overruns.

    On Fri, 18 Aug 2006, Lee Revell wrote:
    > Have you tried it with HZ=100? HZ=1000 might just be too much for that
    > board.

    This indeed was a major problem and a bad choice of mine at the very
    beginning. It goes way better with HZ=100.

    On Fri, 18 Aug 2006, Paul Fulghul wrote:
    > For fun, have you tried playing with the rx FIFO trigger
    > level in the 16550A entry in drivers/serial/8250.c ?
    > You could try replacing UART_FCR_R_TRIG_10 (8 char trigger)
    > with UART_FCR_R_TRIG_01 (4 char trigger) or even
    > UART_FCR_R_TRIG_00 (1 char trigger).
    > That creates more interrupts, but allows
    > more time to activate the ISR before overrun.

    I changed the rx FIFO trigger level to 1 byte (UART_FCR_R_TRIG_00) and it
    helped a lot as well. With this combination I completely resolved the
    problem of overruns at full speed (115200 bauds).

    For the sake of comparison, I made a similar change to the 2.4.31 kernel I
    was using (ie a kernel with low latency/preemptible kernel patches).

    It helped a lot as well: most of the time I wouldn't have overruns (before
    they were very frequent, like at least one overrun in 10k chars received).
    However from time to time I would suffer from a single big overrun (like 30
    chars lost). And using heavily the disk on module will increase the
    likelihood to have a buffer overrun.

    With the 2.6.17.7 kernel (CONFIG_HZ=100 and patched to trigger IRQ at
    1 byte received), I have been completely unable to reproduce the buffer
    overruns whatever read/write operation I've been triggering during the data
    exchange.

    So all in all, the 2.6 kernel behaves better than the 2.4 in this
    case.

    Cheers,
    --
    Raphaël Hertzog

    Premier livre français sur Debian GNU/Linux :
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