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    SubjectRe: How to avoid serial port buffer overruns?
    On Wed, 16 Aug 2006, Paul Fulghum wrote:
    > On Wed, 2006-08-16 at 17:12 -0400, Lee Revell wrote:
    > > 2.6.15 and 2.6.16. Here is the .config:
    >
    > Alan's rework of the receive tty buffering went
    > into 2.6.16 and cured some problems, but clearly not yours.
    > Some more adjustments are in 2.6.18-rc4, so that
    > would be interesting to try for diagnosing this.

    I will try 2.6.18-rc4 and keep you informed.

    > I was wondering if the problem was interrupt latency,
    > the tty receive buffering, or something totally different.
    > I don't know if your problem and Raphael's are caused
    > by the same mechanism. I would still like to know which
    > kernel versions he has tried.

    I tried 2.6.17.7.

    But I'm really not sure that the 2.6 is a regression from 2.4, in fact I
    think it does better by default.

    The stock 2.4.31 kernel I was using had serial overruns at 9600 bauds
    already. Once patched with the low latency/preemptive kernel patchs, it
    was way better and I had only overruns at 115200 bauds.

    With the 2.6.17.7 kernel (configured with CONFIG_PREEMPT and
    CONFIG_HZ=1000), I'm seeing overruns starting at 38400 bauds. So
    compared to plain 2.4, it's better. However compared to the patched
    2.4, it's worse.

    (the figures are *very approximative* as the overruns haven't been
    detected with the same test conditions on 2.4 and on 2.6)

    I have no result with the 2.6.17.7 patched with the real time patch of
    Ingo/Thomas since it currently doesn't work on my card (see my separate
    bugreport).

    (those questions may have been directed to Lee but I'll respond in my case
    as well)
    > Are you using the low_latency flag on the serial device?

    I tried that option with the 2.4 kernel and it didn't improve the situation
    at all, and I haven't retried it with the 2.6 yet. But I will do.

    > What type of UART has been tested (16550? other?)

    I'm using only 16550 and I have no choice here, it's an "off the shelf"
    card.

    > Are you seeing overruns or just lost data?

    I'm seeing overruns.

    Regards,
    --
    Raphaël Hertzog

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