Re: How to avoid serial port buffer overruns?

[Paul Fulghum <paulkf@microgate.com>]

Raphael Hertzog wrote:
> 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.

This tells me your issue is not a problem with the
serial or tty code, but rather a matter of IRQ latency.
(Which you may have already known, but I was unclear on)
I do not expect 2.6.18-rc4 to make a difference.

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.

Lee's issue may still merit investigation into the
serial/tty code.

--
Paul Fulghum
Microgate Systems, Ltd.
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