Re: PPP with PCMCIA modem stalls on 2.6.10 or later

[Paul Fulghum <paulkf@microgate.com>]

Kouji Toriatama wrote:
> ------------------------------------------------------------------
> Feb 16 23:52:40 moka kernel: receive_chars:flip full:low_latency=0
> Feb 16 23:52:40 moka kernel: receive_chars:flip full:discard char
> ------------------------------------------------------------------
> I have got this pair of two lines many times while running the
> wget command.
...
> With 'low_latency' option in 2.6.15 with your patch, the problem
> did not occur and no output from syslog.

Good, the problem is identified:
The old flip buffer code can't keep up unless
it processes received data directly in
the ISR (2.6.9 or 2.6.10+ with low_latency)
instead of in scheduled work (2.6.10+ without low_latency).

The discarded receive chars causes dropped frames
in ppp_async and stalls in data transfer.

I don't remember the details, but I think it is
safe to run low_latency on a uniprocessor.

> I have tried 2.6.16-rc3.  With or without 'low_latency' option,
> the problem did not occur.  It seems to work fine!  I will use
> 2.6.16-rc3 or later.

Alan's new tty buffering code safely handles
high receive rates without data loss, so the
low_latency flag is not necessary.

> If you have any additional plan to pin down this problem, I will
> try your patch.

The fix is already present in 2.6.16 series, so there
is nothing to patch. If you wish to run 2.6.10-2.6.15
you should be able to safely use low_latency on
a uniprocessor machine.

-- 
Paul Fulghum
Microgate Systems, Ltd.