Subject: Re: How to avoid serial port buffer overruns?
From: Lee Revell <rlrevell@joe-job.com>
To: Raphael Hertzog <hertzog@debian.org>
Cc: Linux Kernel ML <linux-kernel@vger.kernel.org>
In-Reply-To: <20060816104559.GF4325@ouaza.com>
References: <20060816104559.GF4325@ouaza.com>
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Date: Wed, 16 Aug 2006 14:44:28 -0400
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On Wed, 2006-08-16 at 12:45 +0200, Raphael Hertzog wrote:
> (Please CC me when replying)
> 
> Hello,
> 
> While using Linux on low-end (semi-embedded) hardware (386 SX 40Mhz, 8Mb
> RAM), I discovered that Linux on that machine would suffer from serial
> port buffer overruns quite easily if I use a baudrate high enough (I start
> loosing bytes at >19200 bauds and I would like to make it reliable up to 
> 115 kbauds). I check if overruns are happening with
> /proc/tty/driver/serial ("oe" field).
> 
> Back when I was using the 2.4 kernel, I reduced dramatically the frequency
> of overruns by using the "low latency" and "preemptible kernel" patch [1]. But
> it still happened sometimes at 115 kbauds if the system was a bit loaded
> (with disk I/O for example).
> 
> Now I switched to stock 2.6 and while the stock kernel improved in
> responsiveness, it still isn't enough by default (even with
> CONFIG_PREEMPT=y and CONFIG_HZ=1000). So I wanted to try the "rt" patch of
> Ingo Molnar and Thomas Gleixner, but the patched kernel doesn't boot (see
> bug report in a separate mail on this list).

Does the serial performance seem to have regressed from 2.4 to 2.6?  I
am chasing a similar issue with a serial MIDI card (supported by the bog
standard 8250 serial driver) that drops notes under 2.6 but works with
2.4.  I don't have details yet, but it sounds like a similar problem.

Lee

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