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Date: Wed, 16 Aug 2006 10:31:47 -0400
To: Raphael Hertzog <hertzog@debian.org>
Cc: Linux Kernel ML <linux-kernel@vger.kernel.org>
Subject: Re: How to avoid serial port buffer overruns?
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From: Lennart Sorensen <lsorense@csclub.uwaterloo.ca>
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On Wed, Aug 16, 2006 at 12:45:59PM +0200, Raphael Hertzog wrote:
> 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).

In my experience, the way to avoid overruns on a serial port was to use
a buffered serial port UART (such as a 16550A for example).  I remember
my 486 wasn't reliable about 19200 or 38400 (depending on how busy the
cpu was) when using an 8250.  Using a 16550A based card and I could do
115200 without any issues since the UART had a 16 byte buffer to help
out the system.  Unless your 386 has an add in card for the serial port,
it almost certainly has a very crappy UART and it would be very hard to
make it work reliably at higher speeds.

--
Len Sorensen
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