[plug] Slow Serial Port

[Peter Wright]

On Mon, Apr 23, 2001 at 08:15:55PM +0800, Linux wrote:
> Hi all 
> I have a problem that I have not been able to resolve and need some help please
> I have been trying to fix this for months
> 
> The problem is my second serial port ttyS1 seems to get hung up almost
> like it has a irq problem.  I can dial into my modem on ttyS0 and there
> is no problem When I dial into ttyS1 I get 45%-90% packet loss pings are
> 140/ms
[ snip ]
> Things I have done or know and have made no difference
[ snip ]
> I have used setserial and tried lots of combinations
> Useing ttyS1 as the dial out port yeilds the same results (huge packet loss)
[ snip ]
> There must be something I have missed or can do 
> Any help would be greatly appreciated

This is almost certainly not what you want to hear :), but even in my
not-hardware-expert opinion it does sound very much like a hardware
problem. Unless you have other information that suggests the hardware is
fine? (eg. the same serial port works fine under Windows)

Well, if you haven't, I'd suggest you try just that (dialing out through a
modem on your second serial port (the problem one) under Windows. If the
problems present under Linux are also apparent under Windows, your ttyS1 is
_probably_ dysfunctional and you'd need someone with reasonable hardware
knowledge/skills to have a chance at fixing it.

*pause as I have another look at Bob's email*

Hang on a tick... you also said (which I initially missed, d'oh!):

> I have tried disabling the second onboard serial port and fitted a serial
> i/o card and tried all possable irq settings i/e  3-4-5

Okay - so I presume this didn't work - in what way didn't it work?
If you'd disabled the second onboard serial port (how?) /dev/ttyS1
shouldn't exist... but does /dev/ttyS2 exist then? 

> I have disabled the mouse GPM
> I have changed motherboards and CPU

Now this is the one that really gets me. You've changed _motherboards_ (so
you've essentially got different serial port hardware) and the problem
still exists? Damn, then I am completely off track with my suggestions
above, it must be a software issue. Pardon me while I go headbut the desk.
*thump* Ow.

> I have used setserial and tried lots of combinations

What CPU(s) and motherboard(s) are you dealing with?

My housemate suggests playing around with irqtune. He has an older machine
(386) that used to have severe lossage problems similar to those you
describe above, dealing with just a single 14400K modem. He used irqtune to
(if I understand correctly) increase the priority of the serial interrupt.
He now runs a 56K modem on it with no significant lossage.

If you haven't already, I'd seriously recommend you check the Serial HOWTO:

http://www.linuxdoc.org/HOWTO/Serial-HOWTO.html

It's flooded with heaps of useful information and a lot of troubleshooting
tips. The HOWTO also mentions irqtune under "Troubleshooting Tools":

  "irqtune" will give serial port interrupts higher priority to improve
  performance.

> Regards Bob Andrews

Hope some of the above helps. Good luck,

Pete.
-- 
http://cygnus.uwa.edu.au/~pete/

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