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	Thu, 5 Jun 1997 21:49:52 -0700 (MST)
Date: Thu, 5 Jun 1997 21:49:52 -0700 (MST)
From: Richard Straka <straka@user1.inficad.com>
To: Bruce Evans <bde@zeta.org.au>
cc: current@FreeBSD.ORG
Subject: Re: sio driver performance
In-Reply-To: <199706042043.GAA16361@godzilla.zeta.org.au>
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> >> When receiving sustained 115200 baud data, an interrupt rate of 
> >> over 26,000/sec is indicated with about 15% of the processor going 
> >> to service interrupts.
> >> 
> >> It appears as though the fifo is not working for the receive. In an
> >> attempt to exhonerate the hardware, I swapped in another 16550 card
> >> I had laying around and got the similar results.
> 
> It works for most users.  Possible problems:
> 
> - noisy cables may cause a modem status interrupt for every character
>   (or every changed bit?).
> - a parity mismatch may cause a line status interrupt for every character.
>   This problem is easy to reproduce using `stty parenb ...' to change the
>   default of 8 bits-no parity to 9 bits-1 parity bit.
> - a hardware bug on UMC8669F devices that prevents initialization of the
>   fifo trigger level while data is arriving.  These devices also have a
>   h/w bug that prevents initialization of the speed while data is arriving.
>   Both bugs are more common in tests than in normal operation.  I always
>   see the fifo bug here if I start the transmitter first.  FreeBSD
>   transmitters do a perfect job of keeping the data flowing, so the
>   receiver never has a chance to recover :-).

Bruce,

Your suggestion of a noisy cable was right on. After swapping out a bad
null modem adapter, the receive fifo seems to be working great
(820 int/sec at 115200 baud). CPU utilization is also greatly reduced.

Thank again for you help and thanks to everyone who responded to my
problem.

Richard Straka
straka@inficad.com