Re: BIOS or kernel APM bug?

From: Erik Mouw (J.A.K.Mouw@its.tudelft.nl)
Date: Wed Sep 25 2002 - 06:51:03 EST

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On Sun, Sep 22, 2002 at 09:31:02PM -0400, James D Strandboge wrote:
> I recently purchased a usb webcam and found that polling /proc/apm
> causes the webcam in xawtv to skip. I can so this either by doing 'cat
> /proc/apm' or using the gnome battstat-applet. Disabling the
> battstat-applet and not touching /proc/apm lets xawtv work fine.
>
> Polling /proc/apm also causes clock drift.
>
> I have a Dell Inspiron 8200 laptop (1.6Ghz Pentium 4). Using kernel
> 2.4.18 with rmap12h and preempt-kernel patch.

BIOS bug. Reading /proc/apm causes the APM BIOS to switch the CPU to
SMM mode with interrupts disabled so it can slowly poll the battery
about it's status. Result: lost interrupts, dropped frames, dropped
serial characters, clock slowdown. Workaround: don't use a battery
monitor, or don't let it poll every two seconds.

Erik

-- 
J.A.K. (Erik) Mouw
Email: J.A.K.Mouw@its.tudelft.nl
WWW: http://www-ict.its.tudelft.nl/~erik/

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