From owner-freebsd-hackers  Thu Jul  8  8:28: 6 1999
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Date: Fri, 9 Jul 1999 01:27:14 +1000
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199907081527.BAA04078@godzilla.zeta.org.au>
To: dfr@nlsystems.com, tanimura@naklab.dnj.ynu.ac.jp
Subject: Re: Rewriting pca(4) using finetimer(9) (was: Re: MPU401 now worksunder New Midi Driver Framework with a Fine Timer)
Cc: freebsd-current@FreeBSD.ORG, freebsd-hackers@FreeBSD.ORG,
	julian@whistle.com
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>dfr> If I understand this correctly, you are suggesting that we program timer0
>dfr> so that we only take interrupts when a finetimer is due to fire? If so,
>dfr> then it sounds very good. The idea of taking 6000+ interrupts/sec made me
>dfr> uneasy, even though most would return without doing any work.

6000+ interrupts/sec is not many, unless it is done all the time.  A
486/33 can handle about 50000 (16000 for pcaudio + 3 * 11520 for sio's).

>There is one problem in this method. acquire_timer0() is only implemented
>for i386. We would need to write something equivalent for alpha...

This is a serious problem.  acquire_timer0() is currently disabled even
for i386's when the i8254 is used for timecounting.  This is not hard
to fix (the hooks into clkintr() work even better with timecounters
since it is not necessary to resynchronise clock interrupts after a
state change), but an i8254 interrupt frequency of 16000 Hz is too fast
to be used routinely if the i8254 is being used for timecounting (even
if the CPU can keep up with the interrupts, the overflow heuristics in
i8254_get_timecount() may break down).  Other systems may have even
more limitations on the timecounters.

Bruce


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