Date: Thu, 2 Jul 1998 07:31:33 -0700 (PDT) From: sos@FreeBSD.ORG To: smoergrd@oslo.geco-prakla.slb.com (Dag-Erling Coidan Smørgrav) Cc: luigi@labinfo.iet.unipi.it, sos@FreeBSD.ORG, nick.hibma@jrc.it, hackers@FreeBSD.ORG Subject: Re: timeout granularity (was: Re: Console driver...) Message-ID: <199807021431.HAA01792@hub.freebsd.org> In-Reply-To: <rx4af6s2x80.fsf@oslo.geco-prakla.slb.com> from =?ISO-8859-1?Q?Dag=2DErling_Coidan_Sm=F8rgrav?= at "Jul 2, 98 03:34:07 pm"
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In reply to Dag-Erling Coidan Smørgrav who wrote:
> Luigi Rizzo <luigi@labinfo.iet.unipi.it> writes:
> > > It's not that simple. You have to know how long to wait, which is
> > > practically impossible to do without hooking the timer interrupt and
> > > reprogramming it to keep pace with the retrace, and even that is
> > > difficult to achieve without a little busy-waiting here and there.
> > a little busy waiting is not a lot. What is bad is 10ms waiting
>
> With a 70 Hz refresh rate, we're talking more like 14 ms in the worst
> case, which is pretty darn bad.
Yup, but...
> > If one can, say, assume that the vertical retrace is never less than
> > 1ms, then HZ=2000 should do the job (wakeup at every tick...).
>
> Ah, I didn't think of it quite like that. Yes, it would work, and you
> wouldn't have to busy-wait at all; just wake up at every tick, check
> if you're in a retrace, and if you are, do your job.
You will have to be more accurate than that...
Its right that you have to wait ~14ms, but the window of opportunity
(ie in the blanking period) is only about 330 usecs long, and you have
to hit it pretty accurately (ie within a few 10 usec) to be able to
do the transfer without shooting over the edge.
So there goes that idea, belive me I've thought of this before :)
Besides rasing the clock that drastically will impact on our
slower customers, I'd say generally its a bad idea...
(And I did the code that allows it for the pca device, hi hi hi)
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Soren Schmidt (sos@FreeBSD.org) FreeBSD Core Team
So much code to hack -- so little time.
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