Date: Thu, 8 Jul 1999 09:54:42 +0100 (BST) From: Doug Rabson <dfr@nlsystems.com> To: Seigo Tanimura <tanimura@naklab.dnj.ynu.ac.jp> Cc: julian@whistle.com, freebsd-hackers@freebsd.org, freebsd-current@freebsd.org Subject: Re: Rewriting pca(4) using finetimer(9) (was: Re: MPU401 now works under New Midi Driver Framework with a Fine Timer) Message-ID: <Pine.BSF.4.10.9907080952480.558-100000@salmon.nlsystems.com> In-Reply-To: <199907080617.PAA23884@rina.naklab.dnj.ynu.ac.jp>
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On Thu, 8 Jul 1999, Seigo Tanimura wrote: > On Wed, 7 Jul 1999 19:46:38 -0700 (PDT), > Julian Elischer <julian@whistle.com> said: > > julian> With your scheme the clock needs to be always running at elevated speed. > julian> Possibly you might have a startup routine that turns on the elevated > julian> frequency, (basically does an 'aquire_timer0()' ) I would say that you > julian> would have more success in implementing your finetimer() by using > julian> "aquiretimer0" than the other way around. > > I agree that acquire_timer0() would give more freedom to the ticks > to callout. Then I tried figuring out how to manage multiple > callouts using acquire_timer0(), which is something like below. > > > Let C the callout queue, and c_i a callout. (0 <= i < I) Next define f(c_i) as > the callout function of c_i, and dt_rem(c_i) the time span between c_(i-1) and > c_i. (dt_rem(c_-1) is defined as zero) We use the time span to avoid traversing > though the queue to update the time tags on the callouts. > > (footnote: I'd better write in TeX :-<) > > Queueing a new callout c' to be made in t' involves a problem to find the > maximum j (which is an integer, j >= 0) satisfying a constraint > > t' > \sum_(k=0)^(j) dt_rem(c_k) > > where the right hand side of the inequality is the time span after which > the callout c_k is made. Then c' is inserted after c_j and new dt_rem(c_(j+1)) > and dt_rem(c_(j+2)) are determined. Now we can acquire_timer0() with dt_rem(c_0). > > In clkintr(), we dequeue c_0 from C, and make a callout to f(c_0). Then > acquire_timer0() is called once more with the new dt_rem(c_0). dt_rem(c_i) is > the difference of callout times, so they need not be updated on every clkintr(). > > > Although the computational cost in clkintr() is generaly O(1), the queueing cost > is O(I). Not sure whether we can reduce it or not (will it really make a trouble?) > > > How does it sound? If I understand this correctly, you are suggesting that we program timer0 so that we only take interrupts when a finetimer is due to fire? If so, then it sounds very good. The idea of taking 6000+ interrupts/sec made me uneasy, even though most would return without doing any work. -- Doug Rabson Mail: dfr@nlsystems.com Nonlinear Systems Ltd. Phone: +44 181 442 9037 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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