Cc: Arjan van de Ven <arjan@...>, Alok Kataria <akataria@...>, Thomas Gleixner <tglx@...>, Larry Finger <Larry.Finger@...>, LKML <linux-kernel@...>, Rafael J. Wysocki <rjw@...>, Michael Buesch <mb@...>, Dan Hecht <dhecht@...>
On Wed, Sep 03, 2008 at 08:59:05PM -0700, Linus Torvalds wrote:
15 years ago when I only knew DOS, I used the PIT a lot for precise
delay calculations. I can attest that it can be a very precise timer
for delays when you run busy loops. You even need very few ticks because
you detect the falling edge with a high accuracy. Basically, I would
do this :
pit1 = readpit();
while (readpit() == pit1);
t1 = rdtsc(); // precise beginning of tick 0
while (readpit() != pit1 - 5000);
t2 = rdtsc(); // precise beginning of tick 5000
(t2 - t1) will be exactly 5000 PIT ticks long, or 4.1904767 ms.
Additional sanity checks are needed of course, such as rollover
detection, and a max loop counter in case we boot on a machine
with a broken PIT.
If someone wants to test this, I'd be interested in the number of
ticks required to get a good accuracy, I bet that even with a few
hundred ones it's already precise by a few ppm (about the precision
of the input clock in fact).
Willy
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