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Date:      Mon, 20 Sep 1999 09:06:26 -0700
From:      Doug <Doug@gorean.org>
To:        David Schwartz <davids@webmaster.com>
Cc:        Mike Smith <mike@smith.net.au>, Poul-Henning Kamp <phk@critter.freebsd.dk>, hackers@FreeBSD.ORG
Subject:   TSC vs. I8254 (Was: RE: cvs commit: src/sys/i386/conf files.i386  src/sys/i386/i386 mp_clock.c)
Message-ID:  <37E65B82.D60552D8@gorean.org>
References:  <000001bf0347$bd39b3e0$021d85d1@youwant.to>

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> > > No, the TSC is far superior on UP, (unless destroyed by APM), it
> > > has roughly 100 times better resolution and is twice as fast to
> > > query.

	If y'all don't mind, I have a slightly related question. I have a 5 year
old Dell computer that started life as a P90, which I overclocked to run as
a P100 by moving the bus speed up to 33 from 30 for a long time without
apparent problems. Then about 2 years ago I got one of those fancy
OverDrive chips (a 150, which was what was rated for my board, which I
overclocked at the same bus speed to run as a 166). After a few months of
running fine about every other boot up freebsd would report the wrong clock
speed. It was always a seemingly random value, with no pattern I could
discern. On a tip from one of the lists I started experimenting with the
CLK_ options in LINT, and finally found one in the 2.2.x branch that
managed to get me the right clock speed on say 9 out of 10 boots, so I was
happy. That option (options "CLK_USE_I586_CALIBRATION") went away in 3.x,
so now I'm down to choosing between options "CLK_USE_I8254_CALIBRATION" and
options CLK_USE_TSC_CALIBRATION. I tried the I8254 option first, and it
gives me what I want, namely the right cpu speed on every boot. However
based on the conversation regarding this thread I'm wondering if I should
try the TSC calibration method instead? 

	I know that someone will be tempted to warn me about the evils of
overclocking, but please don't. :) However, just in case anyone is
interested either the overclocking or the overdrive chip seems to have
fried the on-board serial ports on this machine. I've been trying for
months to get a serial console on it, and couldn't figure out why all I
ever got was garbage no matter how many different configurations I tried.
Well, I'm finally in a position where I have a third computer to try, and
lo and behold my workstation machine can run a serial console just fine,
but the server machine was all scrambled. So, last night I bought a serial
port card and voila, FINALLY I have a serial console. 

TIA for any insights,

Doug


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