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To: Colin Percival <cperciva@freebsd.org>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
In-Reply-To: Your message of "Fri, 28 Oct 2005 06:47:27 PDT."
	<43622BEF.3030006@freebsd.org> 
Date: Fri, 28 Oct 2005 16:26:23 +0200
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Sender: phk@critter.freebsd.dk
Cc: Alexander Leidinger <Alexander@Leidinger.net>, current@freebsd.org
Subject: Re: TSC instead of ACPI: powerd doesn't work anymore (to be
	expected?) 
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In message <43622BEF.3030006@freebsd.org>, Colin Percival writes:
>Alexander Leidinger wrote:
>> Poul-Henning Kamp <phk@phk.freebsd.dk> wrote:
>>> In general, if you run powerd to change your cpu clock (= TSC
>>> frequency), using TSC as timecounter is _not_ what you want to do.
>> 
>> Is it not possible to recalibrate on frequency change, or is it "just" that
>> nobody wrote the code do to it?
>
>It's not possible in any useful way.  When power management code changes
>the TSC frequency, it doesn't change the frequency directly; instead, it
>sends a message to the CPU saying "please, if you feel like it, could you
>change your frequency to <foo> some time soon?"
>
>Even if you manage to handle the change in frequency, the timecounter
>would significantly lose accuracy due to not knowing exactly when the
>change in frequency occurred.

This is stuff you can calibrate your way out of given enough
time, effort and external reference, but a generic solution is
not possible.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.