Re: TSC instead of ACPI: powerd doesn't work anymore (to be expected?)

From: Poul-Henning Kamp (phk_at_phk.freebsd.dk)
Date: 10/28/05

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    To: Colin Percival <cperciva@freebsd.org>
Date: Fri, 28 Oct 2005 16:26:23 +0200

    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.
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