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

From: Colin Percival (cperciva_at_freebsd.org)
Date: 10/28/05

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    Date: Fri, 28 Oct 2005 06:47:27 -0700
To: Alexander Leidinger <Alexander@Leidinger.net>

    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.

    Colin Percival
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