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    SubjectRe: [BUG] APM resume breakage from 2.6.18-rc1 clocksource changes
    On Sun, 09 Jul 2006 14:20:56 -0700, john stultz wrote:
    >> I've traced the cause of this problem to the i386 time-keeping
    >> changes in kernel 2.6.17-git11. What happens is that:
    >> - The kernel autoselects TSC as my clocksource, which is
    >> reasonable since it's a PentiumII. 2.6.17 also chose the TSC.
    >> - Immediately after APM resumes (arch/i386/kernel/apm.c line
    >> 1231 in 2.6.18-rc1) there is an interrupt from the PIT,
    >> which takes us to kernel/timer.c:update_wall_time().
    >> - update_wall_time() does a clocksource_read() and computes
    >> the offset from the previous read. However, the TSC was
    >> reset by HW or BIOS during the APM suspend/resume cycle and
    >> is now smaller than it was at the prevous read. On my machine,
    >> the offset is 0xffffffd598e0a566 at this point, which appears
    >> to throw update_wall_time() into a very very long loop.
    >Huh. It seems you're getting an interrupt before timekeeping_resume()
    >runs (which resets cycle_last). I'll look over the code and see if I can
    >sort out why it works w/ ACPI suspend, but not APM, or if the
    >resume/interrupt-enablement bit is just racy in general.

    I forgot to mention this, but I had a debug printk() in apm.c
    which showed that irqs_disabled() == 0 at the point when APM
    resumes the kernel.

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