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    SubjectRe: [BUG] APM resume breakage from 2.6.18-rc1 clocksource changes
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    On Mon, 2006-07-10 at 20:08 +0200, Pavel Machek wrote:
    > Hi!
    >
    > > > >> 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.
    > >
    > > So it seems possible that the timer tick will be enabled before the
    > > timekeeping resume code runs. I'm not sure why this isn't seen w/ ACPI
    > > suspend/resume, as I think they're using the same
    > > sysdev_class .suspend/.resume bits.
    >
    > ACPI actually keeps interrupts disabled, always.
    >
    > APM can only keep interrupts disabled on non-IBM machines, presumably
    > due to BIOS problems.
    >
    > Could we get some sanity check into looping function? If timesource
    > goes backwards, at least somehow reporting it would be nice...

    Yep, I'm working on a debug patch (similar to the paranoid timekeeping
    debugging option in earlier versions of the TOD patch) that will spit
    out warnings when we see unusual behavior: large numbers of lost ticks,
    timer ticks arriving too early, settimeofday being call, etc.

    thanks
    -john

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