Re: [BUG] APM resume breakage from 2.6.18-rc1 clocksource changes

[john stultz]

On Sun, 2006-07-09 at 22:58 +0200, Mikael Pettersson wrote:
> On Fri, 7 Jul 2006 21:01:26 +0200 (MEST), I wrote:
> >Kernel 2.6.18-rc1 broke resume from APM suspend (to RAM)
> >on my old Dell Latitude CPi laptop. At resume the disk
> >spins up and the screen gets lit, but there is no response
> >to the keyboard, not even sysrq. All other system activity
> >also appears to be halted.
> >
> >I did the obvious test of reverting apm.c to the 2.6.17
> >version and fixing up the fallout from the TIF_POLLING_NRFLAG
> >changes, but it made no difference. So the problem must be
> >somewhere else.
> 
> 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.

Thanks for the bug report!
-john


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