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

[Pavel Machek <pavel@ucw.cz>]

On Sun 2006-07-09 22:58:31, 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.

Step 0: could we get some sanity checks into that loop? I'm pretty
sure we'll face some TSCs going backwards... panic-king the box at
that point is okay, but infinite loop is not...
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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