Messages in this thread | | | Date | Sun, 9 Jul 2006 22:58:31 +0200 (MEST) | From | Mikael Pettersson <> | Subject | [BUG] APM resume breakage from 2.6.18-rc1 clocksource changes |
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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.
Hacks around the problem: - echo jiffies>/sys/devices/system/clocksource/clocksource0/current_clocksource before suspending eliminates the hang. (Note: this doesn't affect the i386 delay loop implementation. Is that a bug or a feature?) - Hacking apm.c to set a flag just before suspending and clear it only after all resume actions are done, and to have update_wall_time() return immediately when this flag is set also eliminates the hang.
I guess the appropriate fix would be to augment either the TSC clocksource driver or the clocksource API to deal with pseudo-monotonic clocks that get reset at suspend.
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