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    SubjectRe: [PATCH] tick: add check for the existence of broadcast clock event device
    On Sat, 6 Jun 2009, Thomas Gleixner wrote:
    > Feng,
    > On Sat, 6 Jun 2009, Feng Tang wrote:
    > > > If your percpu devices are always on (not affected by C3 stop) then
    > > > you never dereference bc. So why do we need an extra check for !bc ?
    > >
    > > Hi tglx,
    > > Thanks for the explanation. But we really ran into the NULL pointer
    > > case, in our platform, there are 2 X86 CPUs which have lapic, also
    > > it has 2 external timers which are pretty similar with HPET timers,
    > > those 2 external timers will be used as per-cpu timers (higher
    > > rating than lapic timer). In system's power cycle of suspend and
    > > resume, disable_nontboot_cpus will be called before goto suspend
    > > state,and enable_nonboot_cpus will be called for the resume process,
    > > so lapic timer of cpu1 will be first registered as per-cpu timer,
    > > and our external timer will be registered later after get a
    > > CPU_ONLINE notifier (similar with HPET), right in this time slot
    > > that lapic is the per-cpu timer, when system get the
    > > CLOCK_EVT_BROADCAST_ENTER/EXIT msg, tick_do_broadcast_on_off() is
    > > called and hit the NULL pointer case.
    > Ok, I can understand now why we need it. I'll apply your patch and add
    > some more info into the commit msg so we do not look at it in a year
    > and scratch our heads. :)

    Hmm, thought more about it.

    1) How do you calibrate the local APIC timer if you do not have some
    initial timer device ?

    2) If you have some initial timer device (PIT/HPET) why isn't it
    registered as broadcast device.

    3) When the CPU uses the local APIC before the external timer is
    initialized what happens if the system goes into a deeper c-state ?



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