linux-kernel - Re: [PATCH] tick: add check for the existence of broadcast clock event device

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Date:	Sat, 6 Jun 2009 18:18:55 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Feng Tang <feng.tang@...el.com>
cc:	"mingo@...e.hu" <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [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 ?
 
Thanks,

	tglx
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