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

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Date:	Sat, 6 Jun 2009 14:54:49 +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

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. :)

> Our external timer driver is very similar with HPET dirver, why HPET
> doesn't see such an issue? becuase HPET has enough number of timers,
> and it use "hpet0" as the bc device, while our platform doesn't have
> a extra one to act as bc.

Correct.

Thanks,

	tglx
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