linux-kernel - Re: [PATCH v2 2/5] cpu_pm: call notifiers during suspend

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Message-ID: <87wrdjku9h.fsf@ti.com>
Date:	Thu, 08 Sep 2011 07:01:46 -0700
From:	Kevin Hilman <khilman@...com>
To:	Santosh <santosh.shilimkar@...com>
Cc:	rjw@...k.pl, linux@....linux.org.uk, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, ccross@...roid.com
Subject: Re: [PATCH v2 2/5] cpu_pm: call notifiers during suspend

Santosh <santosh.shilimkar@...com> writes:

> On Thursday 08 September 2011 01:32 AM, Kevin Hilman wrote:
>> Santosh Shilimkar<santosh.shilimkar@...com>  writes:
>>
>>> From: Colin Cross<ccross@...roid.com>
>>>
>>> Implements syscore_ops in cpu_pm to call the cpu and
>>> cpu cluster notifiers during suspend and resume,
>>> allowing drivers receiving the notifications to
>>> avoid implementing syscore_ops.
>>>
>>> Signed-off-by: Colin Cross<ccross@...roid.com>
>>> [santosh.shilimkar@...com: Rebased against 3.1-rc4]
>>> Signed-off-by: Santosh Shilimkar<santosh.shilimkar@...com>
>>
>> I don't think using syscore_ops is right here.  The platform code should
>> decide where in its own suspend path the notifiers should be triggered.
>>
>> The reason is because while the syscore_ops run late in the suspend
>> path, they still run before some platform-specific decisions about the
>> low-power states are made.  That means that any notifiers that need to
>> use information about the target low-power state (e.g. whether context
>> will be lost or not) cannot do so since that information has not yet
>> been decided until the platform_suspend_ops->enter() runs.
>>
> Initially I thought the same but in general S2R, platform doesn't
> support multiple states like CPUIDLE. On OMAP, we do have a debug
> option to choose the state but on real product, it's always the
> deepest supported state is used. So the driver saving the
> full context for S2R, should be fine.
>
> Ofcourse for CPUIDLE, the notifier call chain decisions are left
> with platform CPUIDLE drivers since there can be multiple low
> power states and the context save/restore has to be done based
> on low power states.
>
> The advantage with this is, the platform code is clean from the
> notfiers calls. CPUIDLE driver needs to call the different notifier
> events based on C-states and that perfectly works.
>
> I liked this simplification for the S2R. Down side is in S2R if you
> don't plan to hit deepest state, drivers end up saving full context
> which is fine I guess.

That's not the downside I'm worried about.

If you have a driver that has a notifier, presumably it has something it
wants to do to prepare for suspend *and* for idle, and you'd only want a
single notifier callback in the driver to be used for both.  That
callback would look something like:

   start_preparing_for_suspend();

   if (next_state == OFF)
      save_context();

   finish_preparing_for_suspend();


The problem with the current cpu_*_pm_enter() calls in syscore_ops is
that they happen before the next states are programmed, so during
suspend the 'if (next_state == off)' above would never be true, but
during idle it might be.

Kevin
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