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Subject: Re: [RFC PATCH v2 4/4] cpuidle: (POWER) Handle power_save=off
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Deepthi Dharwar <deepthi@linux.vnet.ibm.com>
Date: Tue, 29 Nov 2011 07:39:31 +1100
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On Mon, 2011-11-28 at 16:33 +0530, Deepthi Dharwar wrote:

> On an LPAR if cpuidle is disabled, ppc_md.power_save is still set to
> cpuidle_idle_call by default here. This would result in calling of
> cpuidle_idle_call repeatedly, only for the call to return -ENODEV. The
> default idle is never executed.
> This would be a major design flaw. No fallback idle routine.
> 
> We propose to fix this by checking the return value of
> ppc_md.power_save() call from void to int.
> Right now return value is void, but if we change this to int, this
> would solve two problems. One being removing the cast to a function
> pointer in the prev patch and this design flaw stated above.
> 
> So by checking the return value of ppc_md.power_save(), we can invoke
> the default idle on failure. But my only concern is about the effects of
> changing the ppc_md.power_save() to return int on other powerpc
> architectures. Would it be a good idea to change the return type to int
> which would help us flag an error and fallback to default idle?

I would have preferred an approach where the cpuidle module sets
ppc_md.power_save when loaded and restores it when unloaded ... but that
would have to go into the cpuidle core as a powerpc specific tweak and
might not be generally well received.

So go for it, add the return value, but you'll have to update all the
idle functions (grep for power_save in arch/powerpc to find them).

Cheers,
Ben.