linux-kernel - Re: [RFC PATCH v2 4/4] cpuidle: (POWER) Handle power

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Message-ID: <4ED47F48.2070107@linux.vnet.ibm.com>
Date:	Tue, 29 Nov 2011 12:14:24 +0530
From:	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
CC:	linuxppc-dev@...abs.org, linux-pm@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [RFC PATCH v2 4/4] cpuidle: (POWER) Handle power_save=off

On 11/29/2011 02:09 AM, Benjamin Herrenschmidt wrote:

> 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).
> 


Thanks Ben. Yes, I will update all the idle functions under powerpc.
I will re-work these patches with the discussed changes.

Regards,
Deepthi

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