linux-kernel - Re: [linux-pm] [RFC PATCH v2 4/4] cpuidle: (POWER) Handle power_save=off

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Message-ID: <4ED58614.9090204@linux.vnet.ibm.com>
Date:	Wed, 30 Nov 2011 06:55:40 +0530
From:	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>
To:	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>
CC:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linuxppc-dev@...abs.org, linux-pm@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [linux-pm] [RFC PATCH v2 4/4] cpuidle: (POWER) Handle power_save=off

On 11/29/2011 12:14 PM, Deepthi Dharwar wrote:

> 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.
kernel/idle.c:  ppc_md.power_save = NULL;
>>>
>>> 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
> 
> _______________________________________________
> linux-pm mailing list
> linux-pm@...ts.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/linux-pm
> 
> 

Hi Ben,

I was trying to add a return value for power_save for all arch/powepc
idle functions but a few of them directly call *.S  routines, as they
are asm.

What would be a good way to change the return value  for asm routines ?
Do we make a change in asm only, put the return value in r3 or write a
wrapper function which would call these asm routines and return an int ?

Regards,
Deepthi

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