linux-kernel - Re: [PATCH 1/5] cpufreq: Use cpumask_copy instead of cpumask

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Date:	Tue, 13 Oct 2015 12:22:43 -0700
From:	Saravana Kannan <skannan@...eaurora.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	Rafael Wysocki <rjw@...ysocki.net>, linaro-kernel@...ts.linaro.org,
	linux-pm@...r.kernel.org, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] cpufreq: Use cpumask_copy instead of cpumask_or to
 copy a mask

On 10/12/2015 08:23 PM, Viresh Kumar wrote:
> On 12-10-15, 12:12, Saravana Kannan wrote:
>>>   	if (new_policy) {
>>>   		/* related_cpus should at least include policy->cpus. */
>>> -		cpumask_or(policy->related_cpus, policy->related_cpus, policy->cpus);
>>> +		cpumask_copy(policy->related_cpus, policy->cpus);
>>
>> Again, why? It actually seems wrong. A 4 core cluster could come up
>> with just 2 cores when the policy is added. But the related CPUs
>> would be 4 CPUs.
>
> Firstly, the patch hasn't changed anything at all. related_cpus was
> empty until this point, and orring or setting it with ->cpus will
> result in the same output.

I was under the impression that the CPUfreq drivers were expected to 
fill in related_cpus and the or-ing was a safety net. If that's not the 
case, then this change is fine.

> Secondly, this is what we always wanted. related_cpus should contain
> the mask of all possible CPUs for that cluster.

I think the confusion was that I thought the drivers are supposed to do 
this. If this doesn't mess up other CPUfreq drivers that I'm not 
familiar with, then I don't have concerns.

Can you still explain the why in the commit text though? If it's just 
that related_cpus is always empty and copying is more efficient than 
or-ing, then mention that?

Thanks,
Saravana

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