linux-kernel - Re: [RFC 3/9] cpufreq: Add dvfs_possible_from_any

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Date:   Wed, 29 Mar 2017 23:22:20 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linaro-kernel@...ts.linaro.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        smuckle.linux@...il.com, juri.lelli@....com,
        Morten.Rasmussen@....com, patrick.bellasi@....com,
        eas-dev@...ts.linaro.org
Subject: Re: [RFC 3/9] cpufreq: Add dvfs_possible_from_any_cpu policy flag

On Thursday, March 09, 2017 05:15:13 PM Viresh Kumar wrote:
> On many platforms any CPU (from any cpufreq policy) can perform DVFS on
> behalf of other CPUs. Add a flag to identify such cpufreq policies.
> 
> Also enable it for cpufreq-dt driver which is used only on ARM platforms
> currently.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
>  drivers/cpufreq/cpufreq-dt.c | 1 +
>  include/linux/cpufreq.h      | 9 +++++++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/drivers/cpufreq/cpufreq-dt.c b/drivers/cpufreq/cpufreq-dt.c
> index c943787d761e..e57b45f20544 100644
> --- a/drivers/cpufreq/cpufreq-dt.c
> +++ b/drivers/cpufreq/cpufreq-dt.c
> @@ -274,6 +274,7 @@ static int cpufreq_init(struct cpufreq_policy *policy)
>  		transition_latency = CPUFREQ_ETERNAL;
>  
>  	policy->cpuinfo.transition_latency = transition_latency;
> +	policy->dvfs_possible_from_any_cpu = true;
>  
>  	return 0;
>  
> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> index 87165f06a307..9490a314c515 100644
> --- a/include/linux/cpufreq.h
> +++ b/include/linux/cpufreq.h
> @@ -120,6 +120,15 @@ struct cpufreq_policy {
>  	bool			fast_switch_possible;
>  	bool			fast_switch_enabled;
>  
> +	/*
> +	 * Remote DVFS flag (Not added to the driver structure as we don't want
> +	 * to access another structure from scheduler hotpath).
> +	 *
> +	 * Should be set if any CPU (from same or different policy) can do DVFS
> +	 * on behalf of any other CPU.
> +	 */
> +	bool			dvfs_possible_from_any_cpu;

We rely on the assumption that any CPU in a policy can do DVFS in there already.

Why is this flag necessary at all?

> +
>  	 /* Cached frequency lookup from cpufreq_driver_resolve_freq. */
>  	unsigned int cached_target_freq;
>  	int cached_resolved_idx;
> 

Thanks,
Rafael