linux-kernel - Re: [PATCH] cpufreq: CPPC: Correct desired

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Date:   Wed, 12 Oct 2016 09:36:34 -0600
From:   "Prakash, Prashanth" <pprakash@...eaurora.org>
To:     Hoan Tran <hotran@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Al Stone <ahs3@...hat.com>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        lho@....com, Duc Dang <dhdang@....com>
Subject: Re: [PATCH] cpufreq: CPPC: Correct desired_perf calculation

Hi Hoan,

On 10/11/2016 3:12 PM, Hoan Tran wrote:
> The desired_perf is an abstract performance number. Its value should
> be in the range of [lowest perf, highest perf] of CPPC.
> The correct calculation is
>   desired_perf = freq * cppc_highest_perf / cppc_dmi_max_khz
>
> Signed-off-by: Hoan Tran <hotran@....com>
> ---
>  drivers/cpufreq/cppc_cpufreq.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> index 1b2f28f..ab1d4b7 100644
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -84,7 +84,7 @@ static int cppc_cpufreq_set_target(struct cpufreq_policy *policy,
>  
>  	cpu = all_cpu_data[policy->cpu];
>  
> -	cpu->perf_ctrls.desired_perf = (u64)target_freq * policy->max / cppc_dmi_max_khz;
> +	cpu->perf_ctrls.desired_perf = (u64)target_freq * cpu->perf_caps.highest_perf / cppc_dmi_max_khz;
The patch looks good, I suppose we can add a small optimization. We can do a simple check
to see if the newly computed desired_perf is same as old one, If it is same we can just return
here instead of calling cppc_set_perf with same desired_perf value.

Thanks,
Prashanth