linux-kernel - Re: [PATCH] cpufreq: Avoid unnecessary frequency updates due to mismatch

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Date:   Thu, 5 May 2022 09:28:13 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        linux-pm@...r.kernel.org, Rex-BC Chen <rex-bc.chen@...iatek.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH] cpufreq: Avoid unnecessary frequency updates due to mismatch

On Wed, 4 May 2022 at 10:21, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> For some platforms, the frequency returned by hardware may be slightly
> different from what is provided in the frequency table. For example,

Do you have more details ?

Do you mean that between 2 consecutives reads you can get either
500Mhz or 499Mhz ?

Or is it a fixed mismatch between the table and the freq returned by HW ?

> hardware may return 499 MHz instead of 500 MHz. In such cases it is
> better to avoid getting into unnecessary frequency updates, as we may
> end up switching policy->cur between the two and sending unnecessary
> pre/post update notifications, etc.
>
> This patch has chosen allows the hardware frequency and table frequency
> to deviate by 1 MHz for now, we may want to increase it a bit later on
> if someone still complains.
>
> Reported-by: Rex-BC Chen <rex-bc.chen@...iatek.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
>  drivers/cpufreq/cpufreq.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 0d58b0f8f3af..233e8af48848 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -28,6 +28,7 @@
>  #include <linux/suspend.h>
>  #include <linux/syscore_ops.h>
>  #include <linux/tick.h>
> +#include <linux/units.h>
>  #include <trace/events/power.h>
>
>  static LIST_HEAD(cpufreq_policy_list);
> @@ -1708,6 +1709,16 @@ static unsigned int cpufreq_verify_current_freq(struct cpufreq_policy *policy, b
>                 return new_freq;
>
>         if (policy->cur != new_freq) {
> +               /*
> +                * For some platforms, the frequency returned by hardware may be
> +                * slightly different from what is provided in the frequency
> +                * table, for example hardware may return 499 MHz instead of 500
> +                * MHz. In such cases it is better to avoid getting into
> +                * unnecessary frequency updates.
> +                */
> +               if (abs(policy->cur - new_freq) < HZ_PER_MHZ)
> +                       return policy->cur;
> +
>                 cpufreq_out_of_sync(policy, new_freq);
>                 if (update)
>                         schedule_work(&policy->update);
> --
> 2.31.1.272.g89b43f80a514
>