linux-kernel - Re: [PATCH] cpufreq: governors: Remove duplicate check of target freq in supported range

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Date:	Wed, 28 Aug 2013 11:05:19 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Stratos Karafotis <stratosk@...aphore.gr>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: governors: Remove duplicate check of target freq
 in supported range

 27 August 2013 23:04, Stratos Karafotis <stratosk@...aphore.gr> wrote:
> I'm sorry. Let me try to explain this better.

Don't be :)

> With my patch, dbs_info->requested_freq will not be capped within
> policy->min and policy->max in cs_check_cpu.
> So, temporarily it may have a value greater than policy->max
> or lower that policy->min.
> When we call __cpufreq_driver_target, the correct frequency will be selected
> because __cpufreq_driver_target takes care to adjust the
> target frequency within policy range.
> But, eventually, dbs_cpufreq_notifier will adjust dbs_info->requested
> within policy range, if needed.
>
> If we remove
>
>         if (dbs_info->requested_freq == policy->max)
>                 return;
> and
>
>         if (policy->cur == policy->min)
>                 return;
>
> request_freq will keep increasing or decreasing in each iteration and
> finally will overflow or underflow.
>
> Consider, for example, that in a CPU with policy->max = 1000MHz
> the current frequency is 950MHz. With a constant load above
> up_threshold, the requested_freq in first iteration will be 1000MHz
> and __cpufreq_driver_target will select 1000MHz freq.
>
> In second iteration, requested_freq will be 1050MHz, and
> __cpufreq_driver_target will select 1000MHz. dbs_cpufreq_notifier
> will adjust requested_freq back to 1000MHz.
>
> In next iterations, dbs_cpufreq_notifier will not be called, so we
> need the above check (dbs_info->requested_freq == policy->max) to
> prevent requested_freq to grow arbitrary.
>
> I hope my explanation was better now. :)

Yes, your initial patch is fine.
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