linux-kernel - Re: [PATCH] selftests: cpufreq: Check cpuinfo_cur

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Message-ID: <20170719065406.GE352@vireshk-i7>
Date:   Wed, 19 Jul 2017 12:24:06 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Leonard Crestez <leonard.crestez@....com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Shuah Khan <shuah@...nel.org>, linux-kselftest@...r.kernel.org,
        Octavian Purdila <octavian.purdila@....com>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] selftests: cpufreq: Check cpuinfo_cur_freq set as
 expected

On 18-07-17, 22:34, Leonard Crestez wrote:
> The semantics of scaling_cur_freq and cpuinfo_cur_freq are not very
> clear to me.

cpuinfo_cur_freq reads the frequency right from hardware all the time
and so can be slow. It can only be read by root if I remember
correctly.

Whereas scaling_cur_freq tries to read the cached frequency. But it
has changed a bit with the below mentioned patch.

> In my particular case I need to check cpuinfo_cur_freq because this is
> what ends up returning the rate of the arm clk. Otherwise
> scaling_cur_freq just returns policy->cur

Yeah, we may actually need to use cpuinfo_cur_freq as that is what
ends up giving the real freq.

> unless the driver has a
> setpolicy function (I don't understand that condition).

That's because the core doesn't know the cached freq for setpolicy
drivers and so we need to call the ->get() callback. But for non
setpolicy drivers, core already has the cached value.

-- 
viresh