linux-kernel - Re: [PATCH 1/2] cpufreq: tegra186: Fix initial frequency

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Date:   Tue, 4 Aug 2020 10:55:09 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Jon Hunter <jonathanh@...dia.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] cpufreq: tegra186: Fix initial frequency

On 31-07-20, 13:14, Jon Hunter wrote:
> I have been doing some more testing on Tegra, I noticed that when
> reading the current CPU frequency via the sysfs scaling_cur_freq entry,
> this always returns the cached value (at least for Tegra). Looking at
> the implementation of scaling_cur_freq I see ...
> 
> static ssize_t show_scaling_cur_freq(struct cpufreq_policy *policy, char *buf)
> {
>         ssize_t ret; 
>         unsigned int freq;
> 
>         freq = arch_freq_get_on_cpu(policy->cpu);
>         if (freq)
>                 ret = sprintf(buf, "%u\n", freq);
>         else if (cpufreq_driver && cpufreq_driver->setpolicy &&
>                         cpufreq_driver->get)
>                 ret = sprintf(buf, "%u\n", cpufreq_driver->get(policy->cpu));
>         else
>                 ret = sprintf(buf, "%u\n", policy->cur);
>         return ret; 
> }
> 
> The various Tegra CPU frequency drivers do not implement the
> set_policy callback and hence why we always get the cached value. I
> see the following commit added this and before it simply return the
> cached value ...
> 
> commit c034b02e213d271b98c45c4a7b54af8f69aaac1e
> Author: Dirk Brandewie <dirk.j.brandewie@...el.com>
> Date:   Mon Oct 13 08:37:40 2014 -0700
> 
>     cpufreq: expose scaling_cur_freq sysfs file for set_policy() drivers
> 
> Is this intentional? 

Yes, it is.

There are two sysfs files to read the current frequency.

- scaling_cur_freq: as you noticed it returns cached value unless it is for
  setpolicy drivers in whose case cpufreq core doesn't control the frequency and
  so doesn't cache any values.

- cpuinfo_cur_freq: This will return the value as read from hardware using
  ->get() callback.

-- 
viresh