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

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Date:   Fri, 31 Jul 2020 13:14:09 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
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

Hi Viresh,

On 14/07/2020 04:46, Viresh Kumar wrote:

...

> The get() callback is supposed to read the frequency from hardware and
> return it, no cached value here. policy->cur may end up being wrong in
> case there is a bug.

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? 

Cheers
Jon

-- 
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