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

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Message-ID: <20200714034635.2zdv3wzmftjg2t4a@vireshk-i7>
Date:   Tue, 14 Jul 2020 09:16:35 +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 13-07-20, 17:37, Jon Hunter wrote:
> 
> On 13/07/2020 04:25, Viresh Kumar wrote:
> > On 12-07-20, 11:06, Jon Hunter wrote:
> >> Commit 6cc3d0e9a097 ("cpufreq: tegra186: add
> >> CPUFREQ_NEED_INITIAL_FREQ_CHECK flag") fixed CPUFREQ support for
> >> Tegra186 but as a consequence the following warnings are now seen on
> >> boot ...
> >>
> >>  cpufreq: cpufreq_online: CPU0: Running at unlisted freq: 0 KHz
> >>  cpufreq: cpufreq_online: CPU0: Unlisted initial frequency changed to: 2035200 KHz
> >>  cpufreq: cpufreq_online: CPU1: Running at unlisted freq: 0 KHz
> >>  cpufreq: cpufreq_online: CPU1: Unlisted initial frequency changed to: 2035200 KHz
> >>  cpufreq: cpufreq_online: CPU2: Running at unlisted freq: 0 KHz
> >>  cpufreq: cpufreq_online: CPU2: Unlisted initial frequency changed to: 2035200 KHz
> >>  cpufreq: cpufreq_online: CPU3: Running at unlisted freq: 0 KHz
> >>  cpufreq: cpufreq_online: CPU3: Unlisted initial frequency changed to: 2035200 KHz
> >>  cpufreq: cpufreq_online: CPU4: Running at unlisted freq: 0 KHz
> >>  cpufreq: cpufreq_online: CPU4: Unlisted initial frequency changed to: 2035200 KHz
> >>  cpufreq: cpufreq_online: CPU5: Running at unlisted freq: 0 KHz
> >>  cpufreq: cpufreq_online: CPU5: Unlisted initial frequency changed to: 2035200 KHz
> >>
> >> Although we could fix this by adding a 'get' operator for the Tegra186
> >> CPUFREQ driver, there is really little point because the CPUFREQ on
> >> Tegra186 is set by writing a value stored in the frequency table to a
> >> register and we just need to set the initial frequency.
> > 
> > The hardware still runs at the frequency requested by cpufreq core here, right ?
> 
> Yes.
> 
> > It is better to provide the get() callback as it is also used to show the
> > current frequency in userspace.
> 
> I looked at that and I saw that if the get() callback is not provided,
> the current frequency showed by userspace is policy->cur. For this
> device, policy->cur is accurate and so if we added the get() callback we
> essentially just going to return policy->cur. Therefore, given that we
> already know policy->cur, I did not see the point in adding a device
> specific handler to do the same thing.

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.

-- 
viresh