linux-kernel - Re: [PATCH V2] cpufreq: tegra186: Fix initial frequency

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Date:   Wed, 28 Oct 2020 20:22:26 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Jon Hunter <jonathanh@...dia.com>
Cc:     "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Thierry Reding <thierry.reding@...il.com>,
        linux-pm@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] cpufreq: tegra186: Fix initial frequency

On 28-10-20, 12:31, Jon Hunter wrote:
> On 28/10/2020 04:11, Viresh Kumar wrote:
> > When do we fail if the frequency isn't known ? That's the case where
> > we try to set it to one from the table.
> 
> Currently, if the frequency is not known, we fail right before we do the
> initial frequency check [0].

Right, so the frequency returned there is 0. By unknown I assumed that
you are talking about the case where the frequency isn't found in the
table.

> > But (looking at your change), ->get() can't really return 0. We depend
> > on it to get us the exact frequency the hardware is programmed at
> > instead of reading a cached value in the software.
> 
> Actually it can and it does currently. Note in tegra186_cpufreq_get()
> the variable 'freq' is initialised to 0, and if no match is found, then
> it returns 0. This is what happens currently on some Tegra186 boards.

Then there is a problem with the implementation of this helper in your
case. This shouldn't try to go compare the value read from the
register with the table, rather convert that directly to a meaningful
frequency. Its like reading registers and then doing some computations
with the values read with the parent PLLs frequency. Something like
what you will normally do in the implementation of clk_get_rate().

-- 
viresh