linux-kernel - Re: [PATCH v1] soc/tegra: pmc: Query PCLK clock rate at probe time

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Message-ID: <20190725094125.GL12715@pdeschrijver-desktop.Nvidia.com>
Date:   Thu, 25 Jul 2019 12:41:25 +0300
From:   Peter De Schrijver <pdeschrijver@...dia.com>
To:     Jon Hunter <jonathanh@...dia.com>
CC:     Dmitry Osipenko <digetx@...il.com>,
        Thierry Reding <thierry.reding@...il.com>,
        <linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] soc/tegra: pmc: Query PCLK clock rate at probe time

On Thu, Jul 18, 2019 at 10:45:31AM +0100, Jon Hunter wrote:
> 
> On 08/07/2019 00:08, Dmitry Osipenko wrote:
> > The PCLK clock is running off SCLK, which is a critical clock that is
> > very unlikely to randomly change its rate. It's also a bit clumsy (and
> > apparently incorrect) to query the clock's rate with interrupts being
> > disabled because clk_get_rate() takes a mutex and that's the case during
> > suspend/cpuidle entering. Lastly, it's better to always fully reprogram
> > PMC state because it's not obvious whether it could be changed after SC7.
> 
> I agree with the first part, but I would drop the last sentence because
> I see no evidence of this. Maybe Peter can confirm.
> 

SCLK and PCLK certainly can change rate at runtime, although the changes
for this haven't made it upstream yet. It's indeed not very obvious, but
certainly doable.

Peter.