linux-kernel - Re: [PATCH v13 06/35] clk: tegra: Support runtime PM and power domain

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Date:   Thu, 7 Oct 2021 01:03:24 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Viresh Kumar <vireshk@...nel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Peter De Schrijver <pdeschrijver@...dia.com>,
        Mikko Perttunen <mperttunen@...dia.com>,
        Peter Chen <peter.chen@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        Nishanth Menon <nm@...com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Michael Turquette <mturquette@...libre.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-tegra <linux-tegra@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux USB List <linux-usb@...r.kernel.org>,
        linux-staging@...ts.linux.dev, linux-pwm@...r.kernel.org,
        linux-mmc <linux-mmc@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        DTML <devicetree@...r.kernel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        Vignesh Raghavendra <vigneshr@...com>,
        Richard Weinberger <richard@....at>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Lucas Stach <dev@...xeye.de>, Stefan Agner <stefan@...er.ch>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        David Heidelberg <david@...t.cz>
Subject: Re: [PATCH v13 06/35] clk: tegra: Support runtime PM and power domain

06.10.2021 15:38, Ulf Hansson пишет:
>>> Right, so the PM domain managed in tegra_genpd_power_on|off() can
>>> still be powered on/off, as long as the clock remains ungated?
>> Not ungated, but prepared.
> Okay, thanks for clarifying!
> 
> In summary, it sounds like you should be able to fix this problem in
> the I2C driver as I suggested above. If that works, that seems much
> better.

I'll try this variant, thank you.

> Moreover, it would leave the clocks gated/unprepared when the system
> is fully suspended, which I guess is better from an energy point of
> view?

The clocks are kept gated, it wasn't a problem. The problem was that
clocks were needed to be enabled temporarily. In order to enable a
clock, it needs to be prepared first. When clock is prepared, it resumes
clock's device RPM.

Keeping clocks prepared shouldn't make a noticeable difference from the
energy POV since clocks are gated. It's only voltage that is kept high,
but we need to keep it high during suspend anyways in order to resume
successfully. The hardware is mostly gated during suspend, depending on
suspend mode, so the power consumption difference is negligible. At
least I haven't seen any problems, battery doesn't drain during suspend.