linux-kernel - Re: [PATCH v8 01/34] opp: Add dev_pm_opp

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Date:   Wed, 18 Aug 2021 08:21:30 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        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>,
        Mark Brown <broonie@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        Nishanth Menon <nm@...com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Richard Weinberger <richard@....at>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Lucas Stach <dev@...xeye.de>, Stefan Agner <stefan@...er.ch>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        linux-kernel@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-pm@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-staging@...ts.linux.dev, linux-spi@...r.kernel.org,
        linux-pwm@...r.kernel.org, linux-mtd@...ts.infradead.org,
        linux-mmc@...r.kernel.org, linux-media@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
        linux-clk@...r.kernel.org
Subject: Re: [PATCH v8 01/34] opp: Add dev_pm_opp_sync() helper

18.08.2021 07:53, Viresh Kumar пишет:
> On 18-08-21, 07:37, Dmitry Osipenko wrote:
>> This will set voltage level without having an actively used hardware.
>> Take a 3d driver for example, if you set the rate on probe and
>> rpm-resume will never be called, then the voltage will be set high,
>> while hardware is kept suspended if userspace will never wake it up by
>> executing a 3d job.
> 
> What exactly are we looking to achieve with this stuff ? Cache the
> current performance state with genpd (based on the state bootloader's
> has set) ?

Yes, GENPD will cache the perf state across suspend/resume and initially
cached value is out of sync with h/w.

> Or anything else as well ?

Nothing else. But let me clarify it all again.

Initially the performance state of all GENPDs is 0 for all devices.

The clock rate is preinitialized for all devices to a some default rate
by clk driver, or by bootloader or by assigned-clocks in DT.

When device is rpm-resumed, the resume callback of a device driver
enables the clock.

Before clock is enabled, the voltage needs to be configured in
accordance to the clk rate.

So now we have a GENPD with pstate=0 on a first rpm-resume, which
doesn't match the h/w configuration. Calling dev_pm_opp_sync() sets the
pstate in accordance to the h/w config.

In a previous v7 I proposed to preset the rpm_pstate of GENPD (perf
level that is restored before device is rpm-resumed) from PD's
attach_dev callback, but Ulf didn't like that because it requires to use
and modify GENPD 'private' variables from a PD driver. We decided that
will be better to make device drivers to explicitly sync the perf state,
which I implemented in this v8.