linux-kernel - Re: [PATCH v2 4/4] PM: domains: Drop/restore performance state votes for devices at system PM

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Message-ID: <YL9oMVqox7GVASen@gerhold.net>
Date:   Tue, 8 Jun 2021 14:53:05 +0200
From:   Stephan Gerhold <stephan@...hold.net>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Dmitry Osipenko <digetx@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Roja Rani Yarubandi <rojay@...eaurora.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Rajendra Nayak <rnayak@...eaurora.org>
Subject: Re: [PATCH v2 4/4] PM: domains: Drop/restore performance state votes
 for devices at system PM

Hi,

On Thu, Jun 03, 2021 at 12:20:57PM +0200, Ulf Hansson wrote:
> + Mark Brown, Dmitry Baryshkov
> 
> On Thu, 3 Jun 2021 at 11:34, Ulf Hansson <ulf.hansson@...aro.org> wrote:
> >
> > Recent changes in genpd drops and restore performance state votes for
> > devices during runtime PM.
> >
> > For the similar reasons, but to avoid the same kind of boilerplate code in
> > device PM callbacks for system sleep in subsystems/drivers, let's drop and
> > restore performance states votes in genpd for the attached devices during
> > system sleep.
> >
> > Signed-off-by: Ulf Hansson <ulf.hansson@...aro.org>
> 
> After a second thought, it looks like we maybe should defer to apply
> this final patch of the series. At least until we figured out how to
> address the below issue:
> 
> So, I noticed that we have things like "regulator-fixed-domain", that
> uses "required-opps" to enable/disable a regulator through the
> dev_pm_set_performance_state() interface.

Not directly related to your concern, but related to another discussion
we had recently: To me, this looks mostly like another solution for
voting for performance states without doing full DVFS, also known as
assigned-performance-states [1] or required-opps on devices [2]. :)

It's just wrapped in a regulator interface here. Actually, if we
implement [2], the regulator-fixed-domain should mostly just become some
sort of simple wrapper around runtime PM for the regulator device, since
the required-opp might be applied automatically then.

[1]: https://lore.kernel.org/linux-arm-msm/1622095949-2014-1-git-send-email-rnayak@codeaurora.org/
[2]: https://lore.kernel.org/linux-arm-msm/YLYV3ov%2FiBffZMg4@gerhold.net/

> We likely don't want to drop the performance state internally in genpd
> when genpd_suspend_noirq() gets called, for the corresponding struct
> device for the regulator.
> 

So your concern is that the performance state is dropped during suspend
even though the regulator core thinks the regulator stays enabled?

I played with regulator-fixed-domain a bit and I would say this is
already broken (unless you rely on one of the side effects I mentioned
in [3]). The power domain gets powered off entirely during system
suspend, and then the performance state won't have any effect either.

I guess we would need some way to say that this device should only be
managed through runtime PM and never automatically suspended during
system suspend?

Stephan

[3]: https://lore.kernel.org/linux-pm/YLkOAyydZMnxkEy+@gerhold.net/