linux-kernel - Re: [PATCH v2 2/3] cpufreq: qcom-nvmem: Enable virtual power domain devices

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Date:   Fri, 20 Oct 2023 12:20:11 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Stephan Gerhold <stephan@...hold.net>
Cc:     Stephan Gerhold <stephan.gerhold@...nkonzept.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Ilia Lin <ilia.lin@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>, linux-pm@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 2/3] cpufreq: qcom-nvmem: Enable virtual power domain devices

On Thu, 19 Oct 2023 at 19:08, Stephan Gerhold <stephan@...hold.net> wrote:
>
> On Thu, Oct 19, 2023 at 05:19:53PM +0200, Ulf Hansson wrote:
> > On Thu, 19 Oct 2023 at 16:49, Stephan Gerhold <stephan@...hold.net> wrote:
> > > On Thu, Oct 19, 2023 at 04:12:56PM +0200, Ulf Hansson wrote:
> > > > On Thu, 19 Oct 2023 at 15:05, Stephan Gerhold <stephan@...hold.net> wrote:
> > > > > On Thu, Oct 19, 2023 at 01:26:19PM +0200, Ulf Hansson wrote:
> > > > > > BTW, if you really need something like the above, the proper way to do
> > > > > > it would instead be to call device_set_awake_path() for the device.
> > > > > >
> > > > > > This informs genpd that the device needs to stay powered-on during
> > > > > > system suspend (assuming that GENPD_FLAG_ACTIVE_WAKEUP has been set
> > > > > > for it), hence it will keep the corresponding PM domain powered-on
> > > > > > too.
> > > > >
> > > > > Thanks, I can try if this works as alternative to the
> > > > > dev_pm_syscore_device()!
> > > >
> > > > Yes, please. We don't want to abuse the dev_pm_syscore_device() thingy.
> > >
> > > Could you clarify the idea behind GENPD_FLAG_ACTIVE_WAKEUP? Would I set
> > > it conditionally for all RPMPDs or just the ones consumed by the CPU?
> > > How does the genpd *provider* know if one of its *consumer* devices
> > > needs to have its power domain kept on for wakeup?
> >
> > We are thinking of the GENPD_FLAG_ACTIVE_WAKEUP as a platform
> > configuration type of flag for the genpd in question. The consumer
> > driver shouldn't need to know about the details of what is happening
> > on the PM domain level - only whether it needs its device to remain
> > powered-on during system suspend or not.
> >
>
> Thanks! I will test if this works for RPMPD and post new versions of the
> patches. By coincidence I think this flag might actually be useful as
> temporary solution for CPR. If I:
>
>  1. Change $subject patch to use device_set_awake_path() instead, and
>  2. Set GENPD_FLAG_ACTIVE_WAKEUP for the RPMPD genpds, but
>  3. Do *not* set GENPD_FLAG_ACTIVE_WAKEUP for the CPR genpd.
>
> Then the genpd ->power_on|off() callbacks should still be called
> for CPR during system suspend, right? :D

Yes, correct, that should work fine!

>
> > I suspect that the GENPD_FLAG_ACTIVE_WAKEUP is probably okay to set
> > for most genpds, but there may be some exceptions.
> >
>
> Out of curiosity, do you have an example for such an exception where
> GENPD_FLAG_ACTIVE_WAKEUP shouldn't be set, aside from workarounds like
> I just described?
>
> As you said, the consumer device should just say that it wants to stay
> powered for wakeup during suspend. But if its power domains get powered
> off, I would expect that to break. How could a genpd driver still
> provide power without being powered on? Wouldn't that rather be a low
> performance state?

I think this boils down to how the power-rail that the genpd manages,
is handled by the platform during system suspend.

In principle there could be some other separate logic that helps a
FW/PMIC to understand whether it needs to keep the power-rail on or
not - no matter whether the genpd releases its vote for it during
system suspend.

This becomes mostly hypothetical, but clearly there are a lot of
genpd/platforms that don't use GENPD_FLAG_ACTIVE_WAKEUP too. If those
are just mistakes or just not needed, I don't actually know.

Kind regards
Uffe