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

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Message-ID: <ZTFiXJ2XO4WQN_gu@gerhold.net>
Date:   Thu, 19 Oct 2023 19:07:40 +0200
From:   Stephan Gerhold <stephan@...hold.net>
To:     Ulf Hansson <ulf.hansson@...aro.org>
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, 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

> 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?

Thanks,
Stephan