linux-kernel - Re: [PATCH 1/4] cpufreq: qcom-nvmem: Enable virtual power domain devices

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Message-ID: <CAPDyKFpwZdx=vyuAZSv1WGYCyiohfnt87LM1jw=fhKsF5Ks1Yw@mail.gmail.com>
Date:   Tue, 17 Oct 2023 23:50:21 +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 1/4] cpufreq: qcom-nvmem: Enable virtual power domain devices

[...]

> >
> > *) The pm_runtime_resume_and_get() works for QCS404 as a fix. It also
> > works fine when there is only one RPMPD that manages the performance
> > scaling.
> >
>
> Agreed.
>
> > **) In cases where we have multiple PM domains to scale performance
> > for, using pm_runtime_resume_and_get() would work fine too. Possibly
> > we want to use device_link_add() to set up suppliers, to avoid calling
> > pm_runtime_resume_and_get() for each and every device.
> >
>
> Hm. What would you use as "supplied" device? The CPU device I guess?

The consumer would be the device that is used to probe the cpureq
driver and the supplier(s) the virtual devices returned from genpd
when attaching.

>
> I'm looking again at my old patch from 2020 where I implemented this
> with device links in the OPP core. Seems like you suggested this back
> then too :)
>
>   https://lore.kernel.org/linux-pm/20200826093328.88268-1-stephan@gerhold.net/
>
> However, for the special case of the CPU I think we don't gain any code
> simplification from using device links. There will just be a single
> resume of each virtual genpd device, as well as one put during remove().
> Exactly the same applies when using device links, we need to set up the
> device links once for each virtual genpd device, and clean them up again
> during remove().
>
> Or can you think of another advantage of using device links?

No, not at this point.

So, in this particular case it may not matter that much. But when the
number of PM domains starts to vary between platforms it could be a
nice way to abstract some logic. I guess starting without using
device-links and seeing how it evolves could be a way forward too.

>
> > ***) Due to the above, we don't need a new mechanism to avoid
> > "caching" performance states for genpd. At least for the time being.
> >
>
> Right. Given *) and **) I'll prepare a v2 of $subject patch with the
> remove() cleanup fixed and an improved commit description.
>
> I'll wait for a bit in case you have more thoughts about the device
> links.

One more thing though that crossed my mind. In the rpmpd case, is
there anything we need to care about during system suspend/resume that
isn't already taken care of correctly?

Kind regards
Uffe