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

From: Ulf Hansson <ulf.hansson@linaro.org>
To: Stephan Gerhold <stephan.gerhold@kernkonzept.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>,
	Andy Gross <agross@kernel.org>,
	Bjorn Andersson <andersson@kernel.org>,
	Konrad Dybcio <konrad.dybcio@linaro.org>,
	Ilia Lin <ilia.lin@kernel.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Rob Herring <robh+dt@kernel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	Conor Dooley <conor+dt@kernel.org>,
	linux-pm@vger.kernel.org, linux-arm-msm@vger.kernel.org,
	linux-kernel@vger.kernel.org, devicetree@vger.kernel.org,
	Stephan Gerhold <stephan@gerhold.net>,
	stable@vger.kernel.org
Subject: Re: [PATCH v2 2/3] cpufreq: qcom-nvmem: Enable virtual power domain devices
Date: Thu, 19 Oct 2023 13:26:19 +0200	[thread overview]
Message-ID: <CAPDyKFr5A-P=UhWs4rUMBWup3pH75WAhcZ56Y2_Sfk3=WfxRCQ@mail.gmail.com> (raw)
In-Reply-To: <CAPDyKFot9=M1ooP_Q1AOgG5o_4DTQ2qsyai1ZdXAzBwf89W4uA@mail.gmail.com>

On Thu, 19 Oct 2023 at 12:24, Ulf Hansson <ulf.hansson@linaro.org> wrote:
>
> On Wed, 18 Oct 2023 at 10:06, Stephan Gerhold
> <stephan.gerhold@kernkonzept.com> wrote:
> >
> > The genpd core caches performance state votes from devices that are
> > runtime suspended as of commit 3c5a272202c2 ("PM: domains: Improve
> > runtime PM performance state handling"). They get applied once the
> > device becomes active again.
> >
> > To attach the power domains needed by qcom-cpufreq-nvmem the OPP core
> > calls genpd_dev_pm_attach_by_id(). This results in "virtual" dummy
> > devices that use runtime PM only to control the enable and performance
> > state for the attached power domain.
> >
> > However, at the moment nothing ever resumes the virtual devices created
> > for qcom-cpufreq-nvmem. They remain permanently runtime suspended. This
> > means that performance state votes made during cpufreq scaling get
> > always cached and never applied to the hardware.
> >
> > Fix this by enabling the devices after attaching them and use
> > dev_pm_syscore_device() to ensure the power domains also stay on when
> > going to suspend. Since it supplies the CPU we can never turn it off
> > from Linux. There are other mechanisms to turn it off when needed,
> > usually in the RPM firmware (RPMPD) or the cpuidle path (CPR genpd).
>
> I believe we discussed using dev_pm_syscore_device() for the previous
> version. It's not intended to be used for things like the above.
>
> Moreover, I was under the impression that it wasn't really needed. In
> fact, I would think that this actually breaks things for system
> suspend/resume, as in this case the cpr driver's genpd
> ->power_on|off() callbacks are no longer getting called due this,
> which means that the cpr state machine isn't going to be restored
> properly. Or did I get this wrong?

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.

[...]

Kind regards
Uffe