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

[Ulf Hansson]

On Thu, 19 Oct 2023 at 12:24, Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote:
>
> On Wed, 18 Oct 2023 at 10:06, Stephan Gerhold
> <stephan.gerhold@xxxxxxxxxxxxxxx> 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