Re: [PATCH v2 0/4] PM: domains: Avoid boilerplate code for DVFS in subsystem/drivers

[Ulf Hansson]

On Thu, 3 Jun 2021 at 13:13, Stephan Gerhold <stephan@xxxxxxxxxxx> wrote:
>
> On Thu, Jun 03, 2021 at 11:34:34AM +0200, Ulf Hansson wrote:
> > Various discussions on LKML have pointed out that many subsystem/drivers for
> > devices that may be attached to a genpd and which manages DVFS/OPP though the
> > genpd performance states, would need very similar updates.
> >
> > More precisely, they would likely have to call dev_pm_opp_set_rate|opp() to
> > drop and restore OPPs (which propagates upwards into performance states votes
> > in genpd), every time their devices should enter/exit a low power state, via
> > their device PM callbacks.
> >
> > Rather than having to add the boilerplate code for these things into the
> > subsystems/drivers, this series implements the logic internally into genpd.
> >
> > Concerns have been raised about this approach, mostly by myself, around that it
> > limits flexibility. On the other hand, it starts to look like more and more
> > people are requesting this to be manged internally in genpd, for good reasons.
> > So, I think it's worth to give this a try.
> >
> > In the long run, if it turns out that the flexibility was indeed needed, we can
> > always deal with that as special cases on top.
> >
>
> Do I understand your patch set correctly that you basically make the
> performance state votes conditional to the "power-on" vote of the device
> (which is automatically toggled during runtime/system PM)?

The series can be considered as a step in that direction, but no, this
series doesn't change that behaviour.

Users of dev_pm_genpd_set_performance_state() are still free to set a
performance state, orthogonally to whether the PM domain is powered on
or off.

>
> If yes, I think that's a good thing. It was always really confusing to me
> that a device can make performance state votes if it doesn't actually
> want the power domain to be powered on.

I share your view, it's a bit confusing.

Just adding the condition internally to genpd to prevent the caller of
dev_pm_genpd_set_performance() from succeeding to set a new state,
unless the genpd is powered on, should be a rather simple thing to
add.

However, to change this, we first need to double check that all the
callers are making sure they have turned on the PM domain (typically
via runtime PM).

>
> What happens if a driver calls dev_pm_genpd_set_performance_state(...)
> while the device is suspended? Will that mess up the performance state
> when the device resumes?

Good question. The idea is:

If genpd in genpd_runtime_suspend() are able to drop an existing vote
for a performance state, it should restore the vote in
genpd_runtime_resume(). This also means, if there is no vote to drop
in genpd_runtime_suspend(), genpd should just leave the vote as is in
genpd_runtime_resume().

When it comes to the system suspend/resume path, being implemented in
patch4, we should probably defer that patch from being merged. It
turned out that we probably need to think more about that approach.

>
> I think this might also go into the direction of my problem with the OPP
> core for CPU DVFS [1] since the OPP core currently does not "power-on"
> the power domains, it just sets a performance state. I got kind of stuck
> with all the complexity of power domains in Linux so I think we never
> solved that.

Hmm, that issue is in a way related.

Although, if I understand correctly, that was rather about at what
layer it makes best sense to activate the device (from runtime PM
point of view). And this was needed due to the fact that the
corresponding genpd provider, requires the PM domain to be power on to
allow changing a performance state for it. Did I get that correct?

>
> Stephan
>
> [1]: https://lore.kernel.org/linux-pm/20200826093328.88268-1-stephan@xxxxxxxxxxx/

Kind regards
Uffe