Re: [PATCH v2 1/4] PM: device: Introduce platform_resources_managed flag

[Maxime Ripard <mripard@kernel.org>]

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Hi,

On Thu, Apr 24, 2025 at 06:51:00PM +0200, Ulf Hansson wrote:
> On Thu, 17 Apr 2025 at 18:19, Michal Wilczynski
> <m.wilczynski@samsung.com> wrote:
> > On 4/16/25 16:48, Rafael J. Wysocki wrote:
> > > On Wed, Apr 16, 2025 at 3:32 PM Michal Wilczynski
> > > <m.wilczynski@samsung.com> wrote:
> > >>
> > >> On 4/15/25 18:42, Rafael J. Wysocki wrote:
> > >>> On Mon, Apr 14, 2025 at 8:53 PM Michal Wilczynski
> > >>> <m.wilczynski@samsung.com> wrote:
> > >>>>
> > >>>> Introduce a new dev_pm_info flag - platform_resources_managed, to
> > >>>> indicate whether platform PM resources such as clocks or resets are
> > >>>> managed externally (e.g. by a generic power domain driver) instead of
> > >>>> directly by the consumer device driver.
> > >>>
> > >>> I think that this is genpd-specific and so I don't think it belongs in
> > >>> struct dev_pm_info.
> > >>>
> > >>> There is dev->power.subsys_data->domain_data, why not use it for this?
> > >>
> > >> Hi Rafael,
> > >>
> > >> Thanks for the feedback.
> > >>
> > >> You're right — this behavior is specific to genpd, so embedding the flag
> > >> directly in struct dev_pm_info may not be the best choice. Using
> > >> dev->power.subsys_data->domain_data makes more sense and avoids bloating
> > >> the core PM structure.
> > >>
> > >>>
> > >>> Also, it should be documented way more comprehensively IMV.
> > >>>
> > >>> Who is supposed to set it and when?  What does it mean when it is set?
> > >>
> > >> To clarify the intended usage, I would propose adding the following
> > >> explanation to the commit message:
> > >>
> > >> "This flag is intended to be set by a generic PM domain driver (e.g.,
> > >> from within its attach_dev callback) to indicate that it will manage
> > >> platform specific runtime power management resources — such as clocks
> > >> and resets — on behalf of the consumer device. This implies a delegation
> > >> of runtime PM control to the PM domain, typically implemented through
> > >> its start and stop callbacks.
> > >>
> > >> When this flag is set, the consumer driver (e.g., drm/imagination) can
> > >> check it and skip managing such resources in its runtime PM callbacks
> > >> (runtime_suspend, runtime_resume), avoiding conflicts or redundant
> > >> operations."
> > >
> > > This sounds good and I would also put it into a code comment somewhere.
> > >
> > > I guess you'll need helpers for setting and testing this flag, so
> > > their kerneldoc comments can be used for that.
> > >
> > >> This could also be included as a code comment near the flag definition
> > >> if you think that’s appropriate.
> > >>
> > >> Also, as discussed earlier with Maxime and Matt [1], this is not about
> > >> full "resource ownership," but more about delegating runtime control of
> > >> PM resources like clocks/resets to the genpd. That nuance may be worth
> > >> reflecting in the flag name as well, I would rename it to let's say
> > >> 'runtime_pm_platform_res_delegated', or more concise
> > >> 'runtime_pm_delegated'.
> > >
> > > Or just "rpm_delegated" I suppose.
> > >
> > > But if the genpd driver is going to set that flag, it will rather mean
> > > that this driver will now control the resources in question, so the
> > > driver should not attempt to manipulate them directly.  Is my
> > > understanding correct?
> >
> > Yes, your understanding is correct — with one minor clarification.
> >
> > When the genpd driver sets the flag, it indicates that it will take over
> > control of the relevant PM resources in the context of runtime PM, i.e.,
> > via its start() and stop() callbacks. As a result, the device driver
> > should not manipulate those resources from within its RUNTIME_PM_OPS
> > (e.g., runtime_suspend, runtime_resume) to avoid conflicts.
> >
> > However, outside of the runtime PM callbacks, the consumer device driver
> > may still access or use those resources if needed e.g for devfreq.
> >
> > >
> > > Assuming that it is correct, how is the device driver going to know
> > > which resources in particular are now controlled by the genpd driver?
> >
> > Good question — to allow finer-grained control, we could replace the
> > current single boolean flag with a u32 bitmask field. Each bit would
> > correspond to a specific category of platform managed resources. For
> > example:
> >
> > #define RPM_TAKEOVER_CLK        BIT(0)
> > #define RPM_TAKEOVER_RESET      BIT(1)
> >
> > This would allow a PM domain driver to selectively declare which
> > resources it is taking over and let the consumer driver query only the
> > relevant parts.
> 
> Assuming we are targeting device specific resources for runtime PM;
> why would we want the driver to be responsible for some resources and
> the genpd provider for some others? I would assume we want to handle
> all these RPM-resources from the genpd provider, if/when possible,
> right?
> 
> The tricky part though (maybe Stephen had some ideas in his talk [a]
> at OSS), is to teach the genpd provider about what resources it should
> handle. In principle the genpd provider will need some kind of device
> specific knowledge, perhaps based on the device's compatible-string
> and description in DT.
> 
> My point is, using a bitmask doesn't scale as it would end up having
> one bit for each clock (a device may have multiple clocks), regulator,
> pinctrl, phy, etc. In principle, reflecting the description in DT.

My understanding is that it's to address a situation where a "generic"
driver interacts with some platform specific code. I think it's tied to
the discussion with the imagination GPU driver handling his clocks, and
the platform genpd clocks overlapping a bit.

But then, my question is: does it matter? clocks are refcounted, and
resets are as well iirc, so why do we need a transition at all? Can't we
just let the platform genpd code take a reference on the clock, the GPU
driver take one as well, and it's all good, right?

Maxime

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