linux-kernel - Re: [PATCH v2 3/3] PM: runtime: Clarify documentation when callbacks are unassigned

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Date:   Tue, 8 Jun 2021 16:30:48 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Alan Stern <stern@...land.harvard.edu>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        Saravana Kannan <saravanak@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Tony Lindgren <tony@...mide.com>,
        Kevin Hilman <khilman@...nel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] PM: runtime: Clarify documentation when callbacks
 are unassigned

On Tue, 8 Jun 2021 at 16:23, Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Tue, Jun 08, 2021 at 11:02:50AM +0200, Ulf Hansson wrote:
> > Recent changes to the PM core allows ->runtime_suspend|resume callbacks to
> > be unassigned.
> >
> > In the earlier behaviour the PM core would return -ENOSYS, when trying to
> > runtime resume a device, for example. Let's update the documentation to
> > clarify this.
> >
> > Signed-off-by: Ulf Hansson <ulf.hansson@...aro.org>
> > ---
> >
> > Changes in v2:
> >       - Added a new patch for the updating the docs, as pointed out by Alan.
> >
> > ---
> >  Documentation/power/runtime_pm.rst | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/Documentation/power/runtime_pm.rst b/Documentation/power/runtime_pm.rst
> > index 18ae21bf7f92..3d09c9fd450d 100644
> > --- a/Documentation/power/runtime_pm.rst
> > +++ b/Documentation/power/runtime_pm.rst
> > @@ -827,6 +827,14 @@ or driver about runtime power changes.  Instead, the driver for the device's
> >  parent must take responsibility for telling the device's driver when the
> >  parent's power state changes.
> >
> > +Note that, in some cases it may not be desirable for subsystems/drivers to call
> > +pm_runtime_no_callbacks() for their devices. This could be because a subset of
> > +the runtime PM callbacks needs to be implemented, a platform dependent PM
> > +domain could get attached to the device or that the device is power manged
> > +through a supplier device link. For these reasons and to avoid boilerplate code
> > +in subsystems/drivers, the PM core allows runtime PM callbacks to be
> > +unassigned.
> > +
>
> You should also mention that if a callback pointer is NULL, the
> runtime PM core will act as though there was a callback and it
> returned 0.  That's an important consideration.

Good point, let me add it.

I send a new version of $subject patch, unless Rafael is happy to do
the amending when/if applying?

>
> Also, notice that this file was carefully edited to make sure that
> none of the lines exceed 80 characters.  Your new addition should
> be the same.

Absolutely, but it should be okay already, no?

>
> Alan Stern

Kind regards
Uffe