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

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Date:   Tue, 8 Jun 2021 10:23:25 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     "Rafael J . Wysocki" <rjw@...ysocki.net>, 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-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] PM: runtime: Clarify documentation when callbacks
 are unassigned

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.

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.

Alan Stern