linux-kernel - Re: [PATCH v4 3/3] PM: runtime: Clarify documentation when callbacks are unassigned

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Date:   Wed, 9 Jun 2021 10:16:23 -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 v4 3/3] PM: runtime: Clarify documentation when callbacks
 are unassigned

On Wed, Jun 09, 2021 at 12:06:10PM +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 v4:
>         - This time, really, fix spelling and further clarified the behaviour,
> 	according to comments from Alan.
> 
> ---
>  Documentation/power/runtime_pm.rst | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/Documentation/power/runtime_pm.rst b/Documentation/power/runtime_pm.rst
> index 18ae21bf7f92..8a0a43811e3a 100644
> --- a/Documentation/power/runtime_pm.rst
> +++ b/Documentation/power/runtime_pm.rst
> @@ -827,6 +827,15 @@ 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 managed
> +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. More precisely, if a callback pointer is NULL, the PM core will act
> +as though there was a callback and it returned 0.
> +
>  9. Autosuspend, or automatically-delayed suspends
>  =================================================

Acked-by: Alan Stern <stern@...land.harvard.edu>

I don't know what happened before.  Maybe the terminal window got 
resized without me noticing, so the lines looked too long when they 
actually weren't.

Alan Stern