Re: [PATCH v2 0/3] PM: runtime: Update behaviour for no callbacks

From: Alan Stern
Date: Tue Jun 08 2021 - 10:25:01 EST

On Tue, Jun 08, 2021 at 11:02:47AM +0200, Ulf Hansson wrote:
> While reviewing a patch on the mmc-list, I ended up inspecting the behaviour of
> how we deal with the no callback case for runtime PM.
> A couple of observations:
> *) When pm_runtime_no_callbacks() have been called, it allows the PM core to
> takes a quicker path, but at the same time, consumer/supplier device links are
> being skipped in rpm_resume|suspend().
> **) Calling pm_runtime_no_callbacks() to avoid boiler plate code (assigning
> empty functions to ->runtime_suspend|resume()), doesn't work if there could be
> consumer/supplier device link being used or a platform dependent PM domain that
> could get attached to the device.
> Therefore, this series suggests to change the behaviour in the PM core, to
> allow the ->runtime_suspend|resume() callbacks to be unassigned. This is already
> supported for ->runtime_idle() callbacks, so it would also move things into a
> more consistent behaviour.
> I have looked at various error paths, in the kernel of callers of
> pm_runtime_get_sync(). I couldn't find anyone that made sense, that looked for
> the special error code, -ENOSYS, which is the error code getting returned when a
> callback is missing. Whether that is sufficient proof that these changes are
> 100% safe, I can't guarantee, but I think it would be worth a try as the
> benefits of avoiding boilerplate code and the corresponding additional code
> paths are quite nice, if you ask me.

In principle I have no objection to these changes. It's likely
that if any problems do crop up, we'll be able to fix them pretty
easily. For patches 1 and 2:

Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>

Alan Stern