linux-kernel - Re: [PATCH v3 2/2] PM / Runtime: Add pm_runtime_enable

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Date:	Fri, 3 Jul 2015 11:11:19 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
cc:	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Kevin Hilman <khilman@...aro.org>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] PM / Runtime: Add pm_runtime_enable_recursive

On Fri, 3 Jul 2015, Tomeu Vizoso wrote:

> >> Yeah, that would remove the need for messing with the runtime PM
> >> enable status of descendant devices, but I wonder why Rafael went that
> >> way initially.
> >
> > I forget the details.  Probably it was just to be safe.  We probably
> > thought that if a device was disabled for runtime PM then its runtime
> > PM status might not be accurate.  But if direct_complete is set then it
> > may be reasonable to assume that the runtime PM status _is_ accurate.
> 
> Cool.

> > Rafael and I briefly discussed ignore_children while the original
> > direct_complete patch was being designed.  We didn't come to any
> > definite conclusion and decided to forget about it for the time being.
> > Maybe now would be a good time to reconsider it.
> 
> I would prefer to have ignore_children ignore whether the children of
> a device were able to do direct_complete, rather than having a
> direct_complete_default flag (plus not requiring that all its
> descendants have runtime PM enabled).

Okay, but remember that sometimes these "virtual" devices will exist 
beneath a device that needs to have ignore_children off.  So this won't 
be a complete solution to your problem.

Let's see what Rafael thinks about these two issues.  It seems to me
that the hardest part is dealing with drivers/subsystems that have no
runtime PM support.  In such cases, we have to be very careful not to
use direct_complete unless we know that the device does no power 
management at all.

Alan Stern

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