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

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Date:	Fri, 3 Jul 2015 10:16:19 -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:

> On 2 July 2015 at 17:21, Alan Stern <stern@...land.harvard.edu> wrote:
> > On Thu, 2 Jul 2015, Tomeu Vizoso wrote:
> >
> >> > Just because these sub-devices are virtual, it doesn't mean you can
> >> > ignore the way they interact with runtime PM.
> >>
> >> Fair enough, but then, how are we expected to be able to use the
> >> direct_complete facility if the core bails out if a descendant doesn't
> >> have runtime PM enabled?
> >>
> >> > In the case of ep_87 this doesn't matter.  Endpoint devices (like all
> >> > devices) are in the SUSPENDED state by default when they are created,
> >> > and they never leave that state.
> >>
> >> I don't see why it doesn't matter for endpoints or the others. They
> >> don't have runtime PM enabled, so no ancestor will be able to do
> >> direct_complete.
> >
> > Ah, you're concerned about these lines near the start of
> > __device_suspend():
> >
> >         if (dev->power.direct_complete) {
> >                 if (pm_runtime_status_suspended(dev)) {
> >                         pm_runtime_disable(dev);
> >                         if (pm_runtime_suspended_if_enabled(dev))
> >                                 goto Complete;
> >
> >                         pm_runtime_enable(dev);
> >                 }
> >                 dev->power.direct_complete = false;
> >         }
> >
> > Perhaps the pm_runtime_suspended_if_enabled() test should be changed to
> > pm_runtime_status_suspended().  Then it won't matter whether the
> > descendant devices are enabled for runtime PM.
> 
> 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.

> >> > A possible way around the problem is to use pm_suspend_ignore_children
> >> > on the uvcvideo interface.  But I'm not sure that would be the right
> >> > thing to do.
> >>
> >> Would that mean that if a device has ignore_children then it could
> >> still do direct_complete even if its descendants weren't able to?
> >
> > I think we could justify that.  The ignore_children flag means we can
> > communicate with the children even when the device is in runtime
> > suspend, so there's no reason to force the device to leave runtime
> > suspend during a system sleep.
> 
> IIUIC, what you are proposing is to use ignore_children in a way
> similar to how force_direct_complete was used in this patch?
> 
> http://thread.gmane.org/gmane.linux.power-management.general/60198/focus=60292

That message doesn't contain a patch.

> That should work as well, but Rafael raised some objections and thus I
> went with the present direct_complete_default, which should work if we
> can relax the check as discussed above.

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.

Alan Stern

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