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

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Date:	Thu, 2 Jul 2015 11:21:17 -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 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.

> > 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.

Alan Stern

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