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

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Date:	Fri, 3 Jul 2015 10:11:14 +0200
From:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
To:	Alan Stern <stern@...land.harvard.edu>
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 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.

>> > 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 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.

Thanks,

Tomeu

> Alan Stern
>
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