linux-kernel - Re: [PATCH v3 1/2] PM / sleep: Let devices force direct

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Message-ID: <CAAObsKAJzu3uJ3ZNKfK3U8Ae4W9STXXdyhVrNYsn0MK2uSoZHA@mail.gmail.com>
Date:	Tue, 28 Apr 2015 16:26:39 +0200
From:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/2] PM / sleep: Let devices force direct_complete

On 20 April 2015 at 16:12, Alan Stern <stern@...land.harvard.edu> wrote:
> On Mon, 20 Apr 2015, Tomeu Vizoso wrote:
>
>> On 17 April 2015 at 19:30, Alan Stern <stern@...land.harvard.edu> wrote:
>> > On Fri, 17 Apr 2015, Laurent Pinchart wrote:
>> >
>> >> Hi Tomeu,
>> >>
>> >> Thank you for the patch.
>> >>
>> >> On Friday 17 April 2015 17:24:49 Tomeu Vizoso wrote:
>> >> > Introduce a new per-device flag power.force_direct_complete that will
>> >> > instruct the PM core to ignore the runtime PM status of its descendants
>> >> > when deciding whether to let this device remain in runtime suspend when
>> >> > the system goes into a sleep power state.
>> >> >
>> >> > This is needed because otherwise it would be needed to get dozens of
>> >> > drivers to implement the prepare() callback and be runtime PM active
>> >> > even if they don't have a 1-to-1 relationship with a piece of HW.
>> >>
>> >> I'll let PM experts comment on the approach, but I believe the new flag would
>> >> benefit from being documented (likely in Documentation/power/devices.txt) :-)
>> >
>> > Documentation/power/runtime_pm.txt is the right place.
>> >
>> > However, I'm not sure that this is the sort of thing Rafael meant when
>> > he suggested adding a new flag.  I thought he meant the PM core would
>> > look at the new flag only if there was no ->prepare method at all.
>> > Then if the new flag was set, the PM core would act as though ->prepare
>> > had returned 1.  That way there would be no need to add silly little
>> > one-line *_prepare() routines all over the place.
>> >
>> > Maybe he had something else in mind, though...
>>
>> Yeah, I also interpreted it like that, but when I started looking at
>> how it would work, I found that it would be awkward if the uvcvideo
>> driver had to track all the devices that get attached below its
>> devices in order to set that flag to them.
>>
>> When thinking about it, it occurred to me that it may make more sense
>> if we model this as a property of the device bound to the uvcvideo
>> driver, as what's happening here is that the uvcvideo driver knows
>> that it's safe to remain in runtime suspend when the system goes to
>> sleep, and that all its descendant devices can be ignored in that
>> regard.
>
> What you're proposing makes sense, but it is a significant change to
> the runtime PM core.  It should be submitted separately, not as part of
> an update to the UVC driver, and it should be discussed at length.
>
> Basically, you want to mark certain devices to say that they will
> _always_ use direct-suspend.  This means that all descendant devices
> will be forced to use direct-suspend also, and therefore any driver
> bound to one of these descendant devices will be unable to communicate
> with it during a system sleep transition.  This is a non-trivial
> restriction.
>
> Among other things, it means that wakeup settings can't be altered
> during a sleep transition.  Therefore this should be allowed only for
> devices that are not wakeup-capable.

Hi Rafael,

do you have any comments on this?

Thanks,

Tomeu
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