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Re: [PATCH v3 2/2] PM / Runtime: Add pm_runtime_enable_recursive

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  Re: [PATCH v3 2/2] PM / Runtime: Add pm_runtime_enable_recursive Alan Stern <stern@rowland.harvard.edu> - 2015-07-03 16:20 +0200
    Re: [PATCH v3 2/2] PM / Runtime: Add pm_runtime_enable_recursive Tomeu Vizoso <tomeu.vizoso@collabora.com> - 2015-07-03 16:30 +0200
      Re: [PATCH v3 2/2] PM / Runtime: Add pm_runtime_enable_recursive Alan Stern <stern@rowland.harvard.edu> - 2015-07-03 17:20 +0200

#1176601 — 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@rowland.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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#1176604

On 3 July 2015 at 16:16, Alan Stern <stern@rowland.harvard.edu> wrote:
> On Fri, 3 Jul 2015, Tomeu Vizoso wrote:
>
>> On 2 July 2015 at 17:21, Alan Stern <stern@rowland.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.

Cool.

>> >> > 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.

The patch is at the top of the thread:

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.
>
> 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).

Thanks,

Tomeu

> Alan Stern
>
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#1176654

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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