Re: [PATCH v3 1/2] PM / sleep: Let devices force direct_complete

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Tuesday, April 28, 2015 04:26:39 PM Tomeu Vizoso wrote:
> On 20 April 2015 at 16:12, Alan Stern <stern@rowland.harvard.edu> wrote:
> > On Mon, 20 Apr 2015, Tomeu Vizoso wrote:
> >
> >> On 17 April 2015 at 19:30, Alan Stern <stern@rowland.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?

Well, what Alan has said sounds correct to me.

Rafael