Re: [PATCH v2] PM / Sleep: Add pm_generic functions to re-use runtime PM callbacks

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On Tue, 26 Nov 2013, Rafael J. Wysocki wrote:

> On Tuesday, November 26, 2013 10:48:56 AM Ulf Hansson wrote:
> > On 26 November 2013 00:10, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> > > On Monday, November 25, 2013 01:40:21 PM Ulf Hansson wrote:
> > >> To put devices into low power state during sleep, it sometimes makes
> > >> sense at subsystem-level to re-use device's runtime PM callbacks.

Ulf, please be more careful about the distinction between "device" and 
"driver".  Devices don't have callbacks; drivers do.  You made this 
mistake a few times in the patch description.

> > >> The PM core will at device_suspend_late disable runtime PM, after that
> > >> we can safely operate on these callbacks. At suspend_late the device
> > >> will be put into low power state by invoking the device's
> > >> runtime_suspend callback, unless the runtime status is already
> > >> suspended. At resume_early the state is restored by invoking the
> > >> device's runtime_resume callback. Soon after the PM core will re-enable
> > >> runtime PM before returning from device_resume_early.
> > >>
> > >> The new pm_generic functions are named pm_generic_suspend_late_runtime
> > >> and pm_generic_resume_early_runtime, they are supposed to be used in
> > >> pairs.
> > >
> > > What happens if the subsystem uses the new functions as its late suspend/
> > > early resume callbacks, but some of its drivers implement .suspend_late()
> > > and .resume_early()?

The same thing that happens whenever any PM-related callback is defined
by both the subsystem and the driver: The PM core uses the subsystem's
callback.  (Or the pm_domain's callback, or the type's callback, or the 
class's callback.)

Besides, Ulf specifically assumed at the top of this message that
re-using the runtime-suspend callback makes sense at the subsystem
level.  This must mean it is appropriate for all drivers in the
subsystem.  So why would a driver want to override the subsystem's 
behavior?

> > Good point. Normally, I think the decision of using these callbacks
> > should be taken at the lowest level, in other words in the driver.
> > Otherwise the lowest layer of .suspend_late will be by-passed.
> 
> I'm not sure what's the point to add them, then.  If the driver has to decide
> anyway, it may simply populate its .suspend_late and .resume_early pointers
> I think.

Another possibility is to have the pm_generic_suspend_late_runtime
routine check to see if the driver has defined a suspend_late callback.  
If the driver has its own callback, we could invoke it instead of
invoking the runtime_suspend callback.

However, I'm not sure that doing this would be worthwhile.  And it
would hang in an infinite loop if the driver set its suspend_late
pointer to pm_generic_suspend_late_runtime!

Alan Stern

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