Re: [RFC PATCH] PM / Runtime: runtime: Add sysfs option for forcing runtime suspend

[Oliver Neukum <oneukum@suse.de>]

On Tue, 2015-09-22 at 11:22 -0400, Alan Stern wrote:
> On Tue, 22 Sep 2015, Oliver Neukum wrote:
>  
> > Cancel, yes, going to low power is a consequence which needn't bother
> > the power subsystem.
> 
> Going to low power needn't involve the power subsystem?  That sounds 
> weird.

Think of it like rfkill. It makes sense to suspend an rfkilled device.
It still is the job of the driver to report that its device is idle.

> >  You need a callback. If there are spurious
> > events, the current heuristics will keep devices awake.
> > You must discard them anyway, as they are spurious. There's no point
> > in transporting over the bus at all. We can cease IO for input.
> > 
> > > This would create a parallel runtime-PM mechanism which is independent
> > > of the existing one.  Is that really a good idea?
> > 
> > It isn't strictly PM. It helps PM to do a better job, but
> > conceptually it is independent.
> 
> So my next question is: _How_ can this help PM to do a better job?  
> That is, what are the mechanisms?

"inhibit" -> driver stops input -> driver sets PM count to zero
-> PM subsystem acts

To go from the first to the second step a callback is needed

> One you have already stated: Lack of spurious events will help prevent 
> unwanted wakeups (or unwanted failures to go to sleep).

That too. We also save CPU cycles.

> But Dmitry made a stronger claim: Inhibiting an input device should 
> allow the device to go to low power.  I would like to know how we can 
> implement this cleanly.  The most straightforward approach is to use 
> runtime PM, but it's not obvious how this can be made to work with the 
> current API.

Yes, we can use the current API.
The key is that you think of the mechanism as induced idleness,
not forced suspend. We already have a perfectly working mechanism
for suspending idle devices.

	Regards
		Oliver