Re: [patch update 3] PM: Introduce core framework for run-time PM of I/O devices

From: Rafael J. Wysocki
Date: Tue Jun 23 2009 - 20:17:23 EST

On Tuesday 23 June 2009, Alan Stern wrote:
> On Tue, 23 Jun 2009, Rafael J. Wysocki wrote:
> > In short, I think suspending (or queuing a suspend request) should fail if the
> > usage counter is nonzero, but the resuming (or queuing up a resume request)
> > should be possible regardless of its value. The reason is that multiple
> > threads may in theory attempt to resume the device at the same time.
> Agreed. Suspends and resumes aren't symmetrical -- a single resume
> request must outweigh numerous suspend requests.
> > However, I'm not sure if the core should manipulate the usage counter by
> > itself, because it's sort of problematic (there's no good approach to decide
> > when to decrement the counter).
> Yes. The idea behind my previous message was that it's not really so
> easy for the core to decide when to _increment_ the counter either.
> > So, I'd let the callers use pm_runtime_get() to increment the counter
> > and pm_runtime_put() to decrement it, possibly queuing up an idle notification
> > if the counter happens to reach 0. Also, I'm not sure if unbalanced
> > pm_runtime_put() should be regarded as a bug.
> It should be. Once the counter is messed up, runtime PM wouldn't be
> able to work properly. But maybe you should add a pm_set_counter call
> so that drivers can recover from imbalances.
> One question still remains: If the counter is 0 at the end of a
> successful pm_runtime_resume, should the core then call pm_notify_idle?
> Or should we make the driver responsible for that too?

Good question. :-)

I think the core may call pm_notify_idle() in that case, but not necessarily in
the synchronous case.

> > At the same time, I'd like the core to use runtime_status and the other
> > fields in dev_pm_info, except for the usage counter, to ensure that all
> > operations are only carried out when it makes sense.
> Yes. In fact, I'd say that when the counter is positive it doesn't
> make sense to allow a runtime suspend -- so you don't need that
> exception in your statement above. :-)

Well, you're right.

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