[linux-pm] Question about expected behavior when PM runtime is disabled

[Rafael J. Wysocki]

On Monday, June 13, 2011, Alan Stern wrote:
> On Mon, 13 Jun 2011, Rafael J. Wysocki wrote:
> 
> > > I think we need to have the PM core call pm_runtime_get_noresume()  
> > > before invoking the resume_noirq (or thaw_noirq or restore_noirq)  
> > > callback, and then call pm_runtime_put_sync() after invoking the
> > > complete callback.  This would solve your race: The put_sync would 
> > > invoke the runtime_idle method, which would start another runtime 
> > > suspend or autosuspend.
> > > 
> > > (It used to be that the PM core called pm_runtime_get_noresume() 
> > > earlier on, before the prepare callback.  This also solved your race, 
> > > but it caused other problems and so was changed.)
> > > 
> > > It's true that subsystems could do this for themselves, but then they'd
> > > _all_ have to do it.  So we might as well put it in the PM core.
> > > 
> > > Rafael, what do you think?
> > 
> > Yes, we can do that.
> > 
> > I even suspect that all subsystems will end up calling pm_runtime_disable()
> > somewhere in the system suspend code path and pm_runtime_enable() during
> > system resume.  It might be a good idea to do that in the core too, after
> > calling the subsystem's .suspend() and before calling its .resume(),
> > respectively.
> 
> Will that bring back Kevin's problems?  There was a specific commit:  
> "PM: Allow pm_runtime_suspend() to succeed during system suspend".  If
> the core disables runtime PM, won't he be right back where he was
> before?

Not exactly, because that commit removed the pm_runtime_get_noresume()
done before .prepare(), which was too early.  As I said before, I don't
see anything wrong with running pm_runtime_ helpers from .prepare() or
.complete().  However, to me, it is highly doubtful if there is any valid
reason for calling them after .suspend() has been executed.  In fact, I
think that .suspend() should ensure that they won't be executed for the
given device after it has returned, so doing pm_runtime_disable() in the
core at this point makes sense.

We really shouldn't allow any runtime PM callbacks to race with
.suspend_noirq() and .resume_noirq(), because allowing that to happen would
be asking for breakage.

Thanks,
Rafael