Re: [PATCH/RFC] MMC: remove unbalanced pm_runtime_suspend()

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On Thu, 21 Apr 2011, Rafael J. Wysocki wrote:

> > > What about making a rule that it is invalid to schedule a future suspend
> > > or queue a resume request of a device whose driver is being removed?
> > > 
> > > Arguably, we can't prevent people from shooting themselves in the foot this
> > > way or another and I'm not sure if this particular case is worth additional
> > > handling.
> > 
> > After thinking about this, I tend to agree.  The synchronization 
> > issues, combined with the unknown needs of the driver, make this very 
> > difficult to handle in the PM core.
> > 
> > Here's another possible approach: If a driver wants to leave its device 
> > in a powered-down state after unbinding then it can invoke its own 
> > runtime_suspend callback directly, in the following way:
> > 
> > 	... unregister all child devices below dev ...
> > 	pm_runtime_disable(dev);
> > 	if (dev->power.runtime_status != RPM_SUSPENDED) {
> > 		pm_set_suspended(dev);
> > 		my_runtime_suspend_callback(dev);
> > 	}
> 
> I think this would work too, but then possibly many drivers would have to
> do the same thing in their "remove" routines.
> 
> > There may be issues regarding coordination with the subsystem or the
> > power domain; at the moment it's not clear what should be done.  Maybe
> > the runtime-PM core should include an API for directly invoking the
> > appropriate callbacks.
> 
> If we choose this approach, then yes, we should provide a suitable API, but
> I'm still thinking it would be simpler to move the pm_runtime_put_sync() before driver_sysfs_remove() and make the rule as I said previously. :-)

The problem is synchronization.  At what point is the driver supposed 
to stop queuing runtime PM requests?  It would have to be sometime 
before the pm_runtime_barrier() call.  How is the driver supposed to 
know when that point is reached?  The remove routine isn't called until 
later.

Alan Stern

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