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

Rafael J. Wysocki
2011-04-21T17:59:57Z

On Thursday, April 21, 2011, Alan Stern wrote:
> On Wed, 20 Apr 2011, Rafael J. Wysocki wrote:
>
> > On Wednesday, April 20, 2011, Alan Stern wrote:
> > > On Wed, 20 Apr 2011, Rafael J. Wysocki wrote:
> > >
> > > > On Wednesday, April 20, 2011, Alan Stern wrote:
> > > > > On Wed, 20 Apr 2011, Guennadi Liakhovetski wrote:
> > > > ...
> > > > > Ah, now I see the problem. It looks like we did not give sufficient
> > > > > thought to the case where a device starts off (and therefore should
> > > > > finish up) in a powered-down state. Calling pm_runtime_put_sync()
> > > > > after unbinding the device driver seems a little futile -- with no
> > > > > driver, the subsystem may not be able to power-down the device!
> > > > >
> > > > > Rafael, how do you think we should handle this? Get rid of the
> > > > > pm_runtime_get_no_resume() and pm_runtime_put_sync() calls in
> > > > > dd.c:__device_release_driver()?
> > > >
> > > > I think we need pm_runtime_barrier() in there. Otherwise we risk
> > > > removing the driver while there's a runtime PM request pending.
> > > >
> > > > But we can move the pm_runtime_put_sync() before driver_sysfs_remove().
> > >
> > > What happens if another runtime PM request is queued between the
> > > put_sync() and the remove callback? We may need a safe way to prevent
> > > async runtime PM requests while still allowing synchronous requests.
> >
> > 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. :-)

Rafael