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

Alan Stern
2011-04-21T13:58:31Z

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);
}

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.

Alan Stern