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

Rafael J. Wysocki
2011-04-21T20:05:42Z

On Thursday, April 21, 2011, Alan Stern wrote:
> 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.

Executing the driver's callback is not an ideal solution either, because
it simply may be insufficient (it may be necessary to execute the power
domain and/or subsystem callbacks, pretty much what rpm_suspend() does,
but without taking the usage counter into consideration).

Moreover, if we want the driver's ->remove() to do the cleanup anyway,
there's not much point in doing any cleanup before in the core. Also,
there's a little problem that the bus ->remove() is called before the
driver's ->remove(), so it may not be entirely possible to power down
the device when the driver's ->remove() is called already.

I think the current code is better than any of the alternatives considered
so far.

Rafael