Re: [patch update] PM: Introduce core framework for run-time PM of I/O devices (rev. 6)

Alan Stern
2009-06-29T15:27:39Z

On Mon, 29 Jun 2009, Rafael J. Wysocki wrote:

> > > So, it seems, pm_request_resume() can't kill suspend requests by itself
> > > and instead it has to queue up resume requests for this purpose, which
> > > brings us right back to the problem of two requests queued up at a time
> > > (a delayed suspend request and a resume request that is supposed to cancel it).
> >
> > No, you're trying to do too much. If the state is RPM_IDLE (i.e., a
> > suspend request is pending) then rpm_request_resume doesn't need to do
> > anything. The device is already resumed! Sure, it can try to kill the
> > request and change the state to RPM_ACTIVE, but it doesn't need to.
>
> I think it does need to do that, because the reuqest may be scheduled way
> in the future and we can't preserve its work structure until it runs.
> pm_request_resume() doesn't know in advance when the suspend work function is
> going to be queued up and run.

It doesn't need to know. All it needs to do is guarantee that the
device will be in a resumed state some time not long after the function
returns. Thus calling rpm_request_resume while the status is RPM_IDLE
is like calling it while the status is RPM_ACTIVE. In neither case
does it have to do anything, because the device will already be resumed
when it returns.

Perhaps instead we should provide a way to kill a pending suspend
request? It's not clear that anyone would need this. The only reason
I can think of is if you wanted to change the timeout duration. But it
wouldn't be able to run in interrupt context.

> > Think about it. Even if the suspend request were killed off, there's
> > always the possibility that someone could call rpm_runtime_suspend
> > right afterward. If the driver really wants to resume the device and
> > prevent it from suspending again, then the driver should call
> > pm_runtime_get before pm_request_resume. Then it won't matter if the
> > suspend request runs.
>
> No, it doesn't matter if the request runs, but it does matter if the work
> structure used for queuing it up may be used for another purpose. :-)

What else would it be used for? If rpm_request_resume returns without
doing anything and leaves the status set to RPM_IDLE, then the work
structure won't be reused until the status changes.

> The problem with pm_<something>_put is that it does two things at a time,
> decrements the resume counter and runs or queues up an idle notification.
> Perhaps it's a good idea to call it after the second thing and change
> pm_runtime_get() to pm_runtime_inuse(), so that we have:
>
> * pm_runtime_inuse() - increment the resume counter
> * pm_runtime_idle() - decrement the resume counter and run idle notification
> * pm_request_idle() - decrement the resume counter and queue idle notification
>
> and __pm_runtime_idle() as the "bare" idle notification function?

I could live with that, but the nice thing about "get" and "put" is
that they directly suggest a counter is being maintained and therefore
the calls have to balance. Maybe we should just call it
rpm_request_put and not worry that the put happens immediately.

Alan Stern