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

Alan Stern
2009-06-29T17:29:37Z

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

> Theoretically, we are, but practically we want to be able to use
> pm_runtime_put() (the asynchronous version) after a pm_runtime_resume()
> that found the device operational, but that would result in queuing a request
> using the same work structure that is used by the pending suspend request.
> Don't you see a problem here?

This is a different situation. pm_runtime_resume does have the luxury
of killing the suspend request, and it should do so.

Let's think about it this way. Why does a driver call
pm_request_resume in the first place? Because an interrupt handler or
spinlocked region wants to do some I/O, so the device has to
be active.

But when will it do the I/O? If the device is currently suspended, the
driver can do the I/O at the end of its runtime_resume callback. But
if the status is RPM_ACTIVE, the callback won't be invoked, so the
interrupt handler will have to do the I/O directly. The same is true
for RPM_IDLE.

Except for one problem: In RPM_IDLE, a suspend might occur at any time.
(In theory the same thing could happen in RPM_ACTIVE.) To prevent
this, the driver can call pm_runtime_get before pm_request_resume.
When the I/O is all finished, it calls pm_request_put.

If the work routine starts running before the pm_request_put, it will
see that the counter is positive so it will set the status back to
RPM_ACTIVE. Then the put will queue an idle notification. If the work
routine hasn't started running before the pm_request_put then the
status will remain RPM_IDLE all along.

Regardless, it's not necessary for pm_request_resume to kill the
suspend request. And even if it did, the driver would still need to
implement both pathways for doing the I/O.

> > As long as the behavior is documented, I think it will be okay for
> > pm_request_resume not to cancel a pending suspend request.
>
> I could agree with that, but what about pm_runtime_resume() happening after
> a suspend request has been scheduled? Should it also ignore the pending
> suspend request?

It could, but probably it shouldn't.

> In which case it would be consistent to allow to schedule suspends even though
> the resume counter is greater than 0.

True enough, although I'm not sure there's a good reason for it. You
certainly can increment the resume counter after scheduling a suspend
request -- the effect would be the same.

Alan Stern