Re: [linux-pm] [patch update] PM: Introduce core framework for run-time PM of I/O devices (rev. 5)

Alan Stern
2009-06-26T18:06:26Z

On Thu, 25 Jun 2009, Rafael J. Wysocki wrote:

> > The whole business about the runtime_notify and RPM_NOTIFY flags is
> > impenetrable. My suggestion: Rename runtime_notify to notify_pending
> > and eliminate RPM_NOTIFY. Then make sure that notify_pending is set
> > whenever a notify work item is queued.
>
> I was going to do exactly that, but I realized it wouldn't work in general,
> because ->runtime_idle() could run __pm_runtime_suspend() in theory.

I'll cut this short by noting the dilemma. If the runtime_idle
callback does a synchronous suspend, and __pm_runtime_suspend sees the
status is already RPM_SUSPENDING, then it will wait for the suspend to
finish. Hence it's not safe to do cancel_work_sync from within
__pm_runtime_suspend; it might deadlock.

It occurs to me that the problem would be solved if were a cancel_work
routine. In the same vein, it ought to be possible for
cancel_delayed_work to run in interrupt context. I'll see what can be
done.

What do you think about adding a version of pm_runtime_put that would
call pm_runtime_idle directly when the counter reaches 0, instead of
queuing an idle request? I feel that drivers should have a choice
about which sort of notification to use.

> > And don't forget to decrement the parent's child_count again if the resume
> > fails.
>
> I didn't _forget_it, because the device can't be RPM_SUSPENDED after
> __pm_runtime_resume().

You're right; that fact escaped me.

> > In __pm_runtime_suspend, you should decrement the parent's child_count
> > before releasing the child's lock.
>
> Why exactly is that necessary?

I guess it isn't. But it won't hurt to keep the parent's counter
synchronized with the child's state as closely as possible.

Alan Stern