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    SubjectRe: [patch update 3] PM: Introduce core framework for run-time PM of I/O devices
    On Wednesday 24 June 2009, Alan Stern wrote:
    > On Wed, 24 Jun 2009, Rafael J. Wysocki wrote:
    > > > One question still remains: If the counter is 0 at the end of a
    > > > successful pm_runtime_resume, should the core then call pm_notify_idle?
    > > > Or should we make the driver responsible for that too?
    > >
    > > Good question. :-)
    > >
    > > I think the core may call pm_notify_idle() in that case, but not necessarily in
    > > the synchronous case.
    > I'm not sure; we may want to do it even for synchronous resumes.
    > Otherwise the callers would be forced to do it.

    I have no strong opinion. We can do it in the sychronous case too.

    > There's also the other side of the coin. What if the counter is 0 at
    > the end of a failed pm_runtime_suspend?
    > For example, suppose the driver's runtime_suspend method decides that
    > the device hasn't been idle for long enough, so it wants to fail the
    > suspend attempt with -EBUSY and queue a new delayed autosuspend
    > request. But at this point the status is RPM_SUSPENDING, so new
    > suspend requests won't be accepted (N.B., the test for this in the most
    > recent patch doesn't look right).

    In fact it was inversed (fixed now), thanks for spotting this!

    > Even with a queued notification, there's no guarantee that the
    > notification won't be sent before the status changes from
    > RPM_SUSPENDING to RPM_ACTIVE. So we really do need the notification to
    > be sent by pm_runtime_suspend, after it has updated the status and
    > dropped the lock.


    > There's another totally separate issue worth discussing here. This
    > will affect the USB implementation of the new runtime PM framework.
    > The difficulty is that some USB interface drivers require remote wakeup
    > to be enabled while their interfaces are suspended. But remote wakeup
    > is a global setting; it doesn't take effect until the entire physical
    > device is suspended. (To put it another way, USB has no notion of
    > suspending interfaces.) This means we must not allow these interfaces
    > to be suspended before the whole device is. But the whole device is
    > the parent of the interfaces -- if we can't suspend the children before
    > suspending the parent then we're stuck.

    Not if we use the power.ignore_children flag on the parent.

    > Clearly this is something the USB stack has to deal with; it shouldn't
    > affect the general PM framework. However the only solution I can think
    > of involves subverting the framework, which isn't very nice. The idea
    > is to ignore runtime_suspend callbacks for these interface drivers;
    > allow them to keep on running even though the PM core thinks they are
    > suspended. Then suspend and resume them as part of the callbacks for
    > the entire device. (For interface drivers that don't require remote
    > wakeup there is no problem; it doesn't matter when they get suspended.)
    > This will work, but it's a hack. Does anybody have a better idea?

    Well, as I said above, you can set power.ignore_children on the device
    and then it can be suspended even if the interfaces aren't.


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