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    SubjectRe: lockdep warning in urb.c:363 usb_submit_urb
    On Tue, 14 Apr 2020, Rafael J. Wysocki wrote:

    > Note to self: avoid replying to technical messages late in the night ...
    >
    > On Mon, Apr 13, 2020 at 11:32 PM Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
    > >
    > > On Saturday, April 11, 2020 4:41:14 AM CEST Alan Stern wrote:
    > > > Okay, this is my attempt to summarize what we have been discussing.
    > > > But first: There is a dev_pm_skip_resume() helper routine which
    > > > subsystems can call to see whether resume-side _early and _noirq driver
    > > > callbacks should be skipped. But there is no corresponding
    > > > dev_pm_skip_suspend() helper routine. Let's add one, or rename
    > > > dev_pm_smart_suspend_and_suspended() to dev_pm_skip_suspend().
    > >
    > > OK
    > >
    > > > Given that, here's my understanding of what should happen. (I'm
    > > > assuming the direct_complete mechanism is not being used.) This tries
    > > > to describe what we _want_ to happen, which is not always the same as
    > > > what the current code actually _does_.
    > >
    > > OK
    > >
    > > > During the suspend side, for each of the
    > > > {suspend,freeze,poweroff}_{late,noirq} phases: If
    > > > dev_pm_skip_suspend() returns true then the subsystem should
    > > > not invoke the driver's callback, and if there is no subsystem
    > > > callback then the core will not invoke the driver's callback.
    > > >
    > > > During the resume side, for each of the
    > > > {resume,thaw,restore}_{early,noirq} phases: If
    > > > dev_pm_skip_resume() returns true then the subsystem should
    > > > not invoke the driver's callback, and if there is no subsystem
    > > > callback then the core will not invoke the driver's callback.
    > > >
    > > > dev_pm_skip_suspend() will return "true" if SMART_SUSPEND is
    > > > set and the device's runtime status is "suspended".
    > >
    > > Agreed with the above.
    > >
    > > > power.must_resume gets set following the suspend-side _noirq
    > > > phase if power.usage_count > 1 (indicating the device was
    > > > in active use before the start of the sleep transition) or
    > > > power.must_resume is set for any of the device's dependents.
    > >
    > > Or MAY_SKIP_RESUME is unset (which means that the driver does not
    > > allow its resume callbacks to be skipped), or power.may_skip_resume
    > > is unset (which means that the subsystem does not allow the
    > > driver callbacks to be skipped).

    Are you certain about that? It contradicts what you said earlier, that
    MAY_SKIP_RESUME doesn't affect THAW transitions. Also, it would mean
    that a device whose subsystem doesn't know about power.may_skip_resume
    would never be allowed to stay in runtime suspend.

    > > > dev_pm_skip_resume() will return "false" if the current
    > > > transition is RESTORE or power.must_resume is set. Otherwise:
    > > > It will return true if the current transition is THAW,
    > > > SMART_SUSPEND is set, and the device's runtime status is
    > > > "suspended".
    > >
    > > The other way around. That is:
    > >
    > > dev_pm_skip_resume() will return "true" if the current transition is
    > > THAW and dev_pm_skip_suspend() returns "true" for that device (so
    > > SMART_SUSPEND is set, and the device's runtime status is "suspended",
    > > as per the definition of that function above).
    >
    > The above is what I wanted to say ->

    So for THAW, dev_pm_skip_resume() can return "true" even if
    power.must_resume is set? That doesn't seem right.

    > > Otherwise, it will return "true" if the current transition is RESTORE
    > > (which means that all devices are resumed) or power.must_resume is not
    > > set (so this particular device need not be resumed).
    >
    > -> but this isn't. In particular, I messed up the RESTORE part, so it
    > should read:
    >
    > Otherwise, it will return "true" if the current transition is *not*
    > RESTORE (in which case all devices would be resumed) *and*
    > power.must_resume is not set (so this particular device need not be
    > resumed).
    >
    > Sorry about that.

    For the RESTORE and THAW cases that is exactly the same as what I
    wrote, apart from the THAW issue noted above.

    > > > It will return "true" if the current transition is
    > > > RESUME, SMART_SUSPEND and MAY_SKIP_RESUME are both set, and
    > > > the device's runtime status is "suspended".
    > >
    > > Unless MAY_SKIP_RESUME is unset for at least one of its descendants (or
    > > dependent devices).
    >
    > That should include the power.may_skip_resume flag, so as to read as follows:
    >
    > Unless MAY_SKIP_RESUME is unset or power.may_skip_resume is unset for
    > at least one of its descendants (or dependent devices).

    What about the runtime PM usage counter?

    > > > For a RESUME
    > > > transition, it will also return "true" if MAY_SKIP_RESUME and
    > > > power.may_skip_resume are both set, regardless of
    > > > SMART_SUSPEND or the current runtime status.
    > >
    > > And if the device was not in active use before suspend (as per its usage
    > > counter) or MAY_SKIP_RESUME is unset for at least one of its descendants (or
    > > dependent devices in general).
    >
    > And analogously here, so what I really should have written is:
    >
    > And if the device was not in active use before suspend (as per its
    > usage counter) or MAY_SKIP_RESUME or power.may_skip_resume is unset
    > for at least one of its descendants (or dependent devices in general).

    In other words, for RESUME transitions you want the MAY_SKIP_RESUME and
    power.may_skip_resume restrictions to propagate up from dependent
    devices. And of course, the way to do that is by adding them into the
    power.must_resume flag.

    How do you want to handle the usage counter restriction.
    Should that also propagate upward?

    And how should the result of dev_pm_skip_resume() be affected by
    SMART_SUSPEND for RESUME transitions?

    Maybe this is getting confusing because of the way I organized it.
    Let's try like this:

    Transition Conditions for dev_pm_skip_resume() to return "true"
    ---------- ----------------------------------------------------

    RESTORE Never

    THAW power.must_resume is clear (which requires
    MAY_SKIP_RESUME and power.may_skip_resume to be set and
    the runtime usage counter to be = 1, and which
    propagates up from dependent devices)
    SMART_SUSPEND is set,
    runtime status is "suspended"

    RESUME Same as THAW? Or maybe don't require SMART_SUSPEND?
    (But if SMART_SUSPEND is clear, how could the runtime
    status be "suspended"?)

    I can't really tell what you want, because your comments at various
    times have been inconsistent.

    Alan Stern

    > > > At the start of the {resume,thaw,restore}_noirq phase, if
    > > > dev_pm_skip_resume() returns true then the core will set the
    > > > runtime status to "suspended". Otherwise it will set the
    > > > runtime status to "active". If this is not what the subsystem
    > > > or driver wants, it must update the runtime status itself.
    > >
    > > Right.
    > >
    > > > Comments and differences with respect to the code in your pm-sleep-core
    > > > branch:
    > > >
    > > > I'm not sure whether we should specify other conditions for
    > > > setting power.must_resume.
    > >
    > > IMO we should.
    >
    > In fact, this is part of the implementation and it helps to
    > "propagate" the "must resume" condition to the parent and the
    > first-order suppliers of the device (which is sufficient, because
    > their power.must_resume "propagates" in the same way and so on).
    >
    > IOW, the important piece is what the return value of
    > dev_pm_skip_resume() should be in particular conditions and that
    > return value is computed with the help of power.must_resume (and it
    > might have been computed in a different, possibly less efficient,
    > way).
    >
    > > Otherwise it is rather hard to catch the case in which one of the
    > > device's descendants has MAY_SKIP_RESUME unset (and so the device
    > > needs to be resumed).
    > >
    > > > dev_pm_skip_resume() doesn't compute the value described
    > > > above. I'm pretty sure the existing code is wrong.
    > >
    > > Well, we don't seem to have reached an agreement on some details
    > > above ...
    >
    > Sorry for failing to be careful enough ...


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