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Date:   Mon, 6 Apr 2020 16:25:08 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     "Rafael J. Wysocki" <rjw@rjwysocki.net>
cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Qais Yousef <qais.yousef@arm.com>,
        USB list <linux-usb@vger.kernel.org>,
        Linux-pm mailing list <linux-pm@vger.kernel.org>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: lockdep warning in urb.c:363 usb_submit_urb
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On Mon, 6 Apr 2020, Rafael J. Wysocki wrote:

> In the meantime I have created a git branch with changes to simplify the code,
> rename some things and clarify the documentation a bit:
> 
>  git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git \
>  pm-sleep-core
> 
> (https://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git/log/?h=pm-sleep-core
> for web access).
> 
> I'm going to post these changes as patches soon.

All right, those are some significant changes.  It'll take me a little 
while to absorb them.

> On Friday, April 3, 2020 10:15:09 PM CEST Alan Stern wrote:

> > Let's put it like this: The resume-side callbacks should have the
> > overall effect of bringing the device back to its initial state, with
> > the following exceptions and complications:
> > 
> > 	Unless SMART_SUSPEND and LEAVE_SUSPEND are both set, a device
> > 	that was in runtime suspend before the suspend_late phase 
> > 	must end up being runtime-active after the matching RESUME.
> >
> > 	Unless SMART_SUSPEND is set, a device that was in runtime 
> > 	suspend before the freeze_late phase must end up being 
> > 	runtime-active after the matching THAW.
> 
> Correct.
>  
> > [I'm not so sure about this.  Wouldn't it make more sense to treat
> > _every_ device as though SMART_SUSPEND was set for FREEZE/THAW
> > transitions, and require subsystems to do the same?]
> 
> Drivers may expect devices to be runtime-active when their suspend
> callbacks are invoked unless they set SMART_SUSPEND.  IOW, without
> SMART_SUSPEND set the device should not be left in runtime suspend
> during system-wide suspend at all unless direct-complete is applied
> to it.

[Let's confine this discussion to the not-direct-complete case.]

Okay, say that SMART_SUSPEND isn't set and the device is initially
runtime-suspended.  Since the core knows all this, shouldn't the core 
then call pm_runtime_resume() immediately before ->suspend?  Why leave 
this up to subsystems or drivers (which can easily get it wrong -- 
not to mention all the code duplication it would require)?

Also, doesn't it make sense for some subsystems or drivers to want 
their devices to remain in runtime suspend throughout a FREEZE/THAW 
transition but not throughout a SUSPEND/RESUME transition?  With only a 
single SMART_SUSPEND flag, how can we accomodate this desire?

Finally, my description above says that LEAVE_SUSPENDED matters for 
SUSPEND/RESUME but not for FREEZE/THAW.  Is that really what you have 
in mind?

> > 	After RESTORE, _every_ device must end up being runtime 
> > 	active.
> 
> Correct.
> 
> > 	In general, each resume-side callback should undo the effect
> > 	of the matching suspend-side callback.  However, because of
> > 	the requirements mentioned in the preceding sentences,
> > 	sometimes a resume-side callback will be issued even though
> > 	the matching suspend-side callback was skipped -- i.e., when
> > 	a device that starts out runtime-suspended ends up being
> > 	runtime-active.
> > 
> > How does that sound?
> 
> It is correct, but in general the other way around is possible too.
> That is, a suspend-side callback may be issued without the matching
> resume-side one and the device's PM runtime status may be changed
> if LEAVE_SUSPENDED is set and SMART_SUSPEND is unset.

This is inconsistent with what I wrote above (the "Unless SMART_SUSPEND
and LEAVE_SUSPENDED are both set" part).  Are you saying that text
should be changed?

> > Are you certain you want the subsystem callback to be responsible for
> > setting the runtime status to "active"?  Isn't this an example of
> > something the core could do in order to help simplify subsystems?
> 
> The rationale here is that whoever decides whether or not to skip the
> driver-level callbacks, should also set the PM-runtime status of the
> device to match that decision.

Well, that's not really a fair description.  The decision about
skipping driver-level callbacks is being made right here, by us, now.  
(Or if you prefer, by the developers who originally added the
SMART_SUSPEND flag.)  We require subsystems to obey the decisions being
outlined in this discussion.

Given that fact, this is again a case of having the core do something 
rather than forcing subsystems/drivers to do it (possibly getting it 
wrong and certainly creating a lot of code duplication).

If a subsystem really wants to override our decision, it can always
call pm_runtime_set_{active|suspended} to override the core's setting.

> > And this brings up another thing the core might do to help simplify
> > drivers and subsystems: If SMART_SUSPEND isn't set and the device is in
> > runtime suspend, couldn't the core do a pm_runtime_resume before
> > issuing the ->suspend or ->suspend_late callback?
> 
> It could, but sometimes that is not desirable.  Like when the drivver points its
> suspend callback to pm_runtime_force_suspend().

This seems to contradict what you wrote above: "Drivers may expect
devices to be runtime-active when their suspend callbacks are invoked
unless they set SMART_SUSPEND.  IOW, without SMART_SUSPEND set the
device should not be left in runtime suspend during system-wide suspend
at all unless direct-complete is applied to it."

If you stand by that statement then drivers should never point their
suspend callback to pm_runtime_force_suspend() unless they also set
SMART_SUSPEND.

Alan Stern