On Tuesday, April 14, 2020 7:47:35 PM CEST Alan Stern wrote:
> 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@xxxxxxxxxxxxx> 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.
Yes, MAY_SKIP_RESUME, as well as power.may_skip_resume for that matter, really
should not affect the THAW transition at all. I overlooked that when I was
writing the above (and earlier).
This means that the dev_pm_skip_resume() logic really is relatively
straightforward:
- If the current transition is RESTORE, return "false".
- Otherwise, if the current transition is THAW, return the return value
of dev_pm_skip_suspend().
- Otherwise (so the current transition is RESUME which is the only remaining
case), return the logical negation of power.must_resume.
> 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.
Not really, because I want the core to set power.may_skip_resume for the
devices for which dev_pm_skip_suspend() returns "true" if the "suspend_late"
subsystem-level callback is not present. [It might be more consistent
to simply set it for all devices for which dev_pm_skip_suspend() returns
"true" and let the subsystems update it should they want to? IOW, the
default value of power.may_skip_resume could be the return value of
dev_pm_skip_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.
But it cannot be the other way around.
For example, invoking ->thaw_early() from the driver without the corresponding
->freeze_late() would be a bug in general, unless they point to the same
routines as ->runtime_resume() and ->runtime_suspend() (or equivalent),
respectively, but that need not be the case.
> > > 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.
OK then.
> > > > 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?
Yes, it applies to that too.
Of course, if dev_pm_skip_suspend() returns "true", the usage counter cannot
be greater than 1 (for the given device as well as for any dependent devices).
> > > > 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.
Yes, I do.
> And of course, the way to do that is by adding them into the
> power.must_resume flag.
Right.
> How do you want to handle the usage counter restriction.
> Should that also propagate upward?
Yes, it should.
> And how should the result of dev_pm_skip_resume() be affected by
> SMART_SUSPEND for RESUME transitions?
Not directly, just through power.must_resume.
> 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
Right.
> 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"
Like I said above:
THAW dev_pm_skip_suspend() returns "true".
>
> RESUME Same as THAW? Or maybe don't require SMART_SUSPEND?
> (But if SMART_SUSPEND is clear, how could the runtime
> status be "suspended"?)
RESUME 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)
Nothing else is really strictly required IMO.
>
> I can't really tell what you want, because your comments at various
> times have been inconsistent.
Sorry for the inconsistencies, I hope that it's more clear now.
Cheers!
