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SubjectRe: [PATCH] PM: Document rules on using pm_runtime_resume() in system suspend callbacks
On Thu, 21 Sep 2017, Johannes Stezenbach wrote:

> On Thu, Sep 21, 2017 at 02:39:30AM +0200, Rafael J. Wysocki wrote:
> > On Wed, Sep 20, 2017 at 6:27 PM, Johannes Stezenbach <js@sig21.net> wrote:
> > >
> > > E.g. an audio codec could keep running
> > > while the i2c bus used to program its registers can be runtime suspended.
> > > If this is correct I think it would be useful to spell it out explicitly
> > > in the documentation.
> >
> > That's because the i2c bus uses the ignore_children flag that allows
> > it to override the general rules. :-)
>
> Ah! I was looking at Documentation/driver-api/pm only (which is
> changed by your patch), but this is documented in Documentation/power
> (and obviously I hadn't checked the code, shame on me).
>
> > direct_complete has nothing to do with this.
>
> Oh? Reading again, do I get this right:
>
> 1. simple method: always call pm_runtime_resume() in ->suspend(),
> then suspend the driver again
> 2. optimization: if pm_runtime_suspended(), the driver's ->suspend()
> can possibly do nothing if conditions permit, otherwise it calls
> pm_runtime_resume() and then suspends
> 3. optimization: tell pm core to skip ->suspend() via return value
> from ->prepare() which sets direct_complete
>
> ...and your patch only deals with 1 and 2.
>
> Sorry to hijack your thread for side discussion, it was
> inadvertant due to my lack of understanding.
>
>
> > First off, the PM core does check the direct_complete flag in
> > __device_suspend() and does more-or-less what you are saying.
> >
> > However, that flag is initialized in device_prepare() with the help of
> > the ->suspend() return value, because whether or not it makes sense to
>
> you mean ->prepare(), right?
>
> > set that flag depends on some conditions that may change between
> > consecutive system suspend-resume cycles in general and need to be
> > checked in advance before setting it.
> >
> > HTH
>
> It does, however the question remains *why* it needs to check
> it in ->prepare() and not right before calling ->suspend().
> Using ->prepare() for the purpose seems wrong since it traverses
> the hierarchy in the "wrong" order.

No, it is the _right_ order. If a device's ->prepare() says that
direct_complete is okay, but one of its descendants disallows
direct_complete, we then want to clear the direct_complete flag in the
original device structure. We couldn't do this if we checked the
descendant's driver first.

> Only right before
> calling ->suspend() the driver knows if its current state
> allows it to skip any further actions for suspend, because
> suspending children or other users may cause pm_runtime_resume()
> for it.

If the device gets runtime-resumed before ->suspend() would be called
then the direct_complete setting doesn't matter. The PM core follows
the direct_complete path only if the device is already in runtime
suspend when the ->suspend() callback would normally be invoked.

And if the device does get runtime-resumed like this, it can't be
runtime-suspended again. The PM core makes sure of that.

Alan Stern

> (In the back of my head I have the scenario of
> bug #196861, some completely different driver uses
> i2c via ACPI OpRegion during its suspend.)
>
>
> Thanks,
> Johannes


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