lkml.org 
[lkml]   [2017]   [Sep]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH] PM: Document rules on using pm_runtime_resume() in system suspend callbacks
Date
On Thursday, September 21, 2017 11:27:13 AM CEST 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

Right.

> 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

Well, that may work in some cases. :-)

> 3. optimization: tell pm core to skip ->suspend() via return value
> from ->prepare() which sets direct_complete

Yup.

> ...and your patch only deals with 1 and 2.

Yes, basically.

> 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?

Right (sorry).

> > 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().

Becuase the core needs input from middle layers in some cases before
it decides to call ->suspend().

> Using ->prepare() for the purpose seems wrong since it traverses
> the hierarchy in the "wrong" order.

That doesn't matter. What matters is whether or not the device's
state is "compatible" with system sleep at the ->prepare() time (which is
checked by some middle layers in ->prepare()).

BTW, did you notice the pm_runtime_status_suspended() checks in
__device_suspend()? They are in there in case the device has been
resumed from runtime suspend after ->prepare().

> 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. (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.)

Yes, that's a special case (again, because of the way i2c handles
runtime PM overall).

Thanks,
Rafael

\
 
 \ /
  Last update: 2017-09-21 19:34    [W:0.062 / U:29.456 seconds]
©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site