Re: [PATCH v2 1/2] PCI / PM: Add needs_resume flag to avoid suspend complete optimization

[Lukas Wunner <lukas@wunner.de>]

On Mon, Apr 24, 2017 at 10:56:40PM +0200, Rafael J. Wysocki wrote:
> On Monday, April 24, 2017 10:42:42 PM Lukas Wunner wrote:
> > On Mon, Apr 24, 2017 at 10:02:30PM +0200, Lukas Wunner wrote:
> > > On Mon, Apr 24, 2017 at 05:27:42PM +0300, Imre Deak wrote:
> > > > Some drivers - like i915 - may not support the system suspend direct
> > > > complete optimization due to differences in their runtime and system
> > > > suspend sequence. Add a flag that when set resumes the device before
> > > > calling the driver's system suspend handlers which effectively disables
> > > > the optimization.
> > > 
> > > FWIW, there are at least two alternative solutions to this problem which
> > > do not require changes to the PCI core:
> > > 
> > > (1) Add a ->prepare hook to i915_pm_ops which calls pm_runtime_get_sync()
> > >     and a ->complete hook which calls pm_runtime_put().
> > 
> > Thinking a bit more about this, it's even simpler:  The PM core acquires
> > a runtime PM ref in device_prepare() and releases it in device_complete(),
> > so it's sufficient to just call pm_runtime_resume() in a ->prepare hook
> > that's newly added to i915.  No ->complete hook necessary.  Tentative
> > patch below, based on drm-intel-fixes, would replace both of your patches.
> 
> Calling it in ->prepare() means that everybody is now waiting for you to
> resume.
> 
> Not quite optimal IMO.

Okay, understood.

However in the absence of a device_link from the HDA device (consumer)
to the GPU (supplier), there's no guarantee that the GPU is resumed
when the HDA device needs it due to the asynchronous invocation of
the ->suspend hooks.  This is assuming that the HDA device already
needs the GPU during ->suspend phase.

Thanks,

Lukas