Re: [PATCH v2 1/3] PCI / ACPI: Use cached ACPI device state to get PCI device power state

["Rafael J. Wysocki" <rafael@kernel.org>]

On Tue, Jun 25, 2019 at 11:46 AM Mika Westerberg
<mika.westerberg@linux.intel.com> wrote:
>
> On Mon, Jun 24, 2019 at 01:14:47PM +0200, Rafael J. Wysocki wrote:
> > > The problem here is that acpi_device_get_power() really only should be
> > > used for two purposes: (1) To initialize adev->power.state, or to
> > > update it via acpi_device_update_power(), and (2) by the
> > > "real_power_state" sysfs attribute (of ACPI device objects).  The
> > > adev->power.state value should be used anywhere else, in principle, so
> > > the Mika's patch is correct.
> >
> > Well, it is an improvement, but it is not sufficient.
> >
> > > [Note that adev->power.state cannot be updated after calling
> > > acpi_device_get_power() to the value returned by it without updating
> > > the reference counters of the power resources that are "on" *exactly*
> > > because of the problem at hand here.]
> >
> > That is obviously correct, but ->
> >
> > > > but that's just an idle thought, not a suggestion.
> > >
> > > After the initialization of the ACPI subsystem, the authoritative
> > > source of the ACPI device power state information is
> > > adev->power.state.  The ACPI subsystem is expected to update this
> > > value as needed going forward (including system-wide transitions like
> > > resume from S3).
> >
> > -> the "resume from S3 or hibernation" case needs special handling, because
> > in that case the device power state need not reflect the information the ACPI
> > subsystem has.  That only matters if adev->power.state is ACPI_STATE_D0 and
> > the device is actually *not* in D0, because in that case acpi_device_set_power()
> > will not work.
>
> I guess you are talking about the special-cased devices that we leave in
> D0 when system suspend (via firmware) is entered?
>
> > So that case is not covered currently (it should be rare in practice,
> > though, if it happens at all), so something like the patch below (untested) may
> > be needed in addition to the Mika's patch.
>
> Looks good to me.

I actually decided to address this issue in acpi_device_set_power() as
it may affect devices beyond PCI in principle.  I will send a patch
for that shortly.

> > Still, there is also the "power state not matching" case in pci_pm_complete() that's
> > need to be covered and the non-PCI ACPI PM has a similar issue in theory, so I
> > need to think about this a bit more.
>
> Do you want me to hold off sending an updated version of the patch
> series while we figure this one out or is it fine if I send it out now
> and we can add further details on top?

It is independent of the other fix, so it can be sent now just fine IMO.