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

[Mika Westerberg <mika.westerberg@linux.intel.com>]

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.

> 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?