Re: [PATCH 2/2] ACPI: PM: Allow transitions to D0 to occur in special cases

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

On Tue, Jun 25, 2019 at 02:06:13PM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> If a device with ACPI PM is left in D0 during a system-wide
> transition to the S3 (suspend-to-RAM) or S4 (hibernation) sleep
> state, the actual state of the device need not be D0 during resume
> from it, although its power.state value will still reflect D0 (that
> is, the power state from before the system-wide transition).
> 
> In that case, the acpi_device_set_power() call made to ensure that
> the power state of the device will be D0 going forward has no effect,
> because the new state (D0) is equal to the one reflected by the
> device's power.state value.  That does not affect power resources,
> which are taken care of by acpi_resume_power_resources() called from
> acpi_pm_finish() during resume from system-wide sleep states, but it
> still may be necessary to invoke _PS0 for the device on top of that
> in order to finalize its transition to D0.
> 
> For this reason, modify acpi_device_set_power() to allow transitions
> to D0 to occur even if D0 is the current power state of the device
> according to its power.state value.
> 
> That will not affect power resources, which are assumed to be in
> the right configuration already (as reflected by the current values
> of their reference counters), but it may cause _PS0 to be evaluated
> for the device.  However, evaluating _PS0 for a device already in D0
> may lead to confusion in general, so invoke _PSC (if present) to
> check the device's current power state upfront and only evaluate
> _PS0 for it if _PSC has returned a power state different from D0.
> [If _PSC is not present or the evaluation of it fails, the power
> state of the device is assumed to be D0 at this point.]
> 
> Fixes: 20dacb71ad28 (ACPI / PM: Rework device power management to follow ACPI 6)
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>