Re: Intel GemniLake xHCI connected devices can never wake up the system from suspend

[Daniel Drake <drake@endlessm.com>]

> I've studied the ACPI spec trying to understand better, but I'm
> struggling with the question:
> What is the maximum number (lowest power) permitted device power state
> for a device that is configured as able to wake the system from S3,
> **that does not implement the _S3W method**?

Actually the ACPI spec has an answer for the case when _S3D is present.
The lack of clarity is only over the situation when both _S3D and _S3W
are missing - like on the platforms being worked on here.

The _S3D docs say:
> If the device can wake the system from the S3 system sleeping state (see
> _PRW) then the device must support wake in the D-state returned by this
> object. However, OSPM cannot assume wake from the S3 system sleeping state
> is supported in any deeper D-state unless specified by a corresponding
> _S3W object

Looking at the design of the existing Linux code, it seems like this
"max = min" assignment that is causing us trouble originates directly
from an attempt to implement that logic: if we didn't get a response from
_S3W, then we must clamp ourselves to the data we got from _S3D.

If I modify the Linux code to be a little more specific in that logic
(only applying when we actually got something from _S3D) then the
problematic behaviour is avoided and USB wakeups work.

I feel that this change makes the Linux implementation more directly
mirror the wording in the ACPI spec and it's associated lack of clarity
for when both methods are missing. Thoughts?

---
 drivers/acpi/device_pm.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/acpi/device_pm.c b/drivers/acpi/device_pm.c
index a4c8ad98560d..44f12c5c75ee 100644
--- a/drivers/acpi/device_pm.c
+++ b/drivers/acpi/device_pm.c
@@ -543,6 +543,7 @@ static int acpi_dev_pm_get_state(struct device *dev, struct acpi_device *adev,
 	unsigned long long ret;
 	int d_min, d_max;
 	bool wakeup = false;
+	acpi_status sxd_status;
 	acpi_status status;
 
 	/*
@@ -565,8 +566,8 @@ static int acpi_dev_pm_get_state(struct device *dev, struct acpi_device *adev,
 		 * provided if AE_NOT_FOUND is returned.
 		 */
 		ret = d_min;
-		status = acpi_evaluate_integer(handle, method, NULL, &ret);
-		if ((ACPI_FAILURE(status) && status != AE_NOT_FOUND)
+		sxd_status = acpi_evaluate_integer(handle, method, NULL, &ret);
+		if ((ACPI_FAILURE(sxd_status) && sxd_status != AE_NOT_FOUND)
 		    || ret > ACPI_STATE_D3_COLD)
 			return -ENODATA;
 
@@ -599,7 +600,11 @@ static int acpi_dev_pm_get_state(struct device *dev, struct acpi_device *adev,
 		method[3] = 'W';
 		status = acpi_evaluate_integer(handle, method, NULL, &ret);
 		if (status == AE_NOT_FOUND) {
-			if (target_state > ACPI_STATE_S0)
+			/* No _SxW. In this case, the ACPI spec says that we
+			 * must not go into any power state deeper than the
+			 * value returned from _SxD.
+			 */
+			if (sxd_status == AE_OK && target_state > ACPI_STATE_S0)
 				d_max = d_min;
 		} else if (ACPI_SUCCESS(status) && ret <= ACPI_STATE_D3_COLD) {
 			/* Fall back to D3cold if ret is not a valid state. */
-- 
2.14.1