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SubjectRe: Intel GemniLake xHCI connected devices can never wake up the system from suspend
Date
> 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
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