ACPI / CPPC: do not require the _PSD method when using CPPC

According to the ACPI 6.3 specification, the _PSD method is optional when using CPPC. The underlying assumption is that each CPU can change frequency independently from all other CPUs; _PSD is provided to tell the OS that some processors can NOT do that.

However, the acpi_get_psd() function returns ENODEV if there is no _PSD method present, or an ACPI error status if an error occurs when evaluating _PSD, if present. This makes _PSD mandatory when using CPPC, in violation of the specification, and only on Linux.

This has forced some firmware writers to provide a dummy _PSD, even though it is irrelevant, but only because Linux requires it; other OSPMs follow the spec. We really do not want to have OS specific ACPI tables, though.

So, correct acpi_get_psd() so that it does not return an error if there is no _PSD method present, but does return a failure when the method can not be executed properly. This allows _PSD to be optional as it should be.

v3:
-- bodged the simplification in v2; simplified it properly this time -- verified all error statuses are handled

v2:
-- verified simple check for AE_NOT_FOUND was sufficient -- simplified return status check per Rafael's suggestion

---
 drivers/acpi/cppc_acpi.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

index 15f103d7532b..3b2525908dd8 100644
@@ -365,8 +365,10 @@ static int acpi_get_psd(struct cpc_desc *cpc_ptr, acpi_handle handle)
 	union acpi_object  *psd = NULL;
 	struct acpi_psd_package *pdomain;
 
-	status = acpi_evaluate_object_typed(handle, "_PSD", NULL, &buffer,
-			ACPI_TYPE_PACKAGE);
+	status = acpi_evaluate_object_typed(handle, "_PSD", NULL,
+					    &buffer, ACPI_TYPE_PACKAGE);
+	if (status == AE_NOT_FOUND)	/* _PSD is optional */
+		return 0;
 	if (ACPI_FAILURE(status))
 		return -ENODEV;
 

2.21.0