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

[Al Stone <ahs3@redhat.com>]

According to the ACPI 6.3 specification, the _PSD method is optional
when using CPPC.  The underlying assumption appears to be 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 essentially 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.

Signed-off-by: Al Stone <ahs3@redhat.com>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: Len Brown <lenb@kernel.org>
---
 drivers/acpi/cppc_acpi.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 15f103d7532b..e9ecfa13e997 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -365,10 +365,13 @@ 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);
-	if (ACPI_FAILURE(status))
-		return -ENODEV;
+	if (acpi_has_method(handle, "_PSD")) {
+		status = acpi_evaluate_object_typed(handle, "_PSD", NULL,
+						    &buffer, ACPI_TYPE_PACKAGE);
+		if (ACPI_FAILURE(status))
+			return -ENODEV;
+	} else
+		return 0;		/* _PSD is optional */
 
 	psd = buffer.pointer;
 	if (!psd || psd->package.count != 1) {
-- 
2.21.0