ACPI / CPPC: do not require the _PSD method

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.

Signed-off-by: Al Stone <ahs3@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 15f103d7532b..3b2525908dd8 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -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;