[PATCH v3] ACPI / CPPC: do not require the _PSD method when using CPPC

From: Al Stone
Date: Tue Aug 27 2019 - 20:21:31 EST

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

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

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

Signed-off-by: Al Stone <ahs3@xxxxxxxxxx>
drivers/acpi/cppc_acpi.c | 6 ++++--
1 file 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,
+ 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;