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.
v2:
-- verified simple check for AE_NOT_FOUND was sufficient
-- simplified return status check per Rafael's suggestion
Signed-off-by: Al Stone <[email protected]>
Cc: Rafael J. Wysocki <[email protected]>
Cc: Len Brown <[email protected]>
---
drivers/acpi/cppc_acpi.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)
diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 15f103d7532b..7a946f1944ab 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -365,10 +365,12 @@ 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 (status == AE_NOT_FOUND) /* _PSD is optional */
+ return 0;
+ }
psd = buffer.pointer;
if (!psd || psd->package.count != 1) {
--
2.21.0
On Tue, Aug 27, 2019 at 12:30 AM Al Stone <[email protected]> wrote:
>
> 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.
>
> v2:
> -- verified simple check for AE_NOT_FOUND was sufficient
> -- simplified return status check per Rafael's suggestion
>
> Signed-off-by: Al Stone <[email protected]>
> Cc: Rafael J. Wysocki <[email protected]>
> Cc: Len Brown <[email protected]>
> ---
> drivers/acpi/cppc_acpi.c | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index 15f103d7532b..7a946f1944ab 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -365,10 +365,12 @@ 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")) {
This doesn't look necessary any more.
> + status = acpi_evaluate_object_typed(handle, "_PSD", NULL,
> + &buffer, ACPI_TYPE_PACKAGE);
> + if (status == AE_NOT_FOUND) /* _PSD is optional */
> + return 0;
And what about the other possible errors?
> + }
>
> psd = buffer.pointer;
> if (!psd || psd->package.count != 1) {
> --
> 2.21.0
>
On 8/26/19 5:02 PM, Rafael J. Wysocki wrote:
> On Tue, Aug 27, 2019 at 12:30 AM Al Stone <[email protected]> wrote:
>>
>> 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.
>>
>> v2:
>> -- verified simple check for AE_NOT_FOUND was sufficient
>> -- simplified return status check per Rafael's suggestion
>>
>> Signed-off-by: Al Stone <[email protected]>
>> Cc: Rafael J. Wysocki <[email protected]>
>> Cc: Len Brown <[email protected]>
>> ---
>> drivers/acpi/cppc_acpi.c | 10 ++++++----
>> 1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
>> index 15f103d7532b..7a946f1944ab 100644
>> --- a/drivers/acpi/cppc_acpi.c
>> +++ b/drivers/acpi/cppc_acpi.c
>> @@ -365,10 +365,12 @@ 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")) {
>
> This doesn't look necessary any more.
Probably true. I'll look back through acpi_evaluate_object_typed().
>> + status = acpi_evaluate_object_typed(handle, "_PSD", NULL,
>> + &buffer, ACPI_TYPE_PACKAGE);
>> + if (status == AE_NOT_FOUND) /* _PSD is optional */
>> + return 0;
>
> And what about the other possible errors?
Argh. My apologies. I was not paying attention. I'll correct
this and send proper code tomorrow. Really sorry for the noise :(...
>> + }
>>
>> psd = buffer.pointer;
>> if (!psd || psd->package.count != 1) {
>> --
>> 2.21.0
>>
--
ciao,
al
-----------------------------------
Al Stone
Software Engineer
Red Hat, Inc.
[email protected]
-----------------------------------