[PATCH v2] ACPI / CPPC: do not require the _PSD method when using CPPC

[PATCH v2] ACPI / CPPC: do not require the _PSD method when using CPPC

[Al Stone]

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 <ahs3@redhat.com>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: Len Brown <lenb@kernel.org>
---
 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

Re: [PATCH v2] ACPI / CPPC: do not require the _PSD method when using CPPC

[Rafael J. Wysocki]

On Tue, Aug 27, 2019 at 12:30 AM Al Stone <ahs3@redhat.com> 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 <ahs3@redhat.com>
> Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
> Cc: Len Brown <lenb@kernel.org>
> ---
>  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
>

Re: [PATCH v2] ACPI / CPPC: do not require the _PSD method when using CPPC

[Al Stone]

On 8/26/19 5:02 PM, Rafael J. Wysocki wrote:
> On Tue, Aug 27, 2019 at 12:30 AM Al Stone <ahs3@redhat.com> 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 <ahs3@redhat.com>
>> Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
>> Cc: Len Brown <lenb@kernel.org>
>> ---
>>  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.
ahs3@redhat.com
-----------------------------------