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SubjectRe: [PATCH] ACPI / CPPC: do not require the _PSD method when using CPPC
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On 8/13/19 3:59 PM, Rafael J. Wysocki wrote:
> On Monday, August 5, 2019 7:03:38 PM CEST Al Stone wrote:
>> 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")) {
>
> It would be better to compare the status below to AE_NOT_FOUND
> and return 0 if that's the case.
>
> A couple of code lines could be saved this way at least.

D'oh. Good point.

Let me dig back through the ACPICA code again; I had some reason for not
relying on AE_NOT_FOUND alone that I apparently did not write down in my
notes. I'll send out a v2 when I figure out what it was, and if it was
of any consequence.

>> + 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) {
>>
Thanks.

--
ciao,
al
-----------------------------------
Al Stone
Software Engineer
Red Hat, Inc.
ahs3@redhat.com
-----------------------------------

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