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Reply-To: ahs3@redhat.com
Subject: Re: [PATCH v2] ACPI / CPPC: do not require the _PSD method when using
 CPPC
To:     "Rafael J. Wysocki" <rafael@kernel.org>
Cc:     ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        Len Brown <lenb@kernel.org>
References: <20190826223047.13146-1-ahs3@redhat.com>
 <CAJZ5v0hXJNa3Z14MQgvNcVF8YxH+M46mMyC5U6vZqrpBZUjOgw@mail.gmail.com>
From:   Al Stone <ahs3@redhat.com>
Organization: Red Hat, Inc.
Message-ID: <38f21587-f5c9-c831-d7ff-707974178d7f@redhat.com>
Date:   Mon, 26 Aug 2019 20:31:23 -0600
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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
-----------------------------------