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Subject: Re: [PATCH] ACPI / CPPC: do not require the _PSD method when using
 CPPC
To:     "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc:     linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org,
        Len Brown <lenb@kernel.org>
References: <20190805170338.29493-1-ahs3@redhat.com>
 <521915646.RcUJINxfhL@kreacher>
From:   Al Stone <ahs3@redhat.com>
Organization: Red Hat, Inc.
Message-ID: <e103debd-d0f2-5b40-1e6d-7a533ef4ac07@redhat.com>
Date:   Tue, 13 Aug 2019 16:26:38 -0600
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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
-----------------------------------