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Subject: Re: [PATCH] ACPI / CPPC: do not require the _PSD method when using
 CPPC
To:     Sudeep Holla <sudeep.holla@arm.com>
Cc:     linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        Len Brown <lenb@kernel.org>
References: <20190805170338.29493-1-ahs3@redhat.com>
 <20190807114118.GJ16546@e107155-lin>
From:   Al Stone <ahs3@redhat.com>
Organization: Red Hat, Inc.
Message-ID: <b873db68-a130-ed7f-83e0-4491b1c0ff53@redhat.com>
Date:   Sat, 10 Aug 2019 11:25:53 -0600
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On 8/7/19 5:41 AM, Sudeep Holla wrote:
> On Mon, Aug 05, 2019 at 11:03:38AM -0600, 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.
>>
> 
> Makes sense to me. FWIW,
> 
> Reviewed-by: Sudeep Holla < sudeep.holla@arm.com>
> 
> --
> Regards,
> Sudeep
> 

Thanks for the review, Sudeep.  All the ARM systems I've seen seem to
have a _PSD so this hasn't been an issue there.  Some newer platforms
coming out are starting to use CPPC, though, and took the spec at face
value :).

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