linux-kernel - Re: [PATCH] ACPI / CPPC: do not require the

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Date:   Sat, 10 Aug 2019 11:25:53 -0600
From:   Al Stone <ahs3@...hat.com>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>
Subject: Re: [PATCH] ACPI / CPPC: do not require the _PSD method when using
 CPPC

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@....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@...hat.com
-----------------------------------