From: Al Stone <ahs3@redhat.com>
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>
Subject: Re: [PATCH] ACPI / CPPC: do not require the _PSD method when using CPPC
Date: Sat, 10 Aug 2019 11:25:53 -0600 [thread overview]
Message-ID: <b873db68-a130-ed7f-83e0-4491b1c0ff53@redhat.com> (raw)
In-Reply-To: <20190807114118.GJ16546@e107155-lin>
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
-----------------------------------
next prev parent reply other threads:[~2019-08-10 17:25 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-05 17:03 [PATCH] ACPI / CPPC: do not require the _PSD method when using CPPC Al Stone
2019-08-07 11:41 ` Sudeep Holla
2019-08-10 17:25 ` Al Stone [this message]
2019-08-13 14:00 ` Al Stone
2019-08-13 21:57 ` Rafael J. Wysocki
2019-08-13 22:15 ` Al Stone
2019-08-13 21:59 ` Rafael J. Wysocki
2019-08-13 22:26 ` Al Stone
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