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Date:   Wed, 7 Aug 2019 12:41:18 +0100
From:   Sudeep Holla <sudeep.holla@arm.com>
To:     Al Stone <ahs3@redhat.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
Message-ID: <20190807114118.GJ16546@e107155-lin>
References: <20190805170338.29493-1-ahs3@redhat.com>
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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