linux-kernel - Re: [PATCH v2] ACPI: CPPC: Don't require _OSC if X86_FEATURE

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Date:   Tue, 5 Jul 2022 20:11:31 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Mario Limonciello <mario.limonciello@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Pierre Gondois <pierre.gondois@....com>,
        Perry Yuan <perry.yuan@....com>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] ACPI: CPPC: Don't require _OSC if X86_FEATURE_CPPC is supported

On Wed, Jun 29, 2022 at 10:04 PM Mario Limonciello
<mario.limonciello@....com> wrote:
>
> commit 72f2ecb7ece7 ("ACPI: bus: Set CPPC _OSC bits for all and
> when CPPC_LIB is supported") added support for claiming to
> support CPPC in _OSC on non-Intel platforms.
>
> This unfortunately caused a regression on a vartiety of AMD
> platforms in the field because a number of AMD platforms don't set
> the `_OSC` bit 5 or 6 to indicate CPPC or CPPC v2 support.
>
> As these AMD platforms already claim CPPC support via a dedicated
> MSR from `X86_FEATURE_CPPC`, use that to enable this feature rather
> than requiring the `_OSC` on platforms with a dedicated MSR.
>
> If there is additional breakage on the shared memory designs also
> missing this _OSC, additional follow up changes may be needed.
>
> Fixes: 72f2ecb7ece7 ("Set CPPC _OSC bits for all and when CPPC_LIB is supported")
> Reported-by: Perry Yuan <perry.yuan@....com>
> Signed-off-by: Mario Limonciello <mario.limonciello@....com>

I'm still concerned about the possible cases in which there is _CPC,
but it returns garbage, because the firmware thinks that the OS will
not use _CPC due to the _OSC handshake.

> ---
> v1->v2:
>  * Make the code easier to follow (suggested by Rafael)
>  * Update commit message to reflect this is only fixing the MSR case
>    and that any other breakage from 72f2ecb7ece7 will need additional
>    follow ups
>  drivers/acpi/cppc_acpi.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index 903528f7e187..cc154519c608 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -629,7 +629,15 @@ static bool is_cppc_supported(int revision, int num_ent)
>                 return false;
>         }
>
> -       return true;
> +       if (!osc_sb_cppc_not_supported)
> +               return true;
> +
> +       pr_debug("Firmware missing _OSC support\n");
> +#ifdef CONFIG_X86
> +       return boot_cpu_has(X86_FEATURE_CPPC);

So can you please add an additional X86_VENDOR_AMD check to the above?

> +#else
> +       return false;
> +#endif
>  }
>
>  /*
> @@ -684,9 +692,6 @@ int acpi_cppc_processor_probe(struct acpi_processor *pr)
>         acpi_status status;
>         int ret = -ENODATA;
>
> -       if (osc_sb_cppc_not_supported)
> -               return -ENODEV;
> -
>         /* Parse the ACPI _CPC table for this CPU. */
>         status = acpi_evaluate_object_typed(handle, "_CPC", NULL, &output,
>                         ACPI_TYPE_PACKAGE);
> --
> 2.25.1
>