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

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Date:   Wed, 29 Jun 2022 20:41:53 +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>,
        Pierre Gondois <pierre.gondois@....com>,
        Sudeep Holla <sudeep.holla@....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] ACPI: CPPC: Don't require _OSC if X86_FEATURE_CPPC is supported

On Mon, Jun 27, 2022 at 6:58 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 `X86_FEATURE_CPPC`,
> use this enable this feature rather than requiring the `_OSC`.
>
> 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>
> ---
>  drivers/acpi/cppc_acpi.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index 903528f7e187..5463e6309b9a 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -629,6 +629,15 @@ static bool is_cppc_supported(int revision, int num_ent)
>                 return false;
>         }
>
> +       if (osc_sb_cppc_not_supported) {
> +               pr_debug("Firmware missing _OSC support\n");
> +#ifdef CONFIG_X86
> +               return boot_cpu_has(X86_FEATURE_CPPC);
> +#else
> +               return false;
> +#endif

What about doing

if (osc_sb_cppc_not_supported) {
        pr_debug("Firmware missing _OSC support\n");
        return IS_ENABLED(CONFIG_X86) && boot_cpu_has(X86_FEATURE_CPPC);
}

instead for the sake of reducing #ifdeffery?

Also, this is somewhat risky, because even if the given processor has
X86_FEATURE_CPPC set, the platform may still not want to expose CPPC
through ACPI.  How's that going to work after this change?


> +       }
> +
>         return true;
>  }
>
> @@ -684,9 +693,6 @@ 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);
> --
> 2.34.1
>