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

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Date:   Wed, 29 Jun 2022 18:48:57 +0000
From:   "Limonciello, Mario" <Mario.Limonciello@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
CC:     Len Brown <lenb@...nel.org>,
        Pierre Gondois <pierre.gondois@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        "Yuan, Perry" <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

[Public]



> -----Original Message-----
> From: Rafael J. Wysocki <rafael@...nel.org>
> Sent: Wednesday, June 29, 2022 13:42
> To: Limonciello, Mario <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>; Yuan, Perry <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?

I don't think that would compile on non-X86.  X86_FEATURE_CPPC comes as part of
arch/x86/include/asm/cpufeatures.h, which I wouldn't expect is included on !x86.

> 
> 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?
> 

Well actually doing that through _OSC wouldn't have worked before 72f2ecb7ece7 either.
If desirable - a platform could avoid populating _CPC objects in ACPI tables in this case.

I do know of OEM platforms that the underlying APU supports CPPC but the OEM doesn't
populate _CPC.  Presumably for this exact reason.

> 
> > +       }
> > +
> >         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
> >