Re: [PATCH] ACPI: CPPC: Don't require _OSC if X86_FEATURE_CPPC is supported

[Rafael J. Wysocki]

On Wed, Jun 29, 2022 at 8:49 PM Limonciello, Mario
<Mario.Limonciello@xxxxxxx> wrote:
>
> [Public]
>
>
>
> > -----Original Message-----
> > From: Rafael J. Wysocki <rafael@xxxxxxxxxx>
> > Sent: Wednesday, June 29, 2022 13:42
> > To: Limonciello, Mario <Mario.Limonciello@xxxxxxx>
> > Cc: Rafael J. Wysocki <rafael@xxxxxxxxxx>; Len Brown <lenb@xxxxxxxxxx>;
> > Pierre Gondois <pierre.gondois@xxxxxxx>; Sudeep Holla
> > <sudeep.holla@xxxxxxx>; Yuan, Perry <Perry.Yuan@xxxxxxx>; ACPI Devel
> > Maling List <linux-acpi@xxxxxxxxxxxxxxx>; Linux Kernel Mailing List <linux-
> > kernel@xxxxxxxxxxxxxxx>
> > 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@xxxxxxx> 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@xxxxxxx>
> > > Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx>
> > > ---
> > >  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.

Good point.

Something like this would still look better though IMO:

if (!osc_sb_cppc_not_supported)
        return true;

#ifdef CONFIG_X86
        return boot_cpu_has(X86_FEATURE_CPPC);
#else
        return false;
#endif
}


>
> >
> > 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.

That is an option, but there is no requirement that _CPC must not be
populated when CPPC is not supported.

_OSC is the proper mechanism for negotiating CPPC support.

Still, if you know for a fact that on AMD systems X86_FEATURE_CPPC
always means that CPPC is supported, I can live with an extra vendor
check in the code above.