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

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Date:   Tue, 5 Jul 2022 18:28:17 +0000
From:   "Limonciello, Mario" <Mario.Limonciello@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
CC:     Len Brown <lenb@...nel.org>, Sudeep Holla <sudeep.holla@....com>,
        Pierre Gondois <pierre.gondois@....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 v2] ACPI: CPPC: Don't require _OSC if X86_FEATURE_CPPC is
 supported

[Public]



> -----Original Message-----
> From: Rafael J. Wysocki <rafael@...nel.org>
> Sent: Tuesday, July 5, 2022 13:12
> To: Limonciello, Mario <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>; 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 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?

Very well, I'll send out v4 shortly with that modification.

> 
> > +#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
> >