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SubjectRe: [PATCH] ACPI / CPPC: do not require the _PSD method when using CPPC
Date
On Tuesday, August 13, 2019 4:00:56 PM CEST Al Stone wrote:
> On 8/5/19 11:03 AM, Al Stone wrote:
> > According to the ACPI 6.3 specification, the _PSD method is optional
> > when using CPPC. The underlying assumption appears to be that each CPU
> > can change frequency independently from all other CPUs; _PSD is provided
> > to tell the OS that some processors can NOT do that.
> >
> > However, the acpi_get_psd() function returns -ENODEV if there is no _PSD
> > method present, or an ACPI error status if an error occurs when evaluating
> > _PSD, if present. This essentially makes _PSD mandatory when using CPPC,
> > in violation of the specification, and only on Linux.
> >
> > This has forced some firmware writers to provide a dummy _PSD, even though
> > it is irrelevant, but only because Linux requires it; other OSPMs follow
> > the spec. We really do not want to have OS specific ACPI tables, though.
> >
> > So, correct acpi_get_psd() so that it does not return an error if there
> > is no _PSD method present, but does return a failure when the method can
> > not be executed properly. This allows _PSD to be optional as it should
> > be.
> >
> > Signed-off-by: Al Stone <ahs3@redhat.com>
> > Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
> > Cc: Len Brown <lenb@kernel.org>
> > ---
> > drivers/acpi/cppc_acpi.c | 11 +++++++----
> > 1 file changed, 7 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> > index 15f103d7532b..e9ecfa13e997 100644
> > --- a/drivers/acpi/cppc_acpi.c
> > +++ b/drivers/acpi/cppc_acpi.c
> > @@ -365,10 +365,13 @@ static int acpi_get_psd(struct cpc_desc *cpc_ptr, acpi_handle handle)
> > union acpi_object *psd = NULL;
> > struct acpi_psd_package *pdomain;
> >
> > - status = acpi_evaluate_object_typed(handle, "_PSD", NULL, &buffer,
> > - ACPI_TYPE_PACKAGE);
> > - if (ACPI_FAILURE(status))
> > - return -ENODEV;
> > + if (acpi_has_method(handle, "_PSD")) {
> > + status = acpi_evaluate_object_typed(handle, "_PSD", NULL,
> > + &buffer, ACPI_TYPE_PACKAGE);
> > + if (ACPI_FAILURE(status))
> > + return -ENODEV;
> > + } else
> > + return 0; /* _PSD is optional */
> >
> > psd = buffer.pointer;
> > if (!psd || psd->package.count != 1) {
> >
>
> Rafael,
>
> Any other comments?

Yes (they will be sent separately).

> Would it be possible to pull this into an -rc?
> I'd really like to avoid anyone else having to ship Linux-specific
> DSDTs and SSDTs.

You won't achieve that through this patch alone, because they will
also want older kernels that don't include it to run on their platforms.

Thanks,
Rafael



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