On Monday, August 5, 2019 7:03:38 PM CEST 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@xxxxxxxxxx>
> Cc: Rafael J. Wysocki <rjw@xxxxxxxxxxxxx>
> Cc: Len Brown <lenb@xxxxxxxxxx>
> ---
> 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")) {
It would be better to compare the status below to AE_NOT_FOUND
and return 0 if that's the case.
A couple of code lines could be saved this way at least.
> + 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) {
>
