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    Subject[PATCH 4.4 62/99] ACPI / CPPC: do not require the _PSD method
    Date
    From: Al Stone <ahs3@redhat.com>

    [ Upstream commit 4c4cdc4c63853fee48c02e25c8605fb65a6c9924 ]

    According to the ACPI 6.3 specification, the _PSD method is optional
    when using CPPC. The underlying assumption is 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 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>
    Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    drivers/acpi/cppc_acpi.c | 6 ++++--
    1 file changed, 4 insertions(+), 2 deletions(-)

    diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
    index 0afd1981e350b..43c27c04c40ae 100644
    --- a/drivers/acpi/cppc_acpi.c
    +++ b/drivers/acpi/cppc_acpi.c
    @@ -137,8 +137,10 @@ 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);
    + status = acpi_evaluate_object_typed(handle, "_PSD", NULL,
    + &buffer, ACPI_TYPE_PACKAGE);
    + if (status == AE_NOT_FOUND) /* _PSD is optional */
    + return 0;
    if (ACPI_FAILURE(status))
    return -ENODEV;

    --
    2.20.1


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