ACPI: CPPC: Reject desired_perf reads on _CPC revision 4+

When CPPC feedback counters cannot provide a usable sample, cppc-cpufreq
calls cppc_get_desired_perf() because some platforms repurpose Desired
Performance to report actual delivered performance.

ACPI 6.5 defines _CPC revision 3 and lists Read/Write as the Optional
Attribute of Desired Performance. ACPI 6.6 advances _CPC to revision 4 and
lists only Write, so invoking that workaround for revision 4 or later would
require a register read that the interface no longer specifies.

Make cppc_get_desired_perf() return -EOPNOTSUPP for _CPC revision 4 or
later. Use the revision retained in the per-CPU CPC descriptor rather than
the platform-wide FADT revision.

The _CPC revision may still not accurately describe the implemented
register semantics. If a nominally revision 3 platform implements a
non-readable Desired Performance register, a read may return zero and make
cppc_cpufreq_get_rate() report 0 kHz. Treat a zero read as unusable and
fall back to the cached OSPM request, just as for a failed read.

Fixes: c47195631960 ("cppc_cpufreq: Use desired perf if feedback ctrs are 0 or unchanged")
Cc: stable@vger.kernel.org
Suggested-by: Sumit Gupta <sumitg@nvidia.com>
Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Link: https://patch.msgid.link/20260803203531.1268651-2-christian.loehle@arm.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

0 files changed, 0 insertions, 0 deletions