[PATCH 0/2] CPPC: reduce FFH feedback-counter sampling skew on arm64

[Pengjie Zhang]

The legacy CPPC feedback-counter path reads the delivered and reference
performance counters separately.

On arm64 systems using AMU-backed CPPC FFH counters, each FFH read is
served through a cross-CPU counter read helper. Reading the counters
separately therefore widens the sampling window between them and can
skew the delivered/reference ratio used by cpuinfo_cur_freq. Under heavy
load, the skew is observable as transient values that may exceed the
platform maximum, as discussed in [1] and [2].

This series adds a small generic hook for architectures that can obtain
both FFH feedback counters in one operation, while preserving the
existing per-register read path as the fallback.

Patch 1 adds the generic CPPC hook and uses it from cppc_get_perf_ctrs().
Patch 2 implements the hook on arm64 by sampling both AMU counters in a
single operation on the target CPU.

[1] https://lore.kernel.org/all/20231025093847.3740104-4-zengheng4@xxxxxxxxxx/
[2] https://lore.kernel.org/all/20231212072617.14756-1-lihuisong@xxxxxxxxxx/

Signed-off-by: Pengjie Zhang <zhangpengjie2@xxxxxxxxxx>

Pengjie Zhang (2):
  ACPI: CPPC: add paired FFH feedback-counter read hook
  arm64: topology: read CPPC FFH feedback counters in one operation

 arch/arm64/kernel/topology.c | 75 ++++++++++++++++++++++++++++++++----
 drivers/acpi/cppc_acpi.c     | 58 +++++++++++++++++++++++++---
 include/acpi/cppc_acpi.h     |  7 ++++
 3 files changed, 127 insertions(+), 13 deletions(-)

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2.33.0