[PATCH v2 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.

For detailed test results and data demonstrating the observable skew and
the improvements brought by this series, please refer to [3] and [4].

[1] https://lore.kernel.org/all/20231025093847.3740104-4-zengheng4@huawei.com/
[2] https://lore.kernel.org/all/20231212072617.14756-1-lihuisong@huawei.com/
[3] https://lore.kernel.org/all/443104e2-ba6e-454e-8469-909f35817a99@huawei.com/
[4] https://lore.kernel.org/all/317d33d5-8279-4aa8-84b7-6ae1976636ac@huawei.com/

Tested-by: Sumit Gupta <sumitg at nvidia.com>
Reviewed-by: Sumit Gupta <sumitg at nvidia.com>
Tested-by: Vanshidhar Konda <vanshikonda at os.amperecomputing.com>
Reviewed-by: Vanshidhar Konda <vanshikonda at os.amperecomputing.com>
Signed-off-by: Pengjie Zhang <zhangpengjie2 at huawei.com>
---
Changes in v2:
- Simplified the CPPC generic layer fallback logic to prevent pointless single-read retries.
- Added upfront register validation in the arm64 hook to avoid unnecessary IPI overhead.
- Explicitly flipped the -EOPNOTSUPP error to -ENODEV in the arm64 hook when AMU is unsupported, cleanly bypassing redundant CPPC generic fallbacks.
- Addressed other kernel-doc and naming feedbacks from Beata.
- Added Reviewed-by and Tested-by tags from Vanshidhar and Sumit
- Link to v1: https://lore.kernel.org/all/20260410094145.4132082-1-zhangpengjie2@huawei.com/

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 | 92 ++++++++++++++++++++++++++++++++----
 drivers/acpi/cppc_acpi.c     | 50 ++++++++++++++++++--
 include/acpi/cppc_acpi.h     |  7 +++
 3 files changed, 136 insertions(+), 13 deletions(-)

-- 
2.33.0