[PATCH v2 0/2] CPPC: reduce FFH feedback-counter sampling skew on arm64

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/[email protected]/
[2] https://lore.kernel.org/all/[email protected]/
[3] https://lore.kernel.org/all/[email protected]/
[4] https://lore.kernel.org/all/[email protected]/

Tested-by: Sumit Gupta <[email protected]>
Reviewed-by: Sumit Gupta <[email protected]>
Tested-by: Vanshidhar Konda <[email protected]>
Reviewed-by: Vanshidhar Konda <[email protected]>
Signed-off-by: Pengjie Zhang <[email protected]>
---
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/[email protected]/

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(-)

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2.33.0