[PATCH v4 0/2] ACPI: CPPC: Add CPPC v4 support (ACPI 6.6)

[Sumit Gupta <sumitg@nvidia.com>]

Add initial kernel support for CPPC v4 (ACPI 6.6, Section 8.4.6),
which extends the _CPC package from 23 to 25 entries with two
optional fields:

  - OSPM Nominal Performance (8.4.6.1.2.6): register used by OSPM
    to tell the platform what it considers nominal. The platform
    classifies performance above this as boost and below as
    throttle for power/thermal decisions.

  - Resource Priority (8.4.6.1.2.7): Package of Resource Priority
    Register Descriptor sub-packages. Full parsing is not yet
    implemented; such entries are marked as unsupported.

Patch 1: Add v4 _CPC parsing - validate the 25-entry layout,
restrict the Resource Priority package fallback to that slot
only, and mark the two new registers optional.

Patch 2: Add per-policy sysfs attribute ospm_nominal_freq (kHz)
under cpufreq. Writes are converted to perf and applied to every
CPU in policy->cpus.

---
v3[3] -> v4:
- Patch 2:
  - Add a bool cpu_data->ospm_nominal_perf_set to distinguish an
    unwritten cache from a valid perf=0 write.
  - Switch freq_khz to unsigned int and use kstrtouint(). 
  - Add rollback in store_ospm_nominal_freq() to restore the previous
    cached value on a partial multi-CPU write failure.

v2[2] -> v3:
- Patch 1:
  - Accept ACPI_TYPE_PACKAGE only at RESOURCE_PRIORITY; abort otherwise.
  - Added Reviewed-by from Pierre Gondois.
- Patch 2:
  - Sysfs moved from per-CPU acpi_cppc to per-policy cpufreq.
  - Read ospm_nominal_freq sysfs input in kHz and convert to perf.
  - Cached ospm_nominal_perf moved from cpc_desc to cppc_cpudata.
  - Validate input against [Lowest Performance, Nominal Performance]
    in cppc_set_ospm_nominal_perf().
  - Dropped unconditional init in cppc_cpufreq_cpu_init().
  - Remove to skip write if cache matches in cppc_set_ospm_nominal_perf.
  - Early -ENODEV return when no CPC descriptor.
  - Added ABI documentation at
    Documentation/ABI/testing/sysfs-devices-system-cpu.

v1[1] -> v2:
- Patch 1: Added Reviewed-by from Mario Limonciello.
- Patch 2:
  - Make ospm_nominal_perf sysfs read-write; cache last write in
    cpc_desc and skip redundant register writes.
  - Validate input in cppc_set_ospm_nominal_perf.

Sumit Gupta (2):
  ACPI: CPPC: Add support for CPPC v4
  ACPI: CPPC: Add ospm_nominal_perf support

 .../ABI/testing/sysfs-devices-system-cpu      | 17 ++++++
 drivers/acpi/cppc_acpi.c                      | 58 ++++++++++++++++--
 drivers/cpufreq/cppc_cpufreq.c                | 60 +++++++++++++++++++
 include/acpi/cppc_acpi.h                      | 20 ++++++-
 4 files changed, 147 insertions(+), 8 deletions(-)

[1] https://lore.kernel.org/lkml/20260427051823.280419-1-sumitg@nvidia.com/
[2] https://lore.kernel.org/lkml/20260430142430.755437-1-sumitg@nvidia.com/
[3] https://lore.kernel.org/lkml/20260514194822.1841748-1-sumitg@nvidia.com/

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