[PATCH v8 0/3] ACPI / cpufreq: CPPC: Add ospm_nominal_perf support

[Sumit Gupta <sumitg@nvidia.com>]

This series adds support for the OSPM Nominal Performance register
(ACPI 6.6, Section 8.4.6.1.2.6) to the CPPC ACPI and cpufreq drivers.

Unlike the read-only Nominal Performance register, OSPM Nominal
Performance is writable and lets OSPM request a nominal level below
the platform-reported nominal. The platform treats performance above
this level as boost and below as throttle.

 - Patch 1: adds ACPI helpers to write the register and to check that
   the platform supports it.

 - Patch 2: adds the per-policy ospm_nominal_freq attribute in kHz. The
   value is kept across CPU hotplug and suspend/resume. The register
   cannot be read back, so a read returns the last value written.

 - Patch 3: uses the OSPM nominal as the maximum frequency while boost
   is off. Lowering the nominal creates a boost range on a platform
   that has none, and boost can then be enabled.

This series applies on top of v7 of:
  cpufreq: CPPC: Preserve OSPM-set registers across hotplug and unload
  https://lore.kernel.org/lkml/20261030092416.1993889-1-sumitg@nvidia.com/

v7 -> v8:
 - Rebase on Christian's CPPC fixes series.
 - Patch 1:
   - use cpc_is_writable() from that series and drop cpc_reg_writable().
   - add the register to the list of controls the driver writes.
   - take cpc_desc_lock in cppc_ospm_nominal_perf_supported(), so the
     descriptor cannot be removed during the check.
 - Patch 2:
   - make ospm_nominal_freq read-write. A read returns the last value
     written, or "<unset>" before the first write. (Christian, Mario)
   - keep saving the values in the store handler, not at init(), so
     the driver writes the register only after the OS sets it. (Pierre)
 - Patch 3:
   - document in the ABI file how the OSPM nominal sets cpuinfo_max_freq
     and caps scaling_max_freq.

Sumit Gupta (3):
  ACPI: CPPC: Add ospm_nominal_perf support
  cpufreq: CPPC: Add ospm_nominal_freq attribute
  cpufreq: CPPC: Reflect the OSPM nominal in boost and limits

 .../ABI/testing/sysfs-devices-system-cpu      |  31 ++++
 drivers/acpi/cppc_acpi.c                      |  44 +++++
 drivers/cpufreq/cppc_cpufreq.c                | 173 +++++++++++++++++-
 include/acpi/cppc_acpi.h                      |  10 +
 4 files changed, 248 insertions(+), 10 deletions(-)

v7: https://lore.kernel.org/lkml/20260807214837.863209-1-sumitg@nvidia.com/
v6: https://lore.kernel.org/lkml/20260717215330.2215058-1-sumitg@nvidia.com/
v5: https://lore.kernel.org/lkml/20260615185934.2383514-1-sumitg@nvidia.com/
v4: https://lore.kernel.org/lkml/20260527194626.185286-1-sumitg@nvidia.com/
v3: https://lore.kernel.org/lkml/20260514194822.1841748-1-sumitg@nvidia.com/
v2: https://lore.kernel.org/lkml/20260430142430.755437-1-sumitg@nvidia.com/
v1: https://lore.kernel.org/lkml/20260427051823.280419-1-sumitg@nvidia.com/

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2.34.1