[PATCH v4 0/4] cpufreq: CPPC: Preserve OSPM-set registers across hotplug and unload

[Sumit Gupta <sumitg@nvidia.com>]

This series keeps the CPPC cpufreq policy alive across CPU hotplug and
preserves the OSPM-set CPPC registers (Energy Performance Preference,
Autonomous Activity Window, Autonomous Selection - set via sysfs).

Without online()/offline() callbacks, the core tears a policy down when
its last CPU goes offline and rebuilds it on the way back, re-reading the
CPPC capabilities each time. The values written to these registers can
be lost:

 - Across CPU hotplug or suspend/resume: the platform may reset them
   while the CPU is offline.
 - On driver unload: the driver-written value is left in the register
   instead of returning to its pre-driver state.

Handle these with:

 - Patch 1: adds online()/offline() callbacks so the core keeps policy
   alive across CPU hotplug instead of tearing it down and rebuilding it.
 - Patch 2: makes the autonomous selection register helpers take a u64.
 - Patch 3: adds a table-driven mechanism that captures each register's
   firmware value at init(), restores it from offline(), and reapplies
   the OSPM-set value from online().
 - Patch 4: extends the same save/restore to system suspend/resume.

v3[3] -> v4:
 - Patch 1:
   - offline() parks the perf request at lowest_perf, as exit() did.
   - online() resyncs the frequency invariance counters.
   - raise MAX before the perf restore when the saved MIN is above it.
 - Patch 3:
   - write auto_sel first when enabling it and last when disabling it.
   - replace the four save/restore helpers into save_regs() and
     apply_saved_regs(), each taking the firmware or requested type.
   - keep the per-policy saved values in one struct, and name each
     register for the pr_debug diagnostics.
 - Patch 4:
   - suspend() also restores the firmware values and flags it, so
     offline() skips them and resume() only handles still-online policies.

Sumit Gupta (4):
  cpufreq: CPPC: Keep the policy across CPU hotplug
  ACPI: CPPC: Make autonomous selection helpers take a u64
  cpufreq: CPPC: Preserve OSPM-set registers across hotplug and unload
  cpufreq: CPPC: Preserve OSPM-set registers across suspend/resume

 drivers/acpi/cppc_acpi.c       |  20 +-
 drivers/cpufreq/amd-pstate.c   |   2 +-
 drivers/cpufreq/cppc_cpufreq.c | 369 ++++++++++++++++++++++++++++++++-
 include/acpi/cppc_acpi.h       |   8 +-
 4 files changed, 376 insertions(+), 23 deletions(-)

[1] v1: https://lore.kernel.org/lkml/20260623095403.3407436-1-sumitg@nvidia.com/
[2] v2: https://lore.kernel.org/lkml/20260716153820.2007095-1-sumitg@nvidia.com/
[3] v3: https://lore.kernel.org/lkml/20260724215937.3368276-1-sumitg@nvidia.com/

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2.34.1