Re: [PATCH v5 0/4] cpufreq: CPPC: Preserve OSPM-set registers across hotplug and unload

[K Prateek Nayak]

Hello Sumit,

On 9/16/2026 4:08 PM, Sumit Gupta wrote:
> 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.
> 
> v4[4] -> v5:
>  - Patch 1:
>    - offline() stops the frequency invariance updates and online()
>      restarts them, replacing the resync that raced with tick. (Sashiko)
>    - reorder the functions to match cppc_cpufreq_driver. (Jie Zhan)
>  - Patch 3:
>    - reapply the OSPM-set registers even when the performance control
>      write fails. (Sashiko)
>  - Patch 4:
>    - suspend() stops the frequency invariance updates too, completing the
>      patch 1 change for a policy whose CPUs stay online.
> 
> 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 | 378 ++++++++++++++++++++++++++++++++-
>  include/acpi/cppc_acpi.h       |   8 +-
>  4 files changed, 381 insertions(+), 27 deletions(-)

I was able to hack the Kconfig to build cppc_cpufreq.c for x86 too
and test this out on a Zen3 machine that uses shared memory based
CPPC control.

FWIW, I could verify that "auto_select" and
"energy_performance_preference_val" persists correctly across an
offline-online cycle on my system so feel free to include:

Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>

-- 
Thanks and Regards,
Prateek