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

[Sumit Gupta <sumitg@nvidia.com>]

On 17/09/26 00:34, K Prateek Nayak wrote:
> External email: Use caution opening links or attachments
> 
> 
> 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@amd.com>
> 
> --
> Thanks and Regards,
> Prateek
> 

Hi Prateek,

Thanks for testing on x86.
I will add your Tested-by to the series, and Reviewed-by to patch 2.

Thanks,
Sumit