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

[Sumit Gupta <sumitg@nvidia.com>]

On 26/08/26 15:57, Rafael J. Wysocki (Intel) wrote:
> External email: Use caution opening links or attachments
>
>
> On Wed, Aug 26, 2026 at 11:07 AM Christian Loehle
> <christian.loehle@arm.com> wrote:
>> On 8/25/26 22:20, Sumit Gupta wrote:
>>> On 07/08/26 01:38, 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.
>>> Gentle reminder.
>>> Could this be considered for queuing if nothing further needs addressing.
>> FWIW both sashiko findings look legit to me, the feedback counters one I
>> wouldn't consider that drastic, given that these counters are expected to
>> have 'fuzzy' readings anyhow, the second one is worse (losing sysfs settings
>> on cpu_online())
> I agree.
>
> Besides, as I said elsewhere, I want this series to go in before any
> other pending changes related to CPPC:
>
> https://lore.kernel.org/linux-acpi/20260826063019.670240-1-christian.loehle@arm.com/
>
> and I really would like to get some tags on it before it goes in.  It
> clearly is not ready for 7.3, but it may be applicable early for 7.4
> if people care to respond to it.
>
> Thanks!

Thanks for pointing this out.

I did not receive a Sashiko review email for this series, so I was
unaware of the findings. I have found the web review now. I will go
through both findings and address them in v5.

Thanks,
Sumit