Re: [PATCH v3 2/2] cpufreq: CPPC: add autonomous mode boot parameter support

[Sumit Gupta <sumitg@nvidia.com>]

On 18/05/26 19:51, Mario Limonciello wrote:
> External email: Use caution opening links or attachments
>
>
> On 5/18/26 09:15, Sumit Gupta wrote:
>>
>> On 18/05/26 19:20, Mario Limonciello wrote:
>>> External email: Use caution opening links or attachments
>>>
>>>
>>> On 5/18/26 08:44, Sumit Gupta wrote:
>>>> Hi Mario,
>>>>
>>>>
>>>> On 16/05/26 02:43, Mario Limonciello wrote:
>>>>> External email: Use caution opening links or attachments
>>>>>
>>>>>
>>>>> On 5/15/26 07:26, Sumit Gupta wrote:
>>>>>> Add a kernel boot parameter 'cppc_cpufreq.auto_sel_mode' to enable
>>>>>> CPPC autonomous performance selection on all CPUs at system startup.
>>>>>> When autonomous mode is enabled, the hardware automatically adjusts
>>>>>> CPU performance based on workload demands using Energy Performance
>>>>>> Preference (EPP) hints.
>>>>>>
>>>>>> When the parameter is set:
>>>>>> - Configure all CPUs for autonomous operation on first init
>>>>>> - Use HW min/max_perf when available; otherwise initialize from caps
>>>>>> - Initialize desired_perf to max_perf as a starting hint
>>>>>> - Hardware controls frequency instead of the OS governor
>>>>>> - EPP behavior depends on parameter value:
>>>>>>    - performance (or 1): override EPP to performance preference 
>>>>>> (0x0)
>>>>>>    - default_epp (or 2): preserve EPP value programmed by BIOS/
>>>>>> firmware
>>>>>>
>>>>>> The boot parameter is applied only during first policy 
>>>>>> initialization.
>>>>>> Skip applying it on CPU hotplug to preserve runtime sysfs
>>>>>> configuration.
>>>>>>
>>>>>> This patch depends on patch series [1] ("cpufreq: Set policy->min 
>>>>>> and
>>>>>> max as real QoS constraints") so that the policy->min/max set in
>>>>>> cppc_cpufreq_cpu_init() are not overridden by cpufreq_set_policy()
>>>>>> during init.
>>>>>>
>>>>>> Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
>>>>>> ---
>>>>>> [1] https://lore.kernel.org/lkml/20260511135538.522653-1-
>>>>>> pierre.gondois@arm.com/
>>>>>> ---
>>>>>>   .../admin-guide/kernel-parameters.txt         |  16 +++
>>>>>>   drivers/cpufreq/cppc_cpufreq.c                | 122 +++++++++++++
>>>>>> ++++-
>>>>>>   2 files changed, 133 insertions(+), 5 deletions(-)
>>>>>>
>>>>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/
>>>>>> Documentation/admin-guide/kernel-parameters.txt
>>>>>> index 0eb64aab3685..7e4b3a8fd76f 100644
>>>>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>>>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>>>>> @@ -1048,6 +1048,22 @@ Kernel parameters
>>>>>>                       policy to use. This governor must be 
>>>>>> registered
>>>>>> in the
>>>>>>                       kernel before the cpufreq driver probes.
>>>>>>
>>>>>> +     cppc_cpufreq.auto_sel_mode=
>>>>>> +                     [CPU_FREQ] Enable ACPI CPPC autonomous
>>>>>> performance
>>>>>> +                     selection. When enabled, hardware 
>>>>>> automatically
>>>>>> adjusts
>>>>>> +                     CPU frequency on all CPUs based on workload
>>>>>> demands.
>>>>>> +                     In Autonomous mode, Energy Performance
>>>>>> Preference (EPP)
>>>>>> +                     hints guide hardware toward performance (0x0)
>>>>>> or energy
>>>>>> +                     efficiency (0xff).
>>>>>> +                     Requires ACPI CPPC autonomous selection 
>>>>>> register
>>>>>> +                     support.
>>>>>> +                     Accepts:
>>>>>> +                       performance, 1: enable auto_sel + set EPP to
>>>>>> +                                       performance (0x0)
>>>>>> +                       default_epp, 2: enable auto_sel, preserve 
>>>>>> EPP
>>>>>> value
>>>>>> +                                       programmed by BIOS/firmware
>>>>>> +                     Unset: cpufreq governors are used (auto_sel
>>>>>> disabled).
>>>>>
>>>>> Rather than unset doing nothing, have you considered having it take a
>>>>> midpoint like 128?  That's what we do in amd-pstate (default to
>>>>> balance_performance).  I think it turns into a reasonable balance.
>>>>
>>>> Thanks for the suggestion.
>>>> I can add balance_performance that enables auto_sel with EPP=128 in 
>>>> v4.
>>>>
>>>> On changing the driver default (no param behavior) to auto enable
>>>> balance_performance, it would be good to keep the current behavior for
>>>> now since cppc_cpufreq is generic across ARM64/RISC-V platforms where
>>>> EPP and Autonomous Selection registers are optional.
>>>> A default change would affect existing users relying on governors.
>>>>
>>>> Thank you,
>>>> Sumit Gupta
>>>
>>> But couldn't you make the "no module parameter set" follow the behavior
>>> to only set the registers if they're available?
>>>
>>> So the systems that support it start using it, the ones that don't it's
>>> a NOP.
>>>
>>
>> Would it work to add balance_performance as a new mode in v4,
>> and discuss changing the default separately as a follow-up?
>>
>
> Sure.
>
>> Runtime detection helps for unsupported platforms. But platforms which
>> support the registers use OS governors today, and silently switching
>> them to autonomous mode on a kernel update is a behavior change for
>> existing users. They would also have no way to boot into sw governor.
>>
>
> But hopefully it should be better battery life/responsiveness for those
> scenarios too, right?
>

Yes in many cases, but if some workloads rely on specific OS governor
configurations, then that would get impacted.
I will send a separate change later to seek broader consensus on
enabling auto_sel as default without any param.

Thank you,
Sumit Gupta

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