Re: [PATCH v2] cpufreq: CPPC: add autonomous mode boot parameter support

[Sumit Gupta <sumitg@nvidia.com>]

On 08/05/26 08:03, Jie Zhan wrote:
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>
>
> On 5/5/2026 8:29 PM, Sumit Gupta wrote:
>> Hi Jie,
>>
>>
>> On 27/04/26 13:54, Jie Zhan wrote:
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>>>
>>>
>>> Hi Sumit,
>>>
>>> In general, I would expect this parameter only toggles on auto_sel by
>>> default.  IIUC, other CPPC configurations (min/max/desired perf, EPP,
>>> enable) are optional and not closely related to this.
>>>
>>> Why including those stuff here?
>>>
>>>
>>> Please see other questions inline.
>>>
>>> Thanks!
>>> Jie
>> These together provide a known, predictable autonomous mode boot.
>> min/max/desired seeding ensures HW has a known starting bound (BIOS
>> may leave them unset).
>> EPP=PERFORMANCE_PREF is needed as BIOS defaults often bias toward
>> energy saving, and admins on many CPU systems shouldn't have to script
>> per CPU sysfs writes at every boot to undo that.
> Hi Sumit,
>
> The min/max/desired perf might be a slightly different case, but the EPP
> value should probably follow the default if there is?
> Otherwise, users may complain that their BIOS defaults don't work and find
> out it's driver's fault.
>
> Alternatively, we may also make an assumption that we ignore BIOS EPP
> config, but not sure if that's suitable?
>
> (Perhaps let's just discuss in the bottom trunk because the main issue is
> pretty much the same.)
>>> On 4/25/2026 4:18 AM, 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 auto_sel_mode=1:
>>>> - Configure all CPUs for autonomous operation on first init
>>>> - Set EPP to performance preference (0x0)
>>>> - Use HW min/max_perf when available; otherwise initialize from caps
>>>> - Clamp desired_perf to bounds before enabling autonomous mode
>>>> - Hardware controls frequency instead of the OS governor
>>>>
>>>> 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 [2] ("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.
>>>>
>>>> Reviewed-by: Randy Dunlap <rdunlap@infradead.org> (Documentation)
>>>> Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
>>>> ---
>>>> v[1] -> v2:
>>>> - Call cppc_set_enable() unconditionally so CPPC is enabled for both
>>>>     OS-driven and autonomous modes.
>>> Why adding this in v2?
>>> This looks like a separate issue since setting CPPC Enable reg doesn't seem
>>> to be related with autonomous control.
>> In v2, moved it out of the auto_sel specific check.
>> Agree that cppc_set_enable() is general CPPC enablement and can be split
>> into a separate patch in v3 if preferred.
>>
> Yeah it should be a separate patch I think.
>>>> - Init min/max from caps instead of cppc_cpufreq_update_perf_limits()
>>>>     as policy->min/max aren't yet populated.
>>>>
>>>> [1] https://lore.kernel.org/lkml/20260317151053.2361475-1-sumitg@nvidia.com/
>>>> [2] https://lore.kernel.org/lkml/20260423084731.1090384-2-pierre.gondois@arm.com/
>>>> ---
>>>>    .../admin-guide/kernel-parameters.txt         | 13 +++
>>>>    drivers/cpufreq/cppc_cpufreq.c                | 89 +++++++++++++++++--
>>>>    2 files changed, 97 insertions(+), 5 deletions(-)
>>>>
> ...
>>>> +
>>>> +             policy->cur = cppc_perf_to_khz(caps,
>>>> +                                            cpu_data->perf_ctrls.desired_perf);
>>>> +
>>>> +             /* EPP is optional - some platforms may not support it */
>>>> +             ret = cppc_set_epp(cpu, CPPC_EPP_PERFORMANCE_PREF);
>>> Why setting this to PERFORMANCE by default?
>>> A platform can have its own default EPP value.  This would override that.
>> The boot option targets performance oriented use cases on many CPU
>> systems, avoiding sysfs scripting across all CPUs on every boot.
>> The BIOS default EPP (often biased toward energy saving) would otherwise
>> steer HW away from that goal. Admins can still re-tune EPP at runtime via
>> the existing energy_performance_preference_val sysfs.
>>
> Yeah, avoiding scripting makes sense for sure.
> The thing is how do we do this nicely.
>
> For now we need to consider follow either the BIOS default or the driver
> default.
> They seem to be exclusive since there isn't a clear way to support both at
> the same time.
>
> So, are we going to support the BIOS default case?
> For example, users can config a default EPP in the BIOS setup menu.

To consider the BIOS default case, we can extend the parameter to accept
an EPP mode in v3:
- cppc_cpufreq.auto_sel_mode=performance / =1 (EPP=performance)
- cppc_cpufreq.auto_sel_mode=default / =2 (preserve BIOS EPP)

Thank you,
Sumit Gupta

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