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

[Sumit Gupta <sumitg@nvidia.com>]

On 20/04/26 18:37, Sumit Gupta wrote:
>
>>>> On 3/17/26 16:10, Sumit Gupta wrote:
>>>>> Add kernel boot parameter 'cppc_cpufreq.auto_sel_mode' to enable CPPC
>>>>> autonomous performance selection on all CPUs at system startup 
>>>>> without
>>>>> requiring runtime sysfs manipulation. 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 when set; otherwise program from policy limits 
>>>>> (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.
>>>>> On hotplug, skip applying it so that the user's runtime sysfs
>>>>> configuration is preserved.
>>>>>
>>>>> Reviewed-by: Randy Dunlap <rdunlap@infradead.org> (Documentation)
>>>>> Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
>>>>> ---
>>>>> Part 1 [1] of this series was applied for 7.1 and present in next.
>>>>> Sending this patch as reworked version of 'patch 11' from [2] based
>>>>> on next.
>>>>>
>>>>> [1]
>>>>> https://lore.kernel.org/lkml/20260206142658.72583-1-sumitg@nvidia.com/ 
>>>>>
>>>>> [2]
>>>>> https://lore.kernel.org/lkml/20251223121307.711773-1-sumitg@nvidia.com/ 
>>>>>
>>>>> ---
>>>>>    .../admin-guide/kernel-parameters.txt         | 13 +++
>>>>>    drivers/cpufreq/cppc_cpufreq.c                | 84
>>>>> +++++++++++++++++--
>>>>>    2 files changed, 92 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt
>>>>> b/Documentation/admin-guide/kernel-parameters.txt
>>>>> index fa6171b5fdd5..de4b4c89edfe 100644
>>>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>>>> @@ -1060,6 +1060,19 @@ 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.
>>>>> +                     Format: <bool>
>>>>> +                     Default: 0 (disabled)
>>>>> +                     0: use cpufreq governors
>>>>> +                     1: enable if supported by hardware
>>>>> +
>>>>>        cpu_init_udelay=N
>>>>>                        [X86,EARLY] Delay for N microsec between
>>>>> assert and de-assert
>>>>>                        of APIC INIT to start processors. This delay
>>>>> occurs
>>>>> diff --git a/drivers/cpufreq/cppc_cpufreq.c
>>>>> b/drivers/cpufreq/cppc_cpufreq.c
>>>>> index 5dfb109cf1f4..49c148b2a0a4 100644
>>>>> --- a/drivers/cpufreq/cppc_cpufreq.c
>>>>> +++ b/drivers/cpufreq/cppc_cpufreq.c
>>>>> @@ -28,6 +28,9 @@
>>>>>
>>>>>    static struct cpufreq_driver cppc_cpufreq_driver;
>>>>>
>>>>> +/* Autonomous Selection boot parameter */
>>>>> +static bool auto_sel_mode;
>>>>> +
>>>>>    #ifdef CONFIG_ACPI_CPPC_CPUFREQ_FIE
>>>>>    static enum {
>>>>>        FIE_UNSET = -1,
>>>>> @@ -708,11 +711,74 @@ static int cppc_cpufreq_cpu_init(struct
>>>>> cpufreq_policy *policy)
>>>>>        policy->cur = cppc_perf_to_khz(caps, caps->highest_perf);
>>>>>        cpu_data->perf_ctrls.desired_perf = caps->highest_perf;
>>>>>
>>>>> -     ret = cppc_set_perf(cpu, &cpu_data->perf_ctrls);
>>>>> -     if (ret) {
>>>>> -             pr_debug("Err setting perf value:%d on CPU:%d. 
>>>>> ret:%d\n",
>>>>> -                      caps->highest_perf, cpu, ret);
>>>>> -             goto out;
>>>>> +     /*
>>>>> +      * Enable autonomous mode on first init if boot param is set.
>>>>> +      * Check last_governor to detect first init and skip if 
>>>>> auto_sel
>>>>> +      * is already enabled.
>>>>> +      */
>>>> If the goal is to set autosel only once at the driver init,
>>>> shouldn't this be done in cppc_cpufreq_init() ?
>>>> I understand that cpu_data doesn't exist yet in
>>>> cppc_cpufreq_init(), but this seems more appropriate to do
>>>> it there IMO.
>>>>
>>>> This means the cpudata should be updated accordingly
>>>> in this cppc_cpufreq_cpu_init() function.
>>> In an earlier version [1], the setup was in cppc_cpufreq_init() but
>>> was moved to cppc_cpufreq_cpu_init() to improve per-CPU error handling.
>>> Keeping the setup in cppc_cpufreq_init() helps to avoid the 
>>> last_governor
>>> check. We can warn for a CPU failing to enable and continue so other
>>> CPUs keep autonomous mode.
>>> cppc_cpufreq_cpu_init() would then just check the auto_sel state
>>> from register and sync policy limits from min/max_perf registers when
>>> autonomous mode is active.
>>> Please let me know your thoughts.
>> FWIU the auto_sel_mode module parameter allows to
>> configure the default auto_sel_mode when the driver is
>> first loaded, so there should not need to check that again
>> whenever cppc_cpufreq_cpu_init() is called.
>> Maybe Ionela saw something we didn't see ?
>
> AFAIU, the concern in that review [1] was about error handling as the
> earlier version disabled auto_sel on all CPUs if any single CPU failed.
> Per-CPU error handling in cppc_cpufreq_init() (warn and continue)
> addresses that. Can't think of more reason.
> Do you have anything in mind?
>

Actually, one case where cppc_cpufreq_cpu_init() would be needed
is when CPUs are offline at boot. So I will keep the setup in
cppc_cpufreq_cpu_init() in v2 same as present in current version.

Thank you,
Sumit Gupta

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