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

[Pierre Gondois <pierre.gondois@arm.com>]

Hello Sumit,

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.

> +	if (auto_sel_mode && policy->last_governor[0] == '\0' &&
> +	    !cpu_data->perf_ctrls.auto_sel) {
> +		/* Enable CPPC - optional register, some platforms need it */
The documentation of the CPPC Enable Register is subject to
interpretation, but IIUC the field should be set to use the CPPC
controls, so I assume this should be set in cppc_cpufreq_init()
instead ?
> +		ret = cppc_set_enable(cpu, true);
> +		if (ret && ret != -EOPNOTSUPP)
> +			pr_warn("Failed to enable CPPC for CPU%d (%d)\n", cpu, ret);
> +
> +		/*
> +		 * Prefer HW min/max_perf when set; otherwise program from
> +		 * policy limits derived earlier from caps.
> +		 * Clamp desired_perf to bounds and sync policy->cur.
> +		 */
> +		if (!cpu_data->perf_ctrls.min_perf || !cpu_data->perf_ctrls.max_perf)

The function doesn't seem to exist.

> +			cppc_cpufreq_update_perf_limits(cpu_data, policy);
> +
> +		cpu_data->perf_ctrls.desired_perf =
> +			clamp_t(u32, cpu_data->perf_ctrls.desired_perf,
> +				cpu_data->perf_ctrls.min_perf,
> +				cpu_data->perf_ctrls.max_perf);
> +
> +		policy->cur = cppc_perf_to_khz(caps,
> +					       cpu_data->perf_ctrls.desired_perf);
> +

Maybe this should also be done in cppc_cpufreq_init()
if the auto_sel_mode parameter is set ?

> +		/* EPP is optional - some platforms may not support it */
> +		ret = cppc_set_epp(cpu, CPPC_EPP_PERFORMANCE_PREF);
> +		if (ret && ret != -EOPNOTSUPP)
> +			pr_warn("Failed to set EPP for CPU%d (%d)\n", cpu, ret);
> +		else if (!ret)
> +			cpu_data->perf_ctrls.energy_perf = CPPC_EPP_PERFORMANCE_PREF;
> +
> +		ret = cppc_set_perf(cpu, &cpu_data->perf_ctrls);
> +		if (ret) {
> +			pr_debug("Err setting perf for autonomous mode CPU:%d ret:%d\n",
> +				 cpu, ret);
> +			goto out;
> +		}
> +
> +		ret = cppc_set_auto_sel(cpu, true);
> +		if (ret && ret != -EOPNOTSUPP) {
> +			pr_warn("Failed autonomous config for CPU%d (%d)\n",
> +				cpu, ret);
> +			goto out;
> +		}
> +		if (!ret)
> +			cpu_data->perf_ctrls.auto_sel = true;
> +	}
> +
> +	if (cpu_data->perf_ctrls.auto_sel) {

There is a patchset ongoing which tries to remove
setting policy->min/max from driver initialization.
Indeed, these values are only temporarily valid,
until the governor override them.
It is not sure yet the patch will be accepted though.

https://lore.kernel.org/lkml/20260317101753.2284763-4-pierre.gondois@arm.com/


> +		/* Sync policy limits from HW when autonomous mode is active */
> +		policy->min = cppc_perf_to_khz(caps,
> +					       cpu_data->perf_ctrls.min_perf ?:
> +					       caps->lowest_nonlinear_perf);
> +		policy->max = cppc_perf_to_khz(caps,
> +					       cpu_data->perf_ctrls.max_perf ?:
> +					       caps->nominal_perf);
> +	} else {
> +		/* Normal mode: governors control frequency */
> +		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;
> +		}
>   	}
>   
>   	cppc_cpufreq_cpu_fie_init(policy);
> @@ -1038,10 +1104,18 @@ static int __init cppc_cpufreq_init(void)
>   
>   static void __exit cppc_cpufreq_exit(void)
>   {
> +	unsigned int cpu;
> +
> +	for_each_present_cpu(cpu)
> +		cppc_set_auto_sel(cpu, false);

If the firmware has a default EPP value, it means that loading
and the unloading the driver will reset this default EPP value.
Maybe the initial EPP value and/or the auto_sel value should be
cached somewhere and restored on exit ?
I don't know if this is actually an issue, this is just to signal it.

> +
>   	cpufreq_unregister_driver(&cppc_cpufreq_driver);
>   	cppc_freq_invariance_exit();
>   }
>   
> +module_param(auto_sel_mode, bool, 0444);
> +MODULE_PARM_DESC(auto_sel_mode, "Enable CPPC autonomous performance selection at boot");
> +
>   module_exit(cppc_cpufreq_exit);
>   MODULE_AUTHOR("Ashwin Chaugule");
>   MODULE_DESCRIPTION("CPUFreq driver based on the ACPI CPPC v5.0+ spec");