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

[Mario Limonciello <mario.limonciello@amd.com>]

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.

> +
>   	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 6b54427b52e1..5f4d735e7c7d 100644
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -28,6 +28,43 @@
>   
>   static struct cpufreq_driver cppc_cpufreq_driver;
>   
> +/* Autonomous Selection boot parameter modes */
> +enum {
> +	AUTO_SEL_PERFORMANCE = 1,
> +	AUTO_SEL_DEFAULT_EPP = 2,
> +};
> +
> +static int auto_sel_mode;
> +
> +static int auto_sel_mode_set(const char *val, const struct kernel_param *kp)
> +{
> +	if (sysfs_streq(val, "performance") || sysfs_streq(val, "1"))
> +		*(int *)kp->arg = AUTO_SEL_PERFORMANCE;
> +	else if (sysfs_streq(val, "default_epp") || sysfs_streq(val, "2"))
> +		*(int *)kp->arg = AUTO_SEL_DEFAULT_EPP;
> +	else
> +		return -EINVAL;
> +
> +	return 0;
> +}
> +
> +static int auto_sel_mode_get(char *buffer, const struct kernel_param *kp)
> +{
> +	switch (*(int *)kp->arg) {
> +	case AUTO_SEL_PERFORMANCE:
> +		return sysfs_emit(buffer, "performance\n");
> +	case AUTO_SEL_DEFAULT_EPP:
> +		return sysfs_emit(buffer, "default_epp\n");
> +	default:
> +		return sysfs_emit(buffer, "disabled\n");
> +	}
> +}
> +
> +static const struct kernel_param_ops auto_sel_mode_ops = {
> +	.set = auto_sel_mode_set,
> +	.get = auto_sel_mode_get,
> +};
> +
>   #ifdef CONFIG_ACPI_CPPC_CPUFREQ_FIE
>   static enum {
>   	FIE_UNSET = -1,
> @@ -715,11 +752,75 @@ 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 (auto_sel_mode && policy->last_governor[0] == '\0' &&
> +	    !cpu_data->perf_ctrls.auto_sel) {
> +		/* Init min/max_perf from caps if not already set by HW. */
> +		if (!cpu_data->perf_ctrls.min_perf)
> +			cpu_data->perf_ctrls.min_perf = caps->lowest_nonlinear_perf;
> +		if (!cpu_data->perf_ctrls.max_perf)
> +			cpu_data->perf_ctrls.max_perf = policy->boost_enabled ?
> +				caps->highest_perf : caps->nominal_perf;
> +
> +		/*
> +		 * In autonomous mode desired_perf is only a hint; EPP and
> +		 * the platform drive actual selection within [min, max].
> +		 * Initialize it to max_perf so HW starts at the upper bound.
> +		 */
> +		cpu_data->perf_ctrls.desired_perf = cpu_data->perf_ctrls.max_perf;
> +
> +		policy->cur = cppc_perf_to_khz(caps,
> +					       cpu_data->perf_ctrls.desired_perf);
> +
> +		/*
> +		 * Override EPP only in 'performance' mode; 'default_epp' mode
> +		 * preserves the BIOS/firmware programmed EPP value.
> +		 * EPP is optional - some platforms may not support it.
> +		 */
> +		if (auto_sel_mode == AUTO_SEL_PERFORMANCE) {
> +			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;
> +		}
> +
> +		/* Program min/max/desired into CPPC regs (non-fatal on failure). */
> +		ret = cppc_set_perf(cpu, &cpu_data->perf_ctrls);
> +		if (ret)
> +			pr_warn("set_perf failed CPU%d (%d); using HW values\n",
> +				cpu, ret);
> +
> +		ret = cppc_set_auto_sel(cpu, true);
> +		if (ret && ret != -EOPNOTSUPP)
> +			pr_warn("auto_sel CPU%d failed (%d); using OS mode\n",
> +				cpu, ret);
> +		else if (!ret)
> +			cpu_data->perf_ctrls.auto_sel = true;
> +	}
> +
> +	if (cpu_data->perf_ctrls.auto_sel) {
> +		/* 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 ?:
> +					       (policy->boost_enabled ?
> +						caps->highest_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);
> @@ -1079,10 +1180,21 @@ 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);
> +
>   	cpufreq_unregister_driver(&cppc_cpufreq_driver);
>   	cppc_freq_invariance_exit();
>   }
>   
> +module_param_cb(auto_sel_mode, &auto_sel_mode_ops, &auto_sel_mode, 0444);
> +MODULE_PARM_DESC(auto_sel_mode,
> +		 "Enable CPPC autonomous performance selection at boot: "
> +		 "performance or 1 (EPP=performance), "
> +		 "default_epp or 2 (preserve BIOS/firmware EPP)");
> +
>   module_exit(cppc_cpufreq_exit);
>   MODULE_AUTHOR("Ashwin Chaugule");
>   MODULE_DESCRIPTION("CPUFreq driver based on the ACPI CPPC v5.0+ spec");