Re: [PATCH v3 1/4] cpufreq: CPPC: Keep the policy across CPU hotplug

[Christian Loehle <christian.loehle@arm.com>]

On 7/24/26 22:59, Sumit Gupta wrote:
> Without online()/offline() callbacks, the cpufreq core fully tears
> down a policy during exit() when its last online CPU is offlined, and
> rebuilds it during init() when it comes back.
> 
> Add lightweight online()/offline() callbacks so the core instead keeps
> the policy live and reuses the driver's cpu_data across CPU hotplug.
> This avoids re-reading the CPPC capabilities on every offline/online,
> making CPU hotplug faster.
> 
> From online(), re-enable CPPC, as the platform may have disabled it
> while the CPU was offline. Since the policy is no longer rebuilt via
> init(), online() also reprograms the CPPC performance controls
> (desired/min/max).
> 
> Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
> ---
>  drivers/cpufreq/cppc_cpufreq.c | 44 ++++++++++++++++++++++++++++++++++
>  1 file changed, 44 insertions(+)
> 
> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> index f6cea0c54dd9..6dc59f99d880 100644
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -722,6 +722,48 @@ static int cppc_cpufreq_cpu_init(struct cpufreq_policy *policy)
>  	return ret;
>  }
>  
> +/*
> + * With offline() defined, the cpufreq core keeps the policy alive when
> + * a CPU is hotplugged out.
> + */
> +static int cppc_cpufreq_cpu_offline(struct cpufreq_policy *policy)
> +{
> +	return 0;
> +}
> +
> +/*
> + * Re-enable CPPC when the policy's CPU comes back online, since the platform
> + * may have disabled it while the CPU was offline.
> + */
> +static int cppc_cpufreq_cpu_online(struct cpufreq_policy *policy)
> +{
> +	struct cppc_cpudata *cpu_data = policy->driver_data;
> +	unsigned int cpu = policy->cpu;
> +	int ret;
> +
> +	ret = cppc_set_enable(cpu, true);
> +	if (ret && ret != -EOPNOTSUPP)
> +		pr_warn("Failed to re-enable CPPC for CPU%d (%d)\n", cpu, ret);

What's the intention of continuing restoring CPPC register values here?

> +
> +	/*
> +	 * The platform may reset the controls while the CPU is offline, so
> +	 * recompute min/max, clamp desired_perf into range, and reprogram them.
> +	 */
> +	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);
> +
> +	ret = cppc_set_perf(cpu, &cpu_data->perf_ctrls);
> +	if (ret)
> +		pr_debug("Failed to reapply perf request on CPU%d (%d)\n",
> +			 cpu, ret);

I think cppc_set_perf() needs some prep first before using it on reset values.
We assume that reset value may be Autonomous Mode on, right? So we must never
write MIN>MAX and vice versa. I think we may just have to read and write
the 'otherwise-offending' value first on reset.

> +
> +	return 0;
> +}
> +
>  static void cppc_cpufreq_cpu_exit(struct cpufreq_policy *policy)
>  {
>  	struct cppc_cpudata *cpu_data = policy->driver_data;
> @@ -1034,6 +1076,8 @@ static struct cpufreq_driver cppc_cpufreq_driver = {
>  	.fast_switch = cppc_cpufreq_fast_switch,
>  	.init = cppc_cpufreq_cpu_init,
>  	.exit = cppc_cpufreq_cpu_exit,
> +	.online = cppc_cpufreq_cpu_online,
> +	.offline = cppc_cpufreq_cpu_offline,
>  	.set_boost = cppc_cpufreq_set_boost,
>  	.attr = cppc_cpufreq_attr,
>  	.name = "cppc_cpufreq",