Re: [PATCH v2 1/3] cpufreq: CPPC: Keep the policy across CPU hotplug

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

On 7/16/26 16:38, 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.
> 
> Re-enable CPPC from online(), as it may have been disabled while
> the CPU was offline.
> 
> Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
> ---
>  drivers/cpufreq/cppc_cpufreq.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> index f6cea0c54dd9..432c6a6288a7 100644
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -722,6 +722,31 @@ 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)
> +{
> +	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);

Keeping the policy alive means cpu_data->perf_ctrls survives hotplug, but the callback does not
reapply it. A governor restart does not guarantee an immediate target callback, so the controls
may remain reset indefinitely.

I think the online path needs to restore a complete, current performance-control request before
returning. MIN/MAX should be recomputed from the policy and the last requested DESIRED value
clamped into that range.

Also, what about suspend/resume?

> +
> +	return 0;
> +}
> +
>  static void cppc_cpufreq_cpu_exit(struct cpufreq_policy *policy)
>  {
>  	struct cppc_cpudata *cpu_data = policy->driver_data;
> @@ -1034,6 +1059,8 @@ 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",