[REPOST PATCH v6 3/3] arm64: topology: Handle AMU FIE setup on CPU hotplug

[Geert Uytterhoeven]

Hi Lifeng,

On Tue, 30 Dec 2025 at 09:02, Lifeng Zheng <zhenglifeng1 at huawei.com> wrote:
> Currently, when a cpufreq policy is created, the AMU FIE setup process
> checks all CPUs in the policy -- including those that are offline. If any
> of these CPUs are offline at that time, their AMU capability flag hasn't
> been verified yet, leading the check fail. As a result, AMU FIE is not
> enabled, even if the CPUs that are online do support it.
>
> Later, when the previously offline CPUs come online and report AMU support,
> there's no mechanism in place to re-enable AMU FIE for the policy. This
> leaves the entire frequency domain without AMU FIE, despite being eligible.
>
> Restrict the initial AMU FIE check to only those CPUs that are online at
> the time the policy is created, and allow CPUs that come online later to
> join the policy with AMU FIE enabled.
>
> Signed-off-by: Lifeng Zheng <zhenglifeng1 at huawei.com>
> Acked-by: Beata Michalska <beata.michalska at arm.com>

Thanks for your patch, which is now commit 6fd9be0b7b2e957d
("arm64: topology: Handle AMU FIE setup on CPU hotplug") in
arm64/for-next/core (next-20260107 and later).

> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -284,7 +284,7 @@ static int init_amu_fie_callback(struct notifier_block *nb, unsigned long val,
>         struct cpufreq_policy *policy = data;
>
>         if (val == CPUFREQ_CREATE_POLICY)
> -               amu_fie_setup(policy->related_cpus);
> +               amu_fie_setup(policy->cpus);
>
>         /*
>          * We don't need to handle CPUFREQ_REMOVE_POLICY event as the AMU
> @@ -303,10 +303,71 @@ static struct notifier_block init_amu_fie_notifier = {
>         .notifier_call = init_amu_fie_callback,
>  };
>
> +static int cpuhp_topology_online(unsigned int cpu)
> +{
> +       struct cpufreq_policy *policy = cpufreq_cpu_policy(cpu);
> +
> +       /* Those are cheap checks */
> +
> +       /*
> +        * Skip this CPU if:
> +        *  - it has no cpufreq policy assigned yet,
> +        *  - no policy exists that spans CPUs with AMU counters, or
> +        *  - it was already handled.
> +        */
> +       if (unlikely(!policy) || !cpumask_available(amu_fie_cpus) ||
> +           cpumask_test_cpu(cpu, amu_fie_cpus))
> +               return 0;
> +
> +       /*
> +        * Only proceed if all already-online CPUs in this policy
> +        * support AMU counters.
> +        */
> +       if (unlikely(!cpumask_subset(policy->cpus, amu_fie_cpus)))
> +               return 0;
> +
> +       /*
> +        * If the new online CPU cannot pass this check, all the CPUs related to
> +        * the same policy should be clear from amu_fie_cpus mask, otherwise they
> +        * may use different source of the freq scale.
> +        */
> +       if (!freq_counters_valid(cpu)) {
> +               pr_warn("CPU[%u] doesn't support AMU counters\n", cpu);

This is triggered during resume from s2ram on Renesas R-Car H3
(big.LITTLE 4x Cortex-A57 + 4x Cortex-A53), when enabling the first
little core:

    AMU: CPU[4] doesn't support AMU counters

Adding debug code:

    pr_info("Calling
topology_clear_scale_freq_source(SCALE_FREQ_SOURCE_ARCH, %*pbl)\n",
cpumask_pr_args(policy->related_cpus));
    pr_info("Calling cpumask_andnot(..., %*pbl, %*pbl)\n",
cpumask_pr_args(amu_fie_cpus), cpumask_pr_args(policy->related_cpus));

gives:

    AMU: Calling topology_clear_scale_freq_source(SCALE_FREQ_SOURCE_ARCH, 4-7)
    AMU: Calling cpumask_andnot(..., , 4-7)

so AMU is disabled for all little cores.

Since this only happens during s2ram, and not during initial CPU
bring-up on boot, this looks wrong to me?

> +               topology_clear_scale_freq_source(SCALE_FREQ_SOURCE_ARCH,
> +                                                policy->related_cpus);
> +               cpumask_andnot(amu_fie_cpus, amu_fie_cpus, policy->related_cpus);
> +               return 0;
> +       }
> +
> +       cpumask_set_cpu(cpu, amu_fie_cpus);
> +
> +       topology_set_scale_freq_source(&amu_sfd, cpumask_of(cpu));
> +
> +       pr_debug("CPU[%u]: counter will be used for FIE.", cpu);
> +
> +       return 0;
> +}
> +
>  static int __init init_amu_fie(void)
>  {
> -       return cpufreq_register_notifier(&init_amu_fie_notifier,
> +       int ret;
> +
> +       ret = cpufreq_register_notifier(&init_amu_fie_notifier,
>                                         CPUFREQ_POLICY_NOTIFIER);
> +       if (ret)
> +               return ret;
> +
> +       ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
> +                                       "arm64/topology:online",
> +                                       cpuhp_topology_online,
> +                                       NULL);
> +       if (ret < 0) {
> +               cpufreq_unregister_notifier(&init_amu_fie_notifier,
> +                                           CPUFREQ_POLICY_NOTIFIER);
> +               return ret;
> +       }
> +
> +       return 0;
>  }
>  core_initcall(init_amu_fie);
>
> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> index 84ec92bff642..c0ef6ea9c111 100644
> --- a/drivers/base/arch_topology.c
> +++ b/drivers/base/arch_topology.c
> @@ -34,7 +34,14 @@ EXPORT_PER_CPU_SYMBOL_GPL(capacity_freq_ref);
>
>  static bool supports_scale_freq_counters(const struct cpumask *cpus)
>  {
> -       return cpumask_subset(cpus, &scale_freq_counters_mask);
> +       int i;
> +
> +       for_each_cpu(i, cpus) {
> +               if (cpumask_test_cpu(i, &scale_freq_counters_mask))
> +                       return true;
> +       }
> +
> +       return false;
>  }
>
>  bool topology_scale_freq_invariant(void)

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds