+#ifdef CONFIG_ARM64_AMU_EXTN
+
+/*
+ * The "amu_cpus" cpumask only signals that the CPU implementation for the
+ * flagged CPUs supports the Activity Monitors Unit (AMU) but does not provide
+ * information regarding all the events that it supports. When a CPU bit is
+ * set in the cpumask, the user of this feature can only rely on the presence
+ * of the 4 fixed counters for that CPU. But this does not guarantee that the
+ * counters are enabled or access to these counters is enabled by code
+ * executed at higher exception levels (firmware).
+ */
+static struct cpumask amu_cpus __read_mostly;
+
+bool cpu_has_amu_feat(int cpu)
+{
+ return cpumask_test_cpu(cpu, &amu_cpus);
+}
+
+/* Initialize the use of AMU counters for frequency invariance */
+extern void init_cpu_freq_invariance_counters(void);
+
+static void cpu_amu_enable(struct arm64_cpu_capabilities const *cap)
+{
+ if (has_cpuid_feature(cap, SCOPE_LOCAL_CPU)) {
+ pr_info("detected CPU%d: Activity Monitors Unit (AMU)\n",
+ smp_processor_id());
+ cpumask_set_cpu(smp_processor_id(), &amu_cpus);
+ init_cpu_freq_invariance_counters();
+ }
+}
+
+static bool has_amu(const struct arm64_cpu_capabilities *cap,
+ int __unused)
+{
+ /*
+ * The AMU extension is a non-conflicting feature: the kernel can
+ * safely run a mix of CPUs with and without support for the
+ * activity monitors extension. Therefore, unconditionally enable
+ * the capability to allow any late CPU to use the feature.
+ *
+ * With this feature unconditionally enabled, the cpu_enable
+ * function will be called for all CPUs that match the criteria,
+ * including secondary and hotplugged, marking this feature as
+ * present on that respective CPU. The enable function will also
+ * print a detection message.
+ */
+
+ return true;
+}
+#endif
+