arm64: Clarify when cpu_enable() is called

Strengthen the wording in the documentation for cpu_enable() to make it
more obvious to readers not already familiar with the code when the core
will call this callback and that this is intentional.
Signed-off-by: NMark Brown <broonie@kernel.org>
Reviewed-by: NSuzuki K Poulose <suzuki.poulose@arm.com>
[will: minor tweak to emphasis in the comment]
Signed-off-by: NWill Deacon <will@kernel.org>

arch/arm64/include/asm/cpufeature.h

@@ -289,9 +289,16 @@ struct arm64_cpu_capabilities {
 	u16 type;
 	bool (*matches)(const struct arm64_cpu_capabilities *caps, int scope);
 	/*
-	 * Take the appropriate actions to enable this capability for this CPU.
-	 * For each successfully booted CPU, this method is called for each
-	 * globally detected capability.
+	 * Take the appropriate actions to configure this capability
+	 * for this CPU. If the capability is detected by the kernel
+	 * this will be called on all the CPUs in the system,
+	 * including the hotplugged CPUs, regardless of whether the
+	 * capability is available on that specific CPU. This is
+	 * useful for some capabilities (e.g, working around CPU
+	 * errata), where all the CPUs must take some action (e.g,
+	 * changing system control/configuration). Thus, if an action
+	 * is required only if the CPU has the capability, then the
+	 * routine must check it before taking any action.
 	 */
 	void (*cpu_enable)(const struct arm64_cpu_capabilities *cap);
 	union {