ledtrig-cpu: Handle CPU hot(un)plugging

When CPU is hot(un)plugged, no syscore notification is being
generated, nor is cpuidle involved. This leaves the CPU LED
turned on, because the dying thread is doing some work (LED on)
and than it is... well, dying (LED still on :-)

Added notifier block for hot(un)plugging operations, generating
existing trigger events.

Signed-off-by: Pawel Moll <[e-mail]>
Signed-off-by: Bryan Wu <[e-mail]>

drivers/leds/trigger/ledtrig-cpu.c

···
 #include <linux/percpu.h>
 #include <linux/syscore_ops.h>
 #include <linux/rwsem.h>
+#include <linux/cpu.h>
 #include "../leds.h"
 
 #define MAX_NAME_LEN	8
···
 	.resume		= ledtrig_cpu_syscore_resume,
 };
 
+static int ledtrig_cpu_notify(struct notifier_block *self,
+					   unsigned long action, void *hcpu)
+{
+	switch (action & ~CPU_TASKS_FROZEN) {
+	case CPU_STARTING:
+		ledtrig_cpu(CPU_LED_START);
+		break;
+	case CPU_DYING:
+		ledtrig_cpu(CPU_LED_STOP);
+		break;
+	}
+
+	return NOTIFY_OK;
+}
+
+
+static struct notifier_block ledtrig_cpu_nb = {
+	.notifier_call = ledtrig_cpu_notify,
+};
+
 static int __init ledtrig_cpu_init(void)
 {
 	int cpu;
···
 	}
 
 	register_syscore_ops(&ledtrig_cpu_syscore_ops);
+	register_cpu_notifier(&ledtrig_cpu_nb);
 
 	pr_info("ledtrig-cpu: registered to indicate activity on CPUs\n");
 
···
 static void __exit ledtrig_cpu_exit(void)
 {
 	int cpu;
+
+	unregister_cpu_notifier(&ledtrig_cpu_nb);
 
 	for_each_possible_cpu(cpu) {
 		struct led_trigger_cpu *trig = &per_cpu(cpu_trig, cpu);