x86/apb_timer: Convert to hotplug state machine

Install the callbacks via the state machine. There is no setup just one
teardown callback. Remove the silly comment about the workqueue up dependency.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20160713153335.625342983@linutronix.deSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/apb_timer.c

@@ -215,26 +215,18 @@ void apbt_setup_secondary_clock(void)
  * cpu timers during the offline process due to the ordering of notification.
  * the extra interrupt is harmless.
  */
-static int apbt_cpuhp_notify(struct notifier_block *n,
-			     unsigned long action, void *hcpu)
+static int apbt_cpu_dead(unsigned int cpu)
 {
-	unsigned long cpu = (unsigned long)hcpu;
 	struct apbt_dev *adev = &per_cpu(cpu_apbt_dev, cpu);
 
-	switch (action & ~CPU_TASKS_FROZEN) {
-	case CPU_DEAD:
 	dw_apb_clockevent_pause(adev->timer);
 	if (system_state == SYSTEM_RUNNING) {
-			pr_debug("skipping APBT CPU %lu offline\n", cpu);
+		pr_debug("skipping APBT CPU %u offline\n", cpu);
 	} else {
-			pr_debug("APBT clockevent for cpu %lu offline\n", cpu);
+		pr_debug("APBT clockevent for cpu %u offline\n", cpu);
 		dw_apb_clockevent_stop(adev->timer);
 	}
-		break;
-	default:
-		pr_debug("APBT notified %lu, no action\n", action);
-	}
-	return NOTIFY_OK;
+	return 0;
 }
 
 static __init int apbt_late_init(void)
@@ -242,9 +234,8 @@ static __init int apbt_late_init(void)
 	if (intel_mid_timer_options == INTEL_MID_TIMER_LAPIC_APBT ||
 		!apb_timer_block_enabled)
 		return 0;
-	/* This notifier should be called after workqueue is ready */
-	hotcpu_notifier(apbt_cpuhp_notify, -20);
-	return 0;
+	return cpuhp_setup_state(CPUHP_X86_APB_DEAD, "X86_APB_DEAD", NULL,
+				 apbt_cpu_dead);
 }
 fs_initcall(apbt_late_init);
 #else

include/linux/cpuhotplug.h

@@ -13,6 +13,7 @@ enum cpuhp_state {
 	CPUHP_PERF_POWER,
 	CPUHP_PERF_SUPERH,
 	CPUHP_X86_HPET_DEAD,
+	CPUHP_X86_APB_DEAD,
 	CPUHP_WORKQUEUE_PREP,
 	CPUHP_NOTIFY_PREPARE,
 	CPUHP_BRINGUP_CPU,