ARM: arch_timer: Silence debug preempt warnings

Hot-plugging with CONFIG_DEBUG_PREEMPT=y on a device with arm
architected timers causes a slew of "using smp_processor_id() in
preemptible" warnings:

BUG: using smp_processor_id() in preemptible [00000000] code: sh/111
caller is arch_timer_cpu_notify+0x14/0xc8

This happens because sometimes the cpu notifier,
arch_timer_cpu_notify(), is called in preemptible context and
other times in non-preemptible context but we use this_cpu_ptr()
to retrieve the clockevent in all cases. We're only going to
actually use the pointer in non-preemptible context though, so
push the this_cpu_ptr() access down into the cases to force the
checks to occur only in non-preemptible contexts.

Cc: John Stultz <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Acked-by: Marc Zyngier <[e-mail]>
Signed-off-by: Stephen Boyd <[e-mail]>
Signed-off-by: Olof Johansson <[e-mail]>

drivers/clocksource/arm_arch_timer.c

···
 static int __cpuinit arch_timer_cpu_notify(struct notifier_block *self,
 					   unsigned long action, void *hcpu)
 {
-	struct clock_event_device *evt = this_cpu_ptr(arch_timer_evt);
-
+	/*
+	 * Grab cpu pointer in each case to avoid spurious
+	 * preemptible warnings
+	 */
 	switch (action & ~CPU_TASKS_FROZEN) {
 	case CPU_STARTING:
-		arch_timer_setup(evt);
+		arch_timer_setup(this_cpu_ptr(arch_timer_evt));
 		break;
 	case CPU_DYING:
-		arch_timer_stop(evt);
+		arch_timer_stop(this_cpu_ptr(arch_timer_evt));
 		break;
 	}