clockevents: Leave the broadcast device in shutdown mode when not needed

Platforms with Always Running APIC Timer doesn't use the broadcast timer
but the kernel is leaving the broadcast timer (HPET in this case)
in oneshot mode.

On these platforms, before the switch to oneshot mode, broadcast device is
actually in shutdown mode. Code checks for empty tick_broadcast_mask and
avoids going into the periodic mode.

During switch to oneshot mode, add the same tick_broadcast_mask checks in the
tick_broadcast_switch_to_oneshot() and avoid the broadcast device going into
the oneshot mode.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: john stultz <johnstul@us.ibm.com>
Cc: venki@google.com
Link: http://lkml.kernel.org/r/1320452301.15071.16.camel@sbsiddha-desk.sc.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/time/tick-broadcast.c

@@ -575,11 +575,15 @@ void tick_broadcast_switch_to_oneshot(void)
 	unsigned long flags;
 
 	raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
+	if (cpumask_empty(tick_get_broadcast_mask()))
+		goto end;
 
 	tick_broadcast_device.mode = TICKDEV_MODE_ONESHOT;
 	bc = tick_broadcast_device.evtdev;
 	if (bc)
 		tick_broadcast_setup_oneshot(bc);
+
+end:
 	raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
 }