x86: restrict PIT clocksource usage

PIT clocksource is registered unconditionally even when HPET is enabled
or when PIT is replaced by the local APIC timer. In both cases PIT can
not be used as it is stopped and the readout would be stale.

Prevent registering PIT in those cases.

patch depends on:

  x86: offer is_hpet_enabled() on !CONFIG_HPET_TIMER too
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/i8253.c

@@ -197,7 +197,15 @@ static struct clocksource clocksource_pit = {
 
 static int __init init_pit_clocksource(void)
 {
-	if (num_possible_cpus() > 1) /* PIT does not scale! */
+	 /*
+	  * Several reasons not to register PIT as a clocksource:
+	  *
+	  * - On SMP PIT does not scale due to i8253_lock
+	  * - when HPET is enabled
+	  * - when local APIC timer is active (PIT is switched off)
+	  */
+	if (num_possible_cpus() > 1 || is_hpet_enabled() ||
+	    pit_clockevent.mode != CLOCK_EVT_MODE_PERIODIC)
 		return 0;
 
 	clocksource_pit.mult = clocksource_hz2mult(CLOCK_TICK_RATE, 20);