ARM: 6989/1: perf: do not start the PMU when no events are present

armpmu_enable can be called in situations where no events are present
(for example, from the event rotation tick after a profiled task has
exited). In this case, we currently start the PMU anyway which may
leave it active inevitably without any events being monitored.

This patch adds a simple check to the enabling code so that we avoid
starting the PMU when no events are present.

Cc: <stable@kernel.org>
Reported-by: NAshwin Chaugle <ashwinc@codeaurora.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/perf_event.c

@@ -583,7 +583,7 @@ static int armpmu_event_init(struct perf_event *event)
 static void armpmu_enable(struct pmu *pmu)
 {
 	/* Enable all of the perf events on hardware. */
-	int idx;
+	int idx, enabled = 0;
 	struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
 
 	if (!armpmu)
@@ -596,8 +596,10 @@ static void armpmu_enable(struct pmu *pmu)
 			continue;
 
 		armpmu->enable(&event->hw, idx);
+		enabled = 1;
 	}
 
+	if (enabled)
 		armpmu->start();
 }