powerpc/perf: power_pmu_start restores incorrect values, breaking fre…

…quency events

perf on POWER stopped working after commit e050e3f (perf: Fix
broken interrupt rate throttling). That patch exposed a bug in
the POWER perf_events code.

Since the PMCs count upwards and take an exception when the top bit
is set, we want to write 0x80000000 - left in power_pmu_start. We were
instead programming in left which effectively disables the counter
until we eventually hit 0x80000000. This could take seconds or longer.

With the patch applied I get the expected number of samples:

          SAMPLE events:       9948

Signed-off-by: Anton Blanchard <[email protected]>
Acked-by: Paul Mackerras <[email protected]>
Signed-off-by: Benjamin Herrenschmidt <[email protected]>
Cc: <[email protected]>

arch/powerpc/kernel/perf_event.c

@@ -865,6 +865,7 @@ static void power_pmu_start(struct perf_event *event, int ef_flags)
 {
 	unsigned long flags;
 	s64 left;
+	unsigned long val;
 
 	if (!event->hw.idx || !event->hw.sample_period)
 		return;
@@ -880,7 +881,12 @@ static void power_pmu_start(struct perf_event *event, int ef_flags)
 
 	event->hw.state = 0;
 	left = local64_read(&event->hw.period_left);
-	write_pmc(event->hw.idx, left);
+
+	val = 0;
+	if (left < 0x80000000L)
+		val = 0x80000000L - left;
+
+	write_pmc(event->hw.idx, val);
 
 	perf_event_update_userpage(event);
 	perf_pmu_enable(event->pmu);