[PATCH 5/8] ARCv2: perf: set usable max period as a half of real max period

[Vineet Gupta]

From: Alexey Brodkin <abrodkin@xxxxxxxxxxxx>

Overflow interrupt happens when counter reaches a limit which we set as a
maximum value of the counter.

But for better precision counter continues registration of assigned events
even after reaching pre-defined limit. To not really overlap we leave half
of the counter values free.

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Signed-off-by: Alexey Brodkin <abrodkin@xxxxxxxxxxxx>
Signed-off-by: Vineet Gupta <vgupta@xxxxxxxxxxxx>
---
 arch/arc/kernel/perf_event.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arc/kernel/perf_event.c b/arch/arc/kernel/perf_event.c
index 4e3211948467..a7b4e4826533 100644
--- a/arch/arc/kernel/perf_event.c
+++ b/arch/arc/kernel/perf_event.c
@@ -491,7 +491,7 @@ static int arc_pmu_device_probe(struct platform_device *pdev)
 	arc_pmu->n_counters = pct_bcr.c;
 	counter_size = 32 + (pct_bcr.s << 4);
 
-	arc_pmu->max_period = (1ULL << counter_size) - 1ULL;
+	arc_pmu->max_period = (1ULL << counter_size) / 2 - 1ULL;
 
 	pr_info("ARC perf\t: %d counters (%d bits), %d conditions%s\n",
 		arc_pmu->n_counters, counter_size, cc_bcr.c,
-- 
1.9.1

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