linux-kernel - [PATCH v2 5/8] ARCv2: perf: set usable max period as a half of real max period

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Date:	Wed,  5 Aug 2015 18:13:31 +0300
From:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:	linux-arch@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Vineet.Gupta1@...opsys.com,
	arc-linux-dev@...opsys.com, arnd@...db.de, peterz@...radead.org,
	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: [PATCH v2 5/8] ARCv2: perf: set usable max period as a half of real max period

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@...radead.org>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
Signed-off-by: Alexey Brodkin <abrodkin@...opsys.com>
---

No changes since v1.

 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 1a9f922..81847ef 100644
--- a/arch/arc/kernel/perf_event.c
+++ b/arch/arc/kernel/perf_event.c
@@ -459,7 +459,7 @@ 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,
-- 
2.4.3

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