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

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Date:	Tue, 9 Jun 2015 17:49:29 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	<linux-arch@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC:	<arnd@...db.de>, Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	<arc-linux-dev@...opsys.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Subject: [PATCH 5/8] ARCv2: perf: set usable max period as a half of real max period

From: Alexey Brodkin <abrodkin@...opsys.com>

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>
Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
---
 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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