From 6df0726dbe1a4241e7cdf5b1a138e3c9cb7576b7 Mon Sep 17 00:00:00 2001
From: Lorenzo Pieralisi <Lorenzo.Pieralisi@arm.com>
Date: Tue, 6 Mar 2012 17:37:45 +0100
Subject: [PATCH] --- yaml --- r: 288323 b: refs/heads/master c:
 a0feb6db0fe03326d7d2c7a4615ce3289615c023 h: refs/heads/master i:   288321:
 a22d1a157249573b1ffc897fdd2a00f7561a29e5   288319:
 7f8245baeff1a328bec763e8c0deda45ffe6db0e v: v3

---
 [refs]                             |  2 +-
 trunk/arch/arm/kernel/perf_event.c | 23 +++++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/[refs] b/[refs]
index 86dbe86373d6..e4f7dc2d4d70 100644
--- a/[refs]
+++ b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 3f31ae121348afd9ed39700ea2a63c17cd7eeed1
+refs/heads/master: a0feb6db0fe03326d7d2c7a4615ce3289615c023
diff --git a/trunk/arch/arm/kernel/perf_event.c b/trunk/arch/arm/kernel/perf_event.c
index 56173ae16448..b2abfa18f137 100644
--- a/trunk/arch/arm/kernel/perf_event.c
+++ b/trunk/arch/arm/kernel/perf_event.c
@@ -679,6 +679,28 @@ static void __init cpu_pmu_init(struct arm_pmu *armpmu)
 	armpmu->type = ARM_PMU_DEVICE_CPU;
 }
 
+/*
+ * PMU hardware loses all context when a CPU goes offline.
+ * When a CPU is hotplugged back in, since some hardware registers are
+ * UNKNOWN at reset, the PMU must be explicitly reset to avoid reading
+ * junk values out of them.
+ */
+static int __cpuinit pmu_cpu_notify(struct notifier_block *b,
+					unsigned long action, void *hcpu)
+{
+	if ((action & ~CPU_TASKS_FROZEN) != CPU_STARTING)
+		return NOTIFY_DONE;
+
+	if (cpu_pmu && cpu_pmu->reset)
+		cpu_pmu->reset(NULL);
+
+	return NOTIFY_OK;
+}
+
+static struct notifier_block __cpuinitdata pmu_cpu_notifier = {
+	.notifier_call = pmu_cpu_notify,
+};
+
 /*
  * CPU PMU identification and registration.
  */
@@ -730,6 +752,7 @@ init_hw_perf_events(void)
 		pr_info("enabled with %s PMU driver, %d counters available\n",
 			cpu_pmu->name, cpu_pmu->num_events);
 		cpu_pmu_init(cpu_pmu);
+		register_cpu_notifier(&pmu_cpu_notifier);
 		armpmu_register(cpu_pmu, "cpu", PERF_TYPE_RAW);
 	} else {
 		pr_info("no hardware support available\n");