ARM: 7358/1: perf: add PMU hotplug notifier

When a CPU is taken out of reset, either cold booted or hotplugged in,
some of its PMU registers can contain UNKNOWN values.

This patch adds a hotplug notifier to ARM core perf code so that upon
CPU restart the PMU unit is reset and becomes ready to use again.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 23 insertions, 0 deletions

diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
index 56173ae16448..b2abfa18f137 100644
--- a/arch/arm/kernel/perf_event.c
+++ b/arch/arm/kernel/perf_event.c
@@ -680,6 +680,28 @@ static void __init cpu_pmu_init(struct arm_pmu *armpmu)
 }
 
 /*
+ * 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.
  */
 static int __init
@@ -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");