- * Starts/Stops a counter present on the PMU. The PMI handler
- * should stop the counter when perf_event_overflow() returns
- * !0. ->start() will be used to continue.
+ * Starts/Stops a counter present on the PMU.
+ *
+ * The PMI handler should stop the counter when perf_event_overflow()
+ * returns !0. ->start() will be used to continue.
+ *
+ * Also used to change the sample period.
+ *
+ * Called with IRQs disabled and the PMU disabled on the CPU the event
+ * is on -- will be called from NMI context with the PMU generates
+ * NMIs.
+ *
+ * ->stop() with PERF_EF_UPDATE will read the counter and update
+ * period/count values like ->read() would.
+ *
+ * ->start() with PERF_EF_RELOAD will reprogram the the counter
+ * value, must be preceded by a ->stop() with PERF_EF_UPDATE.