Mathieu Poirier [off-list ref] writes:

+static void etm_event_start(struct perf_event *event, int flags)
+{
+	int cpu = smp_processor_id();
+	struct etm_cpu_data *cpu_data;
+	struct etm_event_data *event_data;
+	struct perf_output_handle *handle = this_cpu_ptr(&ctx_handle);
+	struct coresight_device *sink, *csdev = per_cpu(csdev_src, cpu);
+
+	if (!csdev)
+		goto fail;
+
+	/*
+	 * Deal with the ring buffer API and get a handle on the
+	 * session's information.
+	 */
+	event_data = perf_aux_output_begin(handle, event);
+	if (WARN_ON_ONCE(!event_data))
+		goto fail;

Hmm, perf_aux_output_begin() returning NULL is not WARN-worthy and is
actually a normal situation, for example, if the buffer is not
configured yet (which like I suggested in the other email should be a
separate thing from the *event* configuration).

perf_aux_output_begin() will also return NULL if there's no room in the
buffer.

Regards,
--
Alex