Mathieu Poirier [off-list ref] writes:

+static int etm_event_init(struct perf_event *event)
+{
+	if (event->attr.type != etm_pmu.type)
+		return -ENOENT;
+
+	if (event->cpu >= nr_cpu_ids)
+		return -EINVAL;

perf_event_alloc() already does this. Except for this one doesn't cover
the negative space.

[snip]

+static void etm_free_aux(void *data)
+{
+	struct etm_event_data *event_data = data;
+
+	pr_err("Queing work\n");

Probably not pr_err().

+	schedule_work(&event_data->work);
+}

[snip]

+static void etm_event_start(struct perf_event *event, int flags)
+{
+	int cpu = smp_processor_id();
+	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;

There really shouldn't be a warning here. I understand that the 'no
buffer' case is taped over by the !csdev check above, but there are
other ligitimate reasons for perf_aux_output_begin() to return NULL,
like no-space-left.

+
+	/* We need a sink, no need to continue without one */
+	sink = coresight_get_sink(event_data->path[cpu]);
+	if (!sink || !sink_ops(sink)->set_buffer)
+		goto fail_end_stop;

Is this possible after the coresight_build_path() things in setup_aux?
Might be a better candidate for WARN_*ONCE().

+
+	/* Configure the sink */
+	if (sink_ops(sink)->set_buffer(sink, handle,
+				       event_data->snk_config))
+		goto fail_end_stop;
+
+	/* Nothing will happen without a path */
+	if (coresight_enable_path(event_data->path[cpu], CS_MODE_PERF))
+		goto fail_end_stop;

I'd like to understand all the potential failures here, because it's
really a good idea to keep those to a minimum for the sake of
consistency. That is, if the user succeeded in creating an event, about
the only good reason for the event not starting is a filled up buffer.

This is why it makes a lot of sense to keep all the
coresight_build_path()/coresight_enable_path() to the .event_init()
phase and let them fail early, if they should fail.

Regards,
--
Alex