linux-kernel - Re: [PATCH V8 18/23] coresight: etm-perf: new PMU driver for ETM tracers

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Message-ID: <878u3c744r.fsf@ashishki-desk.ger.corp.intel.com>
Date:	Tue, 26 Jan 2016 17:27:16 +0200
From:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:	Mathieu Poirier <mathieu.poirier@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:	linux-doc@...r.kernel.org, zhang.chunyan@...aro.org,
	mike.leach@....com, tor@...com, al.grant@....com, rabin@....in,
	Mathieu Poirier <mathieu.poirier@...aro.org>
Subject: Re: [PATCH V8 18/23] coresight: etm-perf: new PMU driver for ETM tracers

Mathieu Poirier <mathieu.poirier@...aro.org> 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