linux-kernel - Re: [RFC PATCH 14/20] coresight: etm-perf: implementing 'event

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Date:	Tue, 22 Sep 2015 17:29:00 +0300
From:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:	Mathieu Poirier <mathieu.poirier@...aro.org>,
	gregkh@...uxfoundation.org, a.p.zijlstra@...llo.nl,
	acme@...nel.org, mingo@...hat.com, corbet@....net
Cc:	adrian.hunter@...el.com, zhang.chunyan@...aro.org,
	mike.leach@....com, tor@...com, al.grant@....com,
	pawel.moll@....com, linux-arm-kernel@...ts.infradead.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 14/20] coresight: etm-perf: implementing 'event_init()' API

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

> +static void etm_event_destroy(struct perf_event *event)
> +{
> +	/* switching off the source will also tear down the path */
> +	etm_event_power_sources(event->cpu, false);
> +}
> +
> +static int etm_event_init(struct perf_event *event)
> +{
> +	int ret;
> +
> +	if (event->attr.type != etm_pmu.type)
> +		return -ENOENT;
> +
> +	if (event->cpu >= nr_cpu_ids)
> +		return -EINVAL;
> +
> +	/* only one session at a time */
> +	if (etm_event_source_enabled(event->cpu))
> +		return -EBUSY;

Why is this the case? If you were to configure the event in pmu::add()
and deconfigure it in pmu::del(), like you already do with the buffer
part, you could handle as many sessions as you want.

> +
> +	/*
> +	 * Make sure CPUs don't disappear between the
> +	 * power up sequence and configuration.
> +	 */
> +	get_online_cpus();
> +	ret = etm_event_power_sources(event->cpu, true);
> +	if (ret)
> +		goto out;
> +
> +	ret = etm_event_config_sources(event->cpu);

This can be done in pmu::add(), if you can call directly into
etm_configure_cpu() or etm_config_enable() so that there's no cross-cpu
calling in between.

> +
> +	event->destroy = etm_event_destroy;
> +out:
> +	put_online_cpus();
> +	return ret;
> +}
> +
>  static int __init etm_perf_init(void)
>  {
>  	etm_pmu.capabilities	= PERF_PMU_CAP_EXCLUSIVE;
> @@ -59,6 +234,7 @@ static int __init etm_perf_init(void)
>  	etm_pmu.attr_groups	= etm_pmu_attr_groups;
>  	etm_pmu.task_ctx_nr	= perf_sw_context;
>  	etm_pmu.read		= etm_event_read;
> +	etm_pmu.event_init	= etm_event_init;
>  
>  	return perf_pmu_register(&etm_pmu, CORESIGHT_ETM_PMU_NAME, -1);
>  }

One general comment -- it would be slightly easier at least for me if
all the pmu related bits were in one patch.

Thanks,
--
Alex
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