Re: [RESEND PATCH v5 4/6] coresight: Use PMU driver configuration for sink selection

[Suzuki K Poulose <suzuki.poulose@arm.com>]

Hi Mathieu,

On 17/12/2018 17:21, Mathieu Poirier wrote:
> This patch uses the PMU driver configuration held in event::hw::drv_config
> to select a sink for each event that is created (the old sysFS way of
> working is kept around for backward compatibility).
> 
> By proceeding in this way a sink can be used by multiple sessions
> without having to play games with entries in sysFS.
> 
> Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
> ---
>   drivers/hwtracing/coresight/coresight-etm-perf.c | 74 ++++++++++++++++++++----
>   1 file changed, 62 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.c b/drivers/hwtracing/coresight/coresight-etm-perf.c
> index f21eb28b6782..a7e1fdef07f2 100644
> --- a/drivers/hwtracing/coresight/coresight-etm-perf.c
> +++ b/drivers/hwtracing/coresight/coresight-etm-perf.c
> @@ -4,6 +4,7 @@
>    * Author: Mathieu Poirier <mathieu.poirier@linaro.org>
>    */
>   
> +#include <linux/amba/bus.h>
>   #include <linux/coresight.h>
>   #include <linux/coresight-pmu.h>
>   #include <linux/cpumask.h>
> @@ -11,6 +12,7 @@
>   #include <linux/list.h>
>   #include <linux/mm.h>
>   #include <linux/init.h>
> +#include <linux/ioport.h>
>   #include <linux/perf_event.h>
>   #include <linux/percpu-defs.h>
>   #include <linux/slab.h>
> @@ -177,6 +179,26 @@ static void etm_free_aux(void *data)
>   	schedule_work(&event_data->work);
>   }
>   
> +static struct coresight_device *etm_drv_config_sync(struct perf_event *event)

minor nit: The name doesn't quite imply what we do here. Did you mean
s/sync/sink ?

> +{
> +	struct coresight_device *sink = NULL;
> +	struct pmu_drv_config *drv_config = perf_event_get_drv_config(event);
> +
> +	/*
> +	 * Make sure we don't race with perf_drv_config_replace() in
> +	 * kernel/events/core.c.
> +	 */
> +	raw_spin_lock(&drv_config->lock);
> +
> +	/* Copy what we got from user space if applicable. */
> +	if (drv_config->config)
> +		sink = drv_config->config;
> +
> +	raw_spin_unlock(&drv_config->lock);
> +
> +	return sink;
> +}
> +
>   static void *etm_setup_aux(struct perf_event *event, void **pages,
>   			   int nr_pages, bool overwrite)
>   {
> @@ -190,18 +212,11 @@ static void *etm_setup_aux(struct perf_event *event, void **pages,
>   		return NULL;
>   	INIT_WORK(&event_data->work, free_event_data);
>   
> -	/*
> -	 * In theory nothing prevent tracers in a trace session from being
> -	 * associated with different sinks, nor having a sink per tracer.  But
> -	 * until we have HW with this kind of topology we need to assume tracers
> -	 * in a trace session are using the same sink.  Therefore go through
> -	 * the coresight bus and pick the first enabled sink.
> -	 *
> -	 * When operated from sysFS users are responsible to enable the sink
> -	 * while from perf, the perf tools will do it based on the choice made
> -	 * on the cmd line.  As such the "enable_sink" flag in sysFS is reset.
> -	 */
> -	sink = coresight_get_enabled_sink(true);
> +	/* First get the sink config from user space. */
> +	sink = etm_drv_config_sync(event);
> +	if (!sink)
> +		sink = coresight_get_enabled_sink(true);
> +
>   	if (!sink || !sink_ops(sink)->alloc_buffer)
>   		goto err;
>   
> @@ -454,6 +469,40 @@ static void etm_addr_filters_sync(struct perf_event *event)
>   	filters->nr_filters = i;
>   }
>   
> +static int etm_drv_config_find_sink(struct device *dev, void *data)
> +{
> +	struct amba_device *adev = to_amba_device(dev->parent);
> +	struct resource *res = &adev->res;
> +	u64 value = *((u64 *)data);
> +
> +	/*
> +	 * The HW mapping of a component is unique.  If the value we've been
> +	 * given matches the component's start address, then we must have found
> +	 * the device we are looking for.
> +	 */

To be frank, I don't quite like the idea of passing the base address of the
component as the key to locate a device, (even though that is unique and readily
available). I would rather prefer a programmable way to map the keys to the
"sink" devices, which works platform agnostic (e.g, ACPI support, where the base
address is not obvious from the name). Also if we decide to use a platform
agnostic naming scheme, it becomes even more complex.

We could assign a static "id/key" exported either via the device sysfs dir or
the "pmu" dir. I prefer the latter.

Thoughts ?

And whatever we decide to choose, needs to be clearly documented under the 
Documentation/perf/cs_etm.txt.

Cheers
Suzuki

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