Re: [PATCH 06/25] coresight: perf: Simplify flow for CPUs without a path

[James Clark <james.clark@linaro.org>]

On 15/09/2026 16:48, Leo Yan wrote:
> etm_setup_aux() removes a CPU from the event mask when it cannot build a
> path from that CPU's ETM to the selected sink. The start nevertheless
> allocates buffer handle and keeps event data active until the
> corresponding stop callback.
> 
> End the newly acquired handle immediately when the current CPU is not in
> the mask, and do not publish event data in the per-CPU context. The
> event remains logically active so that a task event can trace after
> migrating to an eligible CPU. A stop on the unsupported CPU then has no
> CoreSight modules to disable or trace data to synchronize. Mark its
> hardware state stopped and up to date.
> 
> Assisted-by: Codex:gpt-6
> Signed-off-by: Leo Yan <leo.yan@arm.com>
> ---
>   drivers/hwtracing/coresight/coresight-etm-perf.c | 38 +++++++++++-------------
>   1 file changed, 17 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.c b/drivers/hwtracing/coresight/coresight-etm-perf.c
> index 13d865ca86f1161b2e21174f096473c4829afc8c..afd22fd4a851e600ab69fd5b6883e6f8518b2cab 100644
> --- a/drivers/hwtracing/coresight/coresight-etm-perf.c
> +++ b/drivers/hwtracing/coresight/coresight-etm-perf.c
> @@ -572,13 +572,15 @@ static void etm_event_start(struct perf_event *event, int flags)
>   	 * the sink was specified or hinted to the driver. For
>   	 * now, simply don't record anything on this ETM.
>   	 *
> -	 * As such we pretend that everything is fine, and let
> -	 * it continue without actually tracing. The event could
> -	 * continue tracing when it moves to a CPU where it is
> -	 * reachable to a sink.
> +	 * In that case, leave the event logically active but stop AUX
> +	 * output on this CPU. The event could continue tracing when
> +	 * it moves to a CPU where it is reachable to a sink.
>   	 */
> -	if (!cpumask_test_cpu(cpu, &event_data->mask))
> -		goto out;
> +	if (!cpumask_test_cpu(cpu, &event_data->mask)) {
> +		perf_aux_output_end(handle, 0);
> +		event->hw.state = 0;
> +		return;
> +	}
>   
>   	path = etm_event_cpu_path(event_data, cpu);
>   	path->handle = handle;
> @@ -613,7 +615,6 @@ static void etm_event_start(struct perf_event *event, int flags)
>   		perf_report_aux_output_id(event, hw_id);
>   	}
>   
> -out:
>   	/* Tell the perf core the event is alive */
>   	event->hw.state = 0;
>   	/* Save the event_data for this ETM */
> @@ -709,7 +710,6 @@ static void etm_event_pause(struct coresight_path *path,
>   
>   static void etm_event_stop(struct perf_event *event, int mode)
>   {
> -	int cpu = smp_processor_id();
>   	struct coresight_device *source, *sink;
>   	struct etm_ctxt *ctxt = this_cpu_ptr(&etm_ctxt);
>   	struct perf_output_handle *handle = &ctxt->handle;
> @@ -719,6 +719,15 @@ static void etm_event_stop(struct perf_event *event, int mode)
>   	if (mode & PERF_EF_PAUSE)
>   		return etm_event_pause(path, event, ctxt);
>   
> +	/*
> +	 * With no path enabled, there is no hardware to stop or trace data
> +	 * to synchronize. Mark the event stopped and up to date.
> +	 */
> +	if (!path) {
> +		event->hw.state = PERF_HES_STOPPED | PERF_HES_UPTODATE;
> +		return;
> +	}
> +
>   	/*
>   	 * If we still have access to the event_data via handle,
>   	 * confirm that we haven't messed up the tracking.
> @@ -738,19 +747,6 @@ static void etm_event_stop(struct perf_event *event, int mode)
>   	if (WARN_ON(!event_data))
>   		return;
>   
> -	/*
> -	 * Check if this ETM was allowed to trace, as decided at
> -	 * etm_setup_aux(). If it wasn't allowed to trace, then
> -	 * nothing needs to be torn down other than outputting a
> -	 * zero sized record.
> -	 */
> -	if (handle->event && (mode & PERF_EF_UPDATE) &&
> -	    !cpumask_test_cpu(cpu, &event_data->mask)) {
> -		event->hw.state = PERF_HES_STOPPED;
> -		perf_aux_output_end(handle, 0);
> -		return;
> -	}
> -
>   	source = coresight_get_source(path);
>   	sink = coresight_get_sink(path);
>   	if (!source || !sink)
> 

Reviewed-by: James Clark <james.clark@linaro.org>