Re: [PATCH 16/25] coresight: perf: Support AUX sampling with per-CPU sinks

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

On 15/09/2026 16:49, Leo Yan wrote:
> Add snapshot_aux() for per-CPU sinks with overwrite buffers. Shared
> sinks can mix trace from multiple CPUs, making them unsuitable for
> context-based profiling.
> 
> Pad missing trace with leading zeros, or zero-fill the requested AUX
> payload when a snapshot is unavailable. This ensures that the decoder
> can safely discard the zero-filled trace data.
> 
> Reuse the AUX action guard to defer concurrent stops until the snapshot
> finishes. Skip snapshots during sink interrupt handling. On copy or
> restart failure, mark the AUX output truncated so perf core disables
> the event.
> 
> Assisted-by: Codex:gpt-6
> Signed-off-by: Leo Yan <leo.yan@arm.com>

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

> ---
>   drivers/hwtracing/coresight/coresight-etm-perf.c | 123 +++++++++++++++++++++++
>   drivers/hwtracing/coresight/coresight-etm-perf.h |   2 +
>   2 files changed, 125 insertions(+)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.c b/drivers/hwtracing/coresight/coresight-etm-perf.c
> index 8020a4d8af18bb669441824cfdb687f4fa299eb4..5e90ed49562c117879780f573072e6b522d9d048 100644
> --- a/drivers/hwtracing/coresight/coresight-etm-perf.c
> +++ b/drivers/hwtracing/coresight/coresight-etm-perf.c
> @@ -456,6 +456,7 @@ static void *etm_setup_aux(struct perf_event *event, void **pages,
>   	if (!event_data)
>   		return NULL;
>   	INIT_WORK(&event_data->work, free_event_data);
> +	event_data->overwrite = overwrite;
>   
>   	/* First get the selected sink from user space. */
>   	sink_hash = ATTR_CFG_GET_FLD(&event->attr, sinkid);
> @@ -863,6 +864,127 @@ static void etm_event_del(struct perf_event *event, int mode)
>   	etm_event_stop(event, PERF_EF_UPDATE);
>   }
>   
> +/* A zero trace size pads the sample without accessing the AUX handle. */
> +static long etm_event_snapshot_aux_copy_data(struct perf_output_handle *aux_handle,
> +					     struct perf_output_handle *handle,
> +					     unsigned long trace_size,
> +					     unsigned long size)
> +{
> +	unsigned long pad;
> +	long ret;
> +
> +	trace_size = min(size, trace_size);
> +
> +	/* Pad the front with zeros to keep the latest trace data at the end */
> +	pad = size - trace_size;
> +	while (pad) {
> +		unsigned long chunk = min(pad, PAGE_SIZE);
> +
> +		if (perf_output_copy(handle, page_address(ZERO_PAGE(0)), chunk))
> +			return -EFAULT;
> +		pad -= chunk;
> +	}
> +
> +	/*
> +	 * Skip an empty snapshot, as equal offsets would otherwise copy the
> +	 * entire AUX ring.
> +	 */
> +	if (trace_size) {
> +		unsigned long to = aux_handle->head;
> +
> +		ret = perf_output_copy_aux(aux_handle, handle, to - trace_size, to);
> +		if (ret < 0)
> +			return ret;
> +	}
> +
> +	return size;
> +}
> +
> +static long etm_event_snapshot_aux(struct perf_event *event,
> +				   struct perf_output_handle *handle,
> +				   unsigned long size)
> +{
> +	struct etm_ctxt *ctxt = this_cpu_ptr(&etm_ctxt);
> +	struct perf_output_handle *aux_handle = &ctxt->handle;
> +	struct etm_event_data *event_data;
> +	struct coresight_device *source, *sink;
> +	struct coresight_path *path;
> +	unsigned long trace_size;
> +	long ret = 0;
> +
> +	if (!etm_event_aux_begin(event, ctxt))
> +		goto padding;
> +
> +	if (READ_ONCE(event->hw.aux_paused))
> +		goto out;
> +
> +	event_data = perf_get_aux(aux_handle);
> +	if (!event_data || !event_data->overwrite)
> +		goto out;
> +
> +	path = etm_event_get_ctxt_path(ctxt);
> +	if (!path)
> +		goto out;
> +
> +	source = coresight_get_source(path);
> +	sink = coresight_get_sink(path);
> +	if (WARN_ON_ONCE(!source || !sink))
> +		goto out;
> +
> +	if (!coresight_is_percpu_sink(sink)) {
> +		dev_err_once(&sink->dev, "AUX sampling requires a per-CPU sink\n");
> +		goto out;
> +	}
> +
> +	/* Skip snapshots that preempt the sink's interrupt handler */
> +	if (READ_ONCE(sink->in_interrupt))
> +		goto out;
> +
> +	coresight_pause_source(source);
> +	trace_size = sink_ops(sink)->update_buffer(sink, aux_handle,
> +						   event_data->snk_config);
> +	coresight_disable_path(path);
> +
> +	/*
> +	 * If the AUX handle was closed or marked truncated, the sink is
> +	 * expected to have notified perf core to disable the AUX event.
> +	 */
> +	if (!perf_get_aux(aux_handle) ||
> +	    (aux_handle->aux_flags & PERF_AUX_FLAG_TRUNCATED))
> +		goto fail;
> +
> +	ret = etm_event_snapshot_aux_copy_data(aux_handle, handle, trace_size, size);
> +	if (ret < 0)
> +		goto fail;
> +
> +	if (coresight_enable_path(path, CS_MODE_PERF))
> +		goto fail;
> +
> +	if (coresight_resume_source(source)) {
> +		coresight_disable_path(path);
> +		goto fail;
> +	}
> +
> +	goto out;
> +
> +fail:
> +	/* Set the truncated AUX flag so perf core can disable the event */
> +	if (perf_get_aux(aux_handle)) {
> +		perf_aux_output_flag(aux_handle, PERF_AUX_FLAG_TRUNCATED);
> +		perf_aux_output_end(aux_handle, 0);
> +	}
> +
> +out:
> +	etm_event_aux_end(event, ctxt);
> +
> +padding:
> +	/* Pad the buffer with zeros on an early exit */
> +	if (!ret)
> +		ret = etm_event_snapshot_aux_copy_data(NULL, handle, 0, size);
> +
> +	return ret;
> +}
> +
>   static int etm_addr_filters_validate(struct list_head *filters)
>   {
>   	bool range = false, address = false;
> @@ -1109,6 +1231,7 @@ int __init etm_perf_init(void)
>   	etm_pmu.free_aux		= etm_free_aux;
>   	etm_pmu.start			= etm_event_start;
>   	etm_pmu.stop			= etm_event_stop;
> +	etm_pmu.snapshot_aux		= etm_event_snapshot_aux;
>   	etm_pmu.add			= etm_event_add;
>   	etm_pmu.del			= etm_event_del;
>   	etm_pmu.addr_filters_sync	= etm_addr_filters_sync;
> diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.h b/drivers/hwtracing/coresight/coresight-etm-perf.h
> index 24d929428633f56f24a114d5bb93b8ac4799ca11..a69eb04a597efcfe630342a410cbf8fea3c4501e 100644
> --- a/drivers/hwtracing/coresight/coresight-etm-perf.h
> +++ b/drivers/hwtracing/coresight/coresight-etm-perf.h
> @@ -89,6 +89,7 @@ struct etm_filters {
>    * @aux_hwid_done:	Whether a CPU has emitted the TraceID packet or not.
>    * @snk_config:		The sink configuration.
>    * @cfg_hash:		The hash id of any coresight config selected.
> + * @overwrite:		Whether the AUX buffer uses overwrite mode.
>    * @path:		An array of path, each slot for one CPU.
>    */
>   struct etm_event_data {
> @@ -97,6 +98,7 @@ struct etm_event_data {
>   	cpumask_t aux_hwid_done;
>   	void *snk_config;
>   	u32 cfg_hash;
> +	bool overwrite;
>   	struct coresight_path * __percpu *path;
>   };
>   
>