coresight: tmc-etf: Add comment for store ordering

Since the function CS_LOCK() has contained memory barrier mb(), it
ensures the visibility of the AUX trace data before updating the
aux_head, thus it's needless to add any explicit barrier anymore.

Add comment to make clear for the barrier usage for ETF.
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Link: https://lore.kernel.org/r/20210809111407.596077-4-leo.yan@linaro.orgSigned-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>

drivers/hwtracing/coresight/coresight-tmc-etf.c

@@ -553,6 +553,11 @@ static unsigned long tmc_update_etf_buffer(struct coresight_device *csdev,
 	if (buf->snapshot)
 		handle->head += to_read;
 
+	/*
+	 * CS_LOCK() contains mb() so it can ensure visibility of the AUX trace
+	 * data before the aux_head is updated via perf_aux_output_end(), which
+	 * is expected by the perf ring buffer.
+	 */
 	CS_LOCK(drvdata->base);
 out:
 	spin_unlock_irqrestore(&drvdata->spinlock, flags);