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 <redacted>
---
  drivers/hwtracing/coresight/coresight-tmc-etf.c | 5 +++++
  1 file changed, 5 insertions(+)

diff --git a/drivers/hwtracing/coresight/coresight-tmc-etf.c b/drivers/hwtracing/coresight/coresight-tmc-etf.c
index cd0fb7bfba68..8debd4f40f06 100644
--- a/drivers/hwtracing/coresight/coresight-tmc-etf.c
+++ b/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);