coresight: tmc-etr: Properly set AUX buffer head in snapshot mode

Unify amongst sink drivers how the AUX ring buffer head is communicated
to user space.  That way the same algorithm in user space can be used to
determine where the latest data is and how much of it to access.
Signed-off-by: NMathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: NSuzuki K Poulose <suzuki.poulose@arm.com>
Tested-by: NLeo Yan <leo.yan@linaro.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -1501,14 +1501,13 @@ tmc_update_etr_buffer(struct coresight_device *csdev,
 	tmc_etr_sync_perf_buffer(etr_perf);
 
 	/*
-	 * Update handle->head in snapshot mode. Also update the size to the
-	 * hardware buffer size if there was an overflow.
+	 * In snapshot mode we simply increment the head by the number of byte
+	 * that were written.  User space function  cs_etm_find_snapshot() will
+	 * figure out how many bytes to get from the AUX buffer based on the
+	 * position of the head.
 	 */
-	if (etr_perf->snapshot) {
+	if (etr_perf->snapshot)
 		handle->head += size;
-		if (etr_buf->full)
-			size = etr_buf->size;
-	}
 
 	lost |= etr_buf->full;
 out: