diff --git a/openbsc/include/openbsc/mgcp.h b/openbsc/include/openbsc/mgcp.h
index d4d6140..1790f84 100644
--- a/openbsc/include/openbsc/mgcp.h
+++ b/openbsc/include/openbsc/mgcp.h
@@ -87,6 +87,12 @@
 typedef int (*mgcp_reset)(struct mgcp_trunk_config *cfg);
 typedef int (*mgcp_rqnt)(struct mgcp_endpoint *endp, char tone);
 
+/**
+ * Return:
+ *   <  0 in case no audio was processed
+ *   >= 0 in case audio was processed. The remaining payload
+ *   length will be returned.
+ */
 typedef int (*mgcp_processing)(struct mgcp_endpoint *endp,
 			       struct mgcp_rtp_end *dst_end,
 			       char *data, int *len, int buf_size);