mgcp: Document transcoding semantic and follow it
Transcoding from GSM to PCMA can lead to the MGCP MGW sending
two PCMA packages with the same sequence number and timestamp.
Once with the encoded audio and once completely empty.
This is because "state->dst_packet_duration" is 0 in most cases
(unless a ptime is forced) and we attempt to encode audio even
if there are not enough samples. The encode_audio return will
return 0 in that case which is not trated as an error by the
mgcp network code.
Handle rc == 0 specially and document the semantic.
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);