amr: Avoid toggling the CMR from none and a set one

For LCR and other systems without out-of-band information we need
to indicate the CMR. Not every air message will include the mode
and we sent a stream that had the CMR set and not-set. This lead
to the AudioCodes MGW only playing every second frame.

Remember the last used mode and initialize it to _NONE when we
receive the multirate config. In case of a real error we will
still use AMR_CMR_NONE.

The initial patch is from Harald. I have added the initialization
and moving of the defines to amr.h.

Manually verified by enabling AMR5.9 and looking at two RTP
packages in sequence. In both cases the CMR was 2. I have looked
at "amr.nb.cmr != 2" in wireshark and only found the MGCP dummy
packet.

3 files changed, 14 insertions, 9 deletions

diff --git a/include/osmo-bts/amr.h b/include/osmo-bts/amr.h
index 059f2e03..ba66e4df 100644
--- a/include/osmo-bts/amr.h
+++ b/include/osmo-bts/amr.h
@@ -3,6 +3,9 @@
 
 #include <osmo-bts/gsm_data.h>
 
+#define AMR_TOC_QBIT	0x04
+#define AMR_CMR_NONE	0xF
+
 void amr_log_mr_conf(int ss, int logl, const char *pfx,
 		     struct amr_multirate_conf *amr_mrc);
 
diff --git a/src/common/rsl.c b/src/common/rsl.c
index 6b683bd5..bc7fddb9 100644
--- a/src/common/rsl.c
+++ b/src/common/rsl.c
@@ -811,6 +811,7 @@ static int rsl_rx_chan_activ(struct msgb *msg)
 				  TLVP_LEN(&tp, RSL_IE_MR_CONFIG));
 		amr_log_mr_conf(DRTP, LOGL_DEBUG, gsm_lchan_name(lchan),
 				&lchan->tch.amr_mr);
+		lchan->tch.last_cmr = AMR_CMR_NONE;
 	}
 	/* 9.3.53 MultiRate Control */
 	/* 9.3.54 Supported Codec Types */
@@ -1067,6 +1068,7 @@ static int rsl_rx_mode_modif(struct msgb *msg)
 				  TLVP_LEN(&tp, RSL_IE_MR_CONFIG));
 		amr_log_mr_conf(DRTP, LOGL_DEBUG, gsm_lchan_name(lchan),
 				&lchan->tch.amr_mr);
+		lchan->tch.last_cmr = AMR_CMR_NONE;
 	}
 	/* 9.3.53 MultiRate Control */
 	/* 9.3.54 Supported Codec Types */
diff --git a/src/osmo-bts-sysmo/tch.c b/src/osmo-bts-sysmo/tch.c
index 88c0754b..c3c6ca33 100644
--- a/src/osmo-bts-sysmo/tch.c
+++ b/src/osmo-bts-sysmo/tch.c
@@ -39,6 +39,7 @@
 #include <osmo-bts/bts.h>
 #include <osmo-bts/gsm_data.h>
 #include <osmo-bts/measurement.h>
+#include <osmo-bts/amr.h>
 
 #include <sysmocom/femtobts/superfemto.h>
 #include <sysmocom/femtobts/gsml1prim.h>
@@ -240,12 +241,10 @@ static int rtppayload_to_l1_hr(uint8_t *l1_payload, const uint8_t *rtp_payload,
 	return GSM_HR_BYTES;
 }
 
-#define AMR_TOC_QBIT	0x04
-#define AMR_CMR_NONE	0xF
-
 static struct msgb *l1_to_rtppayload_amr(uint8_t *l1_payload, uint8_t payload_len,
-					 struct amr_multirate_conf *amr_mrc)
+					 struct gsm_lchan *lchan)
 {
+	struct amr_multirate_conf *amr_mrc = &lchan->tch.amr_mr;
 	struct msgb *msg;
 	uint8_t amr_if2_len = payload_len - 2;
 	uint8_t *cur;
@@ -262,14 +261,16 @@ static struct msgb *l1_to_rtppayload_amr(uint8_t *l1_payload, uint8_t payload_le
 	uint8_t ft = l1_payload[2] & 0xF;
 	uint8_t cmr_idx = l1_payload[1];
 	/* CMR == Unset means CMR was not transmitted at this TDMA */
-	if (cmr_idx >= GsmL1_AmrCodecMode_Unset)
-		cmr = AMR_CMR_NONE;
-	else if (cmr_idx >= amr_mrc->num_modes) {
+	if (cmr_idx == GsmL1_AmrCodecMode_Unset)
+		cmr = lchan->tch.last_cmr;
+	else if (cmr_idx >= amr_mrc->num_modes ||
+		 cmr_idx > GsmL1_AmrCodecMode_Unset) {
 		/* Make sure the CMR of the phone is in the active codec set */
 		LOGP(DL1C, LOGL_NOTICE, "L1->RTP: overriding CMR IDX %u\n", cmr_idx);
 		cmr = AMR_CMR_NONE;
 	} else {
 		cmr = amr_mrc->mode[cmr_idx].mode;
+		lchan->tch.last_cmr = cmr;
 	}
 
 	/* RFC 3267  4.4.1 Payload Header */
@@ -606,8 +607,7 @@ int l1if_tch_rx(struct gsm_lchan *lchan, struct msgb *l1p_msg)
 		break;
 #endif
 	case GsmL1_TchPlType_Amr:
-		rmsg = l1_to_rtppayload_amr(payload, payload_len,
-					    &lchan->tch.amr_mr);
+		rmsg = l1_to_rtppayload_amr(payload, payload_len, lchan);
 		break;
 	}