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l1sap: ensure DL RLP frame alignment for TCH/F4.8 NT

Related: OS#6578
Change-Id: Ic8ec1a5d84950d27b22fd7491d5e4a82ace9fd62
---
M include/osmo-bts/csd_v110.h
M src/common/csd_v110.c
M src/common/l1sap.c
M tests/csd/csd_test.c
4 files changed, 31 insertions(+), 7 deletions(-)

git pull ssh://gerrit.osmocom.org:29418/osmo-bts refs/changes/16/38316/1

diff --git a/include/osmo-bts/csd_v110.h b/include/osmo-bts/csd_v110.h
index f1c1027..54a6745 100644
--- a/include/osmo-bts/csd_v110.h
+++ b/include/osmo-bts/csd_v110.h
@@ -22,4 +22,4 @@
 int csd_v110_rtp_encode(const struct gsm_lchan *lchan, uint8_t *rtp,
 			const uint8_t *data, size_t data_len, uint32_t fn);
 int csd_v110_rtp_decode(const struct gsm_lchan *lchan, uint8_t *data,
-			const uint8_t *rtp, size_t rtp_len);
+			const uint8_t *rtp, size_t rtp_len, uint32_t fn);
diff --git a/src/common/csd_v110.c b/src/common/csd_v110.c
index d25eb7b..f214735 100644
--- a/src/common/csd_v110.c
+++ b/src/common/csd_v110.c
@@ -158,7 +158,7 @@
 }
 
 int csd_v110_rtp_decode(const struct gsm_lchan *lchan, uint8_t *data,
-			const uint8_t *rtp, size_t rtp_len)
+			const uint8_t *rtp, size_t rtp_len, uint32_t fn)
 {
 	const struct csd_v110_frame_desc *desc;
 	ubit_t ra_bits[80 * 4];
@@ -187,6 +187,23 @@
 			ra_bits[i] = (rtp[i] >> 7);
 	}
 
+	/* TCH/F4.8 NT: check RLP frame alignment */
+	if (lchan->tch_mode == GSM48_CMODE_DATA_6k0 &&
+	    lchan->csd_mode == LCHAN_CSD_M_NT) {
+		const ubit_t e2 = csd_v110_tchf48_nt_e2_map[fn % 26];
+		const ubit_t *e_bits;
+
+		/* check bits E2/E3 in the 1st V.110 frame */
+		e_bits = &ra_bits[0 * 80 + 5 * 8 + 1];
+		if (e_bits[1] != e2 || e_bits[2] != 0)
+			return -EAGAIN;
+		/* check bits E2/E3 in the 2nd V.110 frame */
+		e_bits = &ra_bits[1 * 80 + 5 * 8 + 1];
+		if (e_bits[1] != e2 || e_bits[2] != 1)
+			return -EAGAIN;
+		/* -EAGAIN means postpone transmission */
+	}
+
 	/* RA1'/RA1: convert from an intermediate rate to radio rate */
 	for (unsigned int i = 0; i < desc->num_blocks; i++) {
 		struct osmo_v110_decoded_frame df;
diff --git a/src/common/l1sap.c b/src/common/l1sap.c
index 85e986d..3f7aa6a 100644
--- a/src/common/l1sap.c
+++ b/src/common/l1sap.c
@@ -1635,11 +1635,11 @@
 
 			memcpy(&rtp[0], msgb_l2(resp_msg),
 			       OSMO_MIN(msgb_l2len(resp_msg), sizeof(rtp)));
-			msgb_get(resp_msg, msgb_l2len(resp_msg));
 
 			int rc = csd_v110_rtp_decode(lchan, msgb_l2(resp_msg),
-						     &rtp[0], sizeof(rtp));
+						     &rtp[0], sizeof(rtp), fn);
 			if (rc > 0) {
+				msgb_get(resp_msg, msgb_l2len(resp_msg));
 				msgb_put(resp_msg, rc);
 				/* 'fake' tch_ind containing all-zero so gsmtap code can be shared
 				 * between UL and DL */
@@ -1648,8 +1648,15 @@
 						       msgb_l2(resp_msg),
 						       msgb_l2len(resp_msg));
 			} else {
-				rate_ctr_inc2(trx->bts->ctrs, BTS_CTR_RTP_RX_DROP_V110_DEC);
-				msgb_free(resp_msg);
+				if (rc == -EAGAIN) {
+					/* enqueue back, postpone transmission
+					 * XXX: we don't have msgb_enqueue_tail[_count]() */
+					llist_add(&resp_msg->list, &lchan->dl_tch_queue);
+					lchan->dl_tch_queue_len++;
+				} else {
+					rate_ctr_inc2(trx->bts->ctrs, BTS_CTR_RTP_RX_DROP_V110_DEC);
+					msgb_free(resp_msg);
+				}
 				resp_msg = NULL;
 				resp_l1sap = &empty_l1sap;
 			}
diff --git a/tests/csd/csd_test.c b/tests/csd/csd_test.c
index bef0ec8..0ca7209 100644
--- a/tests/csd/csd_test.c
+++ b/tests/csd/csd_test.c
@@ -136,7 +136,7 @@
 		fprintf(stderr, "    %s\n", osmo_hexdump(&rtp[i * 16], 16));
 
 	/* decode the encoded RTP frame */
-	rc = csd_v110_rtp_decode(&lchan, &data_dec[0], &rtp[0], sizeof(rtp));
+	rc = csd_v110_rtp_decode(&lchan, &data_dec[0], &rtp[0], sizeof(rtp), 0);
 	fprintf(stderr, "[i] csd_v110_rtp_decode() returns %d\n", rc);
 	if (rc != bit_num)
 		return;

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