rewrite packing of DL PDU in SS

the new algo tries to write more than just one RLC PDU inside a MAC PDU
if this is possible

srsue/test/ttcn3/hdr/ttcn3_syssim.h

@@ -285,13 +285,12 @@ public:
         for (int lcid = 0; lcid < SRSLTE_N_RADIO_BEARERS; lcid++) {
           uint32_t buf_state = rlc.get_buffer_state(lcid);
           // Schedule DL transmission if there is data in RLC buffer or we need to send Msg4
-          if (buf_state > 0 || (msg3_tti != -1 && conres_id != 0)) {
+          if ((buf_state > 0 && bearer_follow_on_map[lcid] == false) || (msg3_tti != -1 && conres_id != 0)) {
             log.debug("LCID=%d, buffer_state=%d\n", lcid, buf_state);
-            const uint32_t mac_header_size = 10; // Add MAC header (10 B for all subheaders, etc)
-            if (tmp_rlc_buffer.get_tailroom() > (buf_state + mac_header_size)) {
-              uint32_t pdu_size = rlc.read_pdu(lcid, tmp_rlc_buffer.msg, buf_state);
             tx_payload_buffer.clear();
-              mac_msg_dl.init_tx(&tx_payload_buffer, pdu_size + mac_header_size, false);
+
+            const uint32_t mac_header_size = 10; // Add MAC header (10 B for all subheaders, etc)
+            mac_msg_dl.init_tx(&tx_payload_buffer, buf_state + mac_header_size, false);
 
             // check if this is Msg4 that needs to contain the contention resolution ID CE
             if (msg3_tti != -1 && lcid == 0 && conres_id != 0) {
@@ -308,16 +307,23 @@ public:
               msg3_tti = -1;
             }
 
-              // Add payload
+            // allocate SDUs
+            while (buf_state > 0) { // there is pending SDU to allocate
               if (mac_msg_dl.new_subh()) {
-                int n = mac_msg_dl.get()->set_sdu(lcid, pdu_size, tmp_rlc_buffer.msg);
+                int n = mac_msg_dl.get()->set_sdu(lcid, buf_state, &rlc);
                 if (n == -1) {
-                  log.error("Error while adding SDU (%d B) to MAC PDU\n", pdu_size);
+                  log.error("Error while adding SDU (%d B) to MAC PDU\n", buf_state);
                   mac_msg_dl.del_subh();
                 }
+
+                // update buffer state
+                buf_state = rlc.get_buffer_state(lcid);
+              }
             }
 
+            // Assemble entire MAC PDU
             uint8_t* mac_pdu_ptr = mac_msg_dl.write_packet(&log);
+
             if (mac_pdu_ptr != nullptr) {
               log.info_hex(mac_pdu_ptr, mac_msg_dl.get_pdu_len(), "DL MAC PDU (%d B):\n", mac_msg_dl.get_pdu_len());
 
@@ -334,9 +340,9 @@ public:
               log.error("Error writing DL MAC PDU\n");
             }
             mac_msg_dl.reset();
-            } else {
-              log.error("Can't fit RLC PDU into buffer (%d > %d)\n", buf_state, tmp_rlc_buffer.get_tailroom());
-            }
+
+          } else if (bearer_follow_on_map[lcid]) {
+            log.info("Waiting for more PDUs for transmission on LCID=%d\n", lcid);
           }
         }
         // Check if we need to provide a UL grant as well
@@ -1140,7 +1146,6 @@ private:
 
   // buffer for DL transmissions
   srslte::byte_buffer_t rar_buffer;
-  srslte::byte_buffer_t tmp_rlc_buffer;    // Used to buffer RLC PDU
   srslte::byte_buffer_t tx_payload_buffer; // Used to buffer final MAC PDU
 
   uint64_t conres_id = 0;