Bluetooth: Controller: Fix conn timeout due to delayed conn upd ind

Unlike legacy Zephyr Controller's double headed Tx queue
linked list, the new split Zephyr Controller's new LLCP does
not place a control PDU at the head of the Tx queue causing
control PDUs to be delayed on air.

Pause the data enqueue until pending PDUs are transmitted
across before enqueuing LE Connection Update Ind PDU, and
thereafter resume other PDUs being enqueued again.

There can still be connection timeout if enqueued PDUs are
not being transmitted/acked during repeated retransmissions.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>

Author: cvinayak

subsys/bluetooth/controller/ll_sw/ull_conn.c

@@ -1478,6 +1478,21 @@ void ull_conn_tx_lll_enqueue(struct ll_conn *conn, uint8_t count)
 	}
 }
 
+uint16_t ull_conn_tx_enqueue_count_get(struct ll_conn *conn)
+{
+	uint16_t count = 0;
+	memq_link_t *curr;
+
+	curr = memq_peek(conn->lll.memq_tx.head, conn->lll.memq_tx.tail, NULL);
+	while (curr != NULL) {
+		count++;
+
+		curr = memq_peek(curr->next, conn->lll.memq_tx.tail, NULL);
+	}
+
+	return count;
+}
+
 void ull_conn_link_tx_release(void *link)
 {
 	mem_release(link, &mem_link_tx.free);

subsys/bluetooth/controller/ll_sw/ull_conn_internal.h

@@ -31,6 +31,7 @@ int ull_conn_llcp(struct ll_conn *conn, uint32_t ticks_at_expire,
 void ull_conn_done(struct node_rx_event_done *done);
 void ull_conn_tx_demux(uint8_t count);
 void ull_conn_tx_lll_enqueue(struct ll_conn *conn, uint8_t count);
+uint16_t ull_conn_tx_enqueue_count_get(struct ll_conn *conn);
 void ull_conn_link_tx_release(void *link);
 uint8_t ull_conn_ack_last_idx_get(void);
 memq_link_t *ull_conn_ack_peek(uint8_t *ack_last, uint16_t *handle,

subsys/bluetooth/controller/ll_sw/ull_llcp_conn_upd.c

@@ -438,7 +438,9 @@ static void lp_cu_send_conn_update_ind_finalize(struct ll_conn *conn, struct pro
 static void lp_cu_send_conn_update_ind(struct ll_conn *conn, struct proc_ctx *ctx, uint8_t evt,
 				       void *param)
 {
-	if (llcp_lr_ispaused(conn) || !llcp_tx_alloc_peek(conn, ctx)) {
+	if (llcp_lr_ispaused(conn) || !llcp_tx_alloc_peek(conn, ctx) ||
+	    ull_conn_tx_enqueue_count_get(conn) != 0U) {
+		llcp_tx_pause_data(conn, LLCP_TX_QUEUE_PAUSE_DATA_CONN_UPD);
 		ctx->state = LP_CU_STATE_WAIT_TX_CONN_UPDATE_IND;
 	} else {
 		/* ensure alloc of TX node, before possibly waiting for NTF node */
@@ -448,6 +450,7 @@ static void lp_cu_send_conn_update_ind(struct ll_conn *conn, struct proc_ctx *ct
 			ctx->state = LP_CU_STATE_WAIT_NTF_AVAIL;
 		} else {
 			lp_cu_send_conn_update_ind_finalize(conn, ctx, evt, param);
+			llcp_tx_resume_data(conn, LLCP_TX_QUEUE_PAUSE_DATA_CONN_UPD);
 		}
 	}
 }
@@ -459,6 +462,7 @@ static void lp_cu_st_wait_ntf_avail(struct ll_conn *conn, struct proc_ctx *ctx,
 	case LP_CU_EVT_RUN:
 		if (llcp_ntf_alloc_is_available()) {
 			lp_cu_send_conn_update_ind_finalize(conn, ctx, evt, param);
+			llcp_tx_resume_data(conn, LLCP_TX_QUEUE_PAUSE_DATA_CONN_UPD);
 		}
 		break;
 	default:

subsys/bluetooth/controller/ll_sw/ull_llcp_internal.h

@@ -47,6 +47,7 @@ enum llcp_tx_q_pause_data_mask {
 	LLCP_TX_QUEUE_PAUSE_DATA_PHY_UPDATE = 0x02,
 	LLCP_TX_QUEUE_PAUSE_DATA_DATA_LENGTH = 0x04,
 	LLCP_TX_QUEUE_PAUSE_DATA_TERMINATE = 0x08,
+	LLCP_TX_QUEUE_PAUSE_DATA_CONN_UPD = 0x10,
 };
 
 #if ((CONFIG_BT_CTLR_LLCP_COMMON_TX_CTRL_BUF_NUM <\