Bluetooth: controller: Fix free Rx PDU queue starvation

Fix for scenarios where in active PHY Update Procedure or
Connection Update Procedure could cause temporary depletion
of the free Rx PDUs in the queue between LL/HCI thread
context to LLL context. Symptoms being on-air NACKing during
the above said procedures, causing supervision timeouts due
to procedures not completing at instants.

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

Author: cvinayak

subsys/bluetooth/controller/ll_sw/ull.c

@@ -1723,57 +1723,64 @@ static inline void rx_alloc(uint8_t max)
 {
 	uint8_t idx;
 
-#if defined(CONFIG_BT_CONN)
-	while (mem_link_rx.quota_pdu &&
-	       MFIFO_ENQUEUE_IDX_GET(ll_pdu_rx_free, &idx)) {
+	if (max > mem_link_rx.quota_pdu) {
+		max = mem_link_rx.quota_pdu;
+	}
+
+	while ((max--) && MFIFO_ENQUEUE_IDX_GET(pdu_rx_free, &idx)) {
 		memq_link_t *link;
 		struct node_rx_hdr *rx;
 
 		link = mem_acquire(&mem_link_rx.free);
 		if (!link) {
-			break;
+			return;
 		}
 
 		rx = mem_acquire(&mem_pdu_rx.free);
 		if (!rx) {
 			mem_release(link, &mem_link_rx.free);
-			break;
+			return;
 		}
 
-		link->mem = NULL;
 		rx->link = link;
 
-		MFIFO_BY_IDX_ENQUEUE(ll_pdu_rx_free, idx, rx);
+		MFIFO_BY_IDX_ENQUEUE(pdu_rx_free, idx, rx);
 
 		ll_rx_link_inc_quota(-1);
 	}
-#endif /* CONFIG_BT_CONN */
 
-	if (max > mem_link_rx.quota_pdu) {
-		max = mem_link_rx.quota_pdu;
+#if defined(CONFIG_BT_CONN)
+	if (!max) {
+		return;
 	}
 
-	while ((max--) && MFIFO_ENQUEUE_IDX_GET(pdu_rx_free, &idx)) {
+	/* Replenish the ULL to LL/HCI free Rx PDU queue after LLL to ULL free
+	 * Rx PDU queue has been filled.
+	 */
+	while (mem_link_rx.quota_pdu &&
+	       MFIFO_ENQUEUE_IDX_GET(ll_pdu_rx_free, &idx)) {
 		memq_link_t *link;
 		struct node_rx_hdr *rx;
 
 		link = mem_acquire(&mem_link_rx.free);
 		if (!link) {
-			break;
+			return;
 		}
 
 		rx = mem_acquire(&mem_pdu_rx.free);
 		if (!rx) {
 			mem_release(link, &mem_link_rx.free);
-			break;
+			return;
 		}
 
+		link->mem = NULL;
 		rx->link = link;
 
-		MFIFO_BY_IDX_ENQUEUE(pdu_rx_free, idx, rx);
+		MFIFO_BY_IDX_ENQUEUE(ll_pdu_rx_free, idx, rx);
 
 		ll_rx_link_inc_quota(-1);
 	}
+#endif /* CONFIG_BT_CONN */
 }
 
 static void rx_demux(void *param)