[PATCH net] eth: fbnic: ring the doorbell if a burst ends in a drop

@@ -128,9 +128,14 @@ struct fbnic_ring {
 		/* Rx BDQs only */
 		struct page_pool *page_pool;
 
-		/* Deferred_head is used to cache the head for TWQ1 if
+		/* TWQ0 only, index of the meta descriptor of the last packet
+		 * placed in the ring without ringing the doorbell, -1 if the
+		 * doorbell is in sync with the tail.
+		 */
+		s32 deferred_meta;
+
+		/* TCQ only, used to cache the head for TWQ1 if
 		 * an attempt is made to clean TWQ1 with zero napi_budget.
-		 * We do not use it for any other ring.
 		 */
 		s32 deferred_head;
 	};

@@ -311,6 +311,29 @@ fbnic_rx_csum(u64 rcd, struct sk_buff *skb, struct fbnic_ring *rcq,
 	}
 }
 
+static void fbnic_tx_doorbell(struct fbnic_ring *ring, __le64 *meta)
+{
+	*meta |= cpu_to_le64(FBNIC_TWD_FLAG_REQ_COMPLETION);
+	ring->deferred_meta = -1;
+
+	/* Force DMA writes to flush before writing to tail */
+	dma_wmb();
+
+	writel(ring->tail, ring->doorbell);
+}
+
+/* Packets handed to us with xmit_more set are left in the ring without a
+ * doorbell, and without a completion request, in the expectation that the
+ * packet ending the burst will ring for all of them. If that packet gets
+ * dropped instead we have to ring here, otherwise the descriptors sit in
+ * the ring until the next transmit, which may never come.
+ */
+static void fbnic_tx_flush_doorbell(struct fbnic_ring *ring)
+{
+	if (ring->deferred_meta >= 0)
+		fbnic_tx_doorbell(ring, &ring->desc[ring->deferred_meta]);
+}
+
 static bool
 fbnic_tx_map(struct fbnic_ring *ring, struct sk_buff *skb, __le64 *meta)
 {

@@ -378,14 +401,10 @@ fbnic_tx_map(struct fbnic_ring *ring, struct sk_buff *skb, __le64 *meta)
 	/* Verify there is room for another packet */
 	fbnic_maybe_stop_tx(skb->dev, ring, FBNIC_MAX_SKB_DESC);
 
-	if (fbnic_tx_sent_queue(skb, ring)) {
-		*meta |= cpu_to_le64(FBNIC_TWD_FLAG_REQ_COMPLETION);
-
-		/* Force DMA writes to flush before writing to tail */
-		dma_wmb();
-
-		writel(tail, ring->doorbell);
-	}
+	if (fbnic_tx_sent_queue(skb, ring))
+		fbnic_tx_doorbell(ring, meta);
+	else
+		ring->deferred_meta = meta - ring->desc;
 
 	return false;
 dma_error:

@@ -425,8 +444,10 @@ fbnic_xmit_frame_ring(struct sk_buff *skb, struct fbnic_ring *ring)
 	 * otherwise try next time
 	 */
 	desc_needed = skb_shinfo(skb)->nr_frags + 10;
-	if (fbnic_maybe_stop_tx(skb->dev, ring, desc_needed))
+	if (fbnic_maybe_stop_tx(skb->dev, ring, desc_needed)) {
+		fbnic_tx_flush_doorbell(ring);
 		return NETDEV_TX_BUSY;
+	}
 
 	*meta = cpu_to_le64(FBNIC_TWD_FLAG_DEST_MAC);
 

@@ -447,6 +468,8 @@ fbnic_xmit_frame_ring(struct sk_buff *skb, struct fbnic_ring *ring)
 err_free:
 	dev_kfree_skb_any(skb);
 err_count:
+	fbnic_tx_flush_doorbell(ring);
+
 	u64_stats_update_begin(&ring->stats.syncp);
 	ring->stats.dropped++;
 	u64_stats_update_end(&ring->stats.syncp);

@@ -2491,6 +2514,7 @@ static void fbnic_enable_twq0(struct fbnic_ring *twq)
 	fbnic_ring_wr32(twq, FBNIC_QUEUE_TWQ0_CTL, FBNIC_QUEUE_TWQ_CTL_RESET);
 	twq->tail = 0;
 	twq->head = 0;
+	twq->deferred_meta = -1;
 
 	/* Store descriptor ring address and size */
 	fbnic_ring_wr32(twq, FBNIC_QUEUE_TWQ0_BAL, lower_32_bits(twq->dma));