linux/ena: Fix possible stuck tx packets when a tx drop occurs

This fix was suggested in [1].

In case there is a tx packet burst, where N packets are to be sent,
if the last packet in the burst is dropped in ena_start_xmit(),
the doorbell is not sent to the device for this burst.

In such a case, the first N-1 packets in the burst will not be sent
until the next time a doorbell is sent.

This commit fixes this issue by sending a doorbell after a dropped
packet if it is the last in a tx burst !netdev_xmit_more() and also
after checking that indeed there are descriptors sent to the device.

[1]: https://lore.kernel.org/netdev/CANn89iJRHuFgPrsezhm2DuAw7JmLL2ZkPhZaf2Ymuq+STUm-8w@mail.gmail.com/

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>

kernel/linux/common/ena_com/ena_eth_com.h

@@ -71,15 +71,14 @@ static inline void ena_com_unmask_intr(struct ena_com_io_cq *io_cq,
 	writel(intr_reg->intr_control, io_cq->unmask_reg);
 }
 
-static inline int ena_com_free_q_entries(struct ena_com_io_sq *io_sq)
+static inline u16 ena_com_used_q_entries(struct ena_com_io_sq *io_sq)
 {
-	u16 tail, next_to_comp, cnt;
-
-	next_to_comp = io_sq->next_to_comp;
-	tail = io_sq->tail;
-	cnt = tail - next_to_comp;
+	return io_sq->tail - io_sq->next_to_comp;
+}
 
-	return io_sq->q_depth - 1 - cnt;
+static inline int ena_com_free_q_entries(struct ena_com_io_sq *io_sq)
+{
+	return io_sq->q_depth - 1 - ena_com_used_q_entries(io_sq);
 }
 
 /* Check if the submission queue has enough space to hold required_buffers */

kernel/linux/ena/ena_netdev.c

@@ -3087,6 +3087,13 @@ static netdev_tx_t ena_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	tx_info->skb = NULL;
 
 error_drop_packet:
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(3, 18, 0)
+	if (!netdev_xmit_more() && ena_com_used_q_entries(tx_ring->ena_com_io_sq))
+#else
+	if (ena_com_used_q_entries(tx_ring->ena_com_io_sq))
+#endif
+		ena_ring_tx_doorbell(tx_ring);
+
 	dev_kfree_skb(skb);
 	return NETDEV_TX_OK;
 }