net: airoha: Take into account out-of-order tx completions in airoha_dev_xmit()

Completion napi can free out-of-order tx descriptors if hw QoS is
enabled and packets with different priority are queued to same DMA ring.
Take into account possible out-of-order reports checking if the tx queue
is full using circular buffer head/tail pointer instead of the number of
queued packets.

Fixes: 23020f0493270 ("net: airoha: Introduce ethernet support for EN7581 SoC")
Suggested-by: Simon Horman <[e-mail]>
Signed-off-by: Lorenzo Bianconi <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

drivers/net/ethernet/airoha/airoha_eth.c

···
 #endif
 }
 
+static bool airoha_dev_tx_queue_busy(struct airoha_queue *q, u32 nr_frags)
+{
+	u32 tail = q->tail <= q->head ? q->tail + q->ndesc : q->tail;
+	u32 index = q->head + nr_frags;
+
+	/* completion napi can free out-of-order tx descriptors if hw QoS is
+	 * enabled and packets with different priorities are queued to the same
+	 * DMA ring. Take into account possible out-of-order reports checking
+	 * if the tx queue is full using circular buffer head/tail pointers
+	 * instead of the number of queued packets.
+	 */
+	return index >= tail;
+}
+
 static netdev_tx_t airoha_dev_xmit(struct sk_buff *skb,
 				   struct net_device *dev)
 {
···
 	txq = netdev_get_tx_queue(dev, qid);
 	nr_frags = 1 + skb_shinfo(skb)->nr_frags;
 
-	if (q->queued + nr_frags > q->ndesc) {
+	if (airoha_dev_tx_queue_busy(q, nr_frags)) {
 		/* not enough space in the queue */
 		netif_tx_stop_queue(txq);
 		spin_unlock_bh(&q->lock);