cns3xxx: ethernet - clean up tx descs only when needed
cns3xxx: ethernet - clean up tx descs only when needed

We already clean up tx descriptors in the napi eth_poll() function so it
would likely be rare to run out of available descriptors in eth_xmit. Thus
we can clean them up only when needed and return busy only when we
still don't have enough.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39762 3c298f89-4303-0410-b956-a3cf2f4a3e73

--- a/target/linux/cns3xxx/files/drivers/net/ethernet/cavium/cns3xxx_eth.c
+++ b/target/linux/cns3xxx/files/drivers/net/ethernet/cavium/cns3xxx_eth.c
@@ -583,7 +583,7 @@
 	}
 }
 
-static void eth_complete_tx(struct sw *sw)
+static int eth_complete_tx(struct sw *sw)
 {
 	struct _tx_ring *tx_ring = &sw->tx_ring;
 	struct tx_desc *desc;
@@ -615,6 +615,8 @@
 	tx_ring->free_index = index;
 	tx_ring->num_used -= i;
 	eth_check_num_used(tx_ring);
+
+	return TX_DESCS - tx_ring->num_used;
 }
 
 static int eth_poll(struct napi_struct *napi, int budget)
@@ -776,11 +778,13 @@
 	skb_walk_frags(skb, skb1)
 		nr_desc++;
 
-	eth_schedule_poll(sw);
 	spin_lock_bh(&tx_lock);
 	if ((tx_ring->num_used + nr_desc + 1) >= TX_DESCS) {
-		spin_unlock_bh(&tx_lock);
-		return NETDEV_TX_BUSY;
+		/* clean up tx descriptors when needed */
+		if (eth_complete_tx(sw) < nr_desc) {
+			spin_unlock_bh(&tx_lock);
+			return NETDEV_TX_BUSY;
+		}
 	}
 
 	index = index0 = tx_ring->cur_index;

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