net: e1000: support txtd update delay via xmit_more

Don't update Tx tail descriptor if we queue hasn't been stopped and
we know at least one more skb will be sent right away.
Signed-off-by: Florian Westphal <fw@strlen.de>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/e1000/e1000_main.c

@@ -2977,7 +2977,6 @@ static void e1000_tx_queue(struct e1000_adapter *adapter,
 			   struct e1000_tx_ring *tx_ring, int tx_flags,
 			   int count)
 {
-	struct e1000_hw *hw = &adapter->hw;
 	struct e1000_tx_desc *tx_desc = NULL;
 	struct e1000_tx_buffer *buffer_info;
 	u32 txd_upper = 0, txd_lower = E1000_TXD_CMD_IFCS;
@@ -3031,11 +3030,6 @@ static void e1000_tx_queue(struct e1000_adapter *adapter,
 	wmb();
 
 	tx_ring->next_to_use = i;
-	writel(i, hw->hw_addr + tx_ring->tdt);
-	/* we need this if more than one processor can write to our tail
-	 * at a time, it synchronizes IO on IA64/Altix systems
-	 */
-	mmiowb();
 }
 
 /* 82547 workaround to avoid controller hang in half-duplex environment.
@@ -3264,6 +3258,15 @@ static netdev_tx_t e1000_xmit_frame(struct sk_buff *skb,
 		/* Make sure there is space in the ring for the next send. */
 		e1000_maybe_stop_tx(netdev, tx_ring, MAX_SKB_FRAGS + 2);
 
+		if (!skb->xmit_more ||
+		    netif_xmit_stopped(netdev_get_tx_queue(netdev, 0))) {
+			writel(tx_ring->next_to_use, hw->hw_addr + tx_ring->tdt);
+			/* we need this if more than one processor can write to
+			 * our tail at a time, it synchronizes IO on IA64/Altix
+			 * systems
+			 */
+			mmiowb();
+		}
 	} else {
 		dev_kfree_skb_any(skb);
 		tx_ring->buffer_info[first].time_stamp = 0;