ixgb: Fix early TSO completion

This fix was already merged in commit 96f9c2e2
but reverted in commit 989316dd. After
stresstesting we found that the fix does not add new regressions and
works around a TX hang spotted by several users.
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>

drivers/net/ixgb/ixgb_main.c

@@ -1287,6 +1287,7 @@ ixgb_tx_map(struct ixgb_adapter *adapter, struct sk_buff *skb,
 	struct ixgb_buffer *buffer_info;
 	int len = skb->len;
 	unsigned int offset = 0, size, count = 0, i;
+	unsigned int mss = skb_shinfo(skb)->gso_size;
 
 	unsigned int nr_frags = skb_shinfo(skb)->nr_frags;
 	unsigned int f;
@@ -1298,6 +1299,11 @@ ixgb_tx_map(struct ixgb_adapter *adapter, struct sk_buff *skb,
 	while(len) {
 		buffer_info = &tx_ring->buffer_info[i];
 		size = min(len, IXGB_MAX_DATA_PER_TXD);
+		/* Workaround for premature desc write-backs
+		 * in TSO mode.  Append 4-byte sentinel desc */
+		if (unlikely(mss && !nr_frags && size == len && size > 8))
+			size -= 4;
+
 		buffer_info->length = size;
 		WARN_ON(buffer_info->dma != 0);
 		buffer_info->dma =
@@ -1324,6 +1330,13 @@ ixgb_tx_map(struct ixgb_adapter *adapter, struct sk_buff *skb,
 		while(len) {
 			buffer_info = &tx_ring->buffer_info[i];
 			size = min(len, IXGB_MAX_DATA_PER_TXD);
+
+			/* Workaround for premature desc write-backs
+			 * in TSO mode.  Append 4-byte sentinel desc */
+			if (unlikely(mss && !nr_frags && size == len
+			             && size > 8))
+				size -= 4;
+
 			buffer_info->length = size;
 			buffer_info->dma =
 				pci_map_page(adapter->pdev,
@@ -1401,8 +1414,9 @@ ixgb_tx_queue(struct ixgb_adapter *adapter, int count, int vlan_id,int tx_flags)
 /* Tx Descriptors needed, worst case */
 #define TXD_USE_COUNT(S) (((S) >> IXGB_MAX_TXD_PWR) + \
 			 (((S) & (IXGB_MAX_DATA_PER_TXD - 1)) ? 1 : 0))
-#define DESC_NEEDED TXD_USE_COUNT(IXGB_MAX_DATA_PER_TXD) + \
-	MAX_SKB_FRAGS * TXD_USE_COUNT(PAGE_SIZE) + 1
+#define DESC_NEEDED TXD_USE_COUNT(IXGB_MAX_DATA_PER_TXD) /* skb->date */ + \
+	MAX_SKB_FRAGS * TXD_USE_COUNT(PAGE_SIZE) + 1 /* for context */ \
+	+ 1 /* one more needed for sentinel TSO workaround */
 
 static int
 ixgb_xmit_frame(struct sk_buff *skb, struct net_device *netdev)