gianfar: reallocate skb when headroom is not enough for fcb

Gianfar uses a hardware header FCB for offloading.  However when used
with bridging or IP forwarding, TX skb might not have enough headroom
for the FCB.  Reallocate skb for such cases.
Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/gianfar.c

@@ -1239,10 +1239,19 @@ static int gfar_enet_open(struct net_device *dev)
 	return err;
 }
 
-static inline struct txfcb *gfar_add_fcb(struct sk_buff *skb)
+static inline struct txfcb *gfar_add_fcb(struct sk_buff **skbp)
 {
-	struct txfcb *fcb = (struct txfcb *)skb_push (skb, GMAC_FCB_LEN);
-
+	struct txfcb *fcb;
+	struct sk_buff *skb = *skbp;
+
+	if (unlikely(skb_headroom(skb) < GMAC_FCB_LEN)) {
+		struct sk_buff *old_skb = skb;
+		skb = skb_realloc_headroom(old_skb, GMAC_FCB_LEN);
+		if (!skb)
+			return NULL;
+		dev_kfree_skb_any(old_skb);
+	}
+	fcb = (struct txfcb *)skb_push(skb, GMAC_FCB_LEN);
 	cacheable_memzero(fcb, GMAC_FCB_LEN);
 
 	return fcb;
@@ -1363,18 +1372,20 @@ static int gfar_start_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	/* Set up checksumming */
 	if (CHECKSUM_PARTIAL == skb->ip_summed) {
-		fcb = gfar_add_fcb(skb);
-		lstatus |= BD_LFLAG(TXBD_TOE);
-		gfar_tx_checksum(skb, fcb);
+		fcb = gfar_add_fcb(&skb);
+		if (likely(fcb != NULL)) {
+			lstatus |= BD_LFLAG(TXBD_TOE);
+			gfar_tx_checksum(skb, fcb);
+		}
 	}
 
 	if (priv->vlgrp && vlan_tx_tag_present(skb)) {
-		if (unlikely(NULL == fcb)) {
-			fcb = gfar_add_fcb(skb);
+		if (unlikely(NULL == fcb))
+			fcb = gfar_add_fcb(&skb);
+		if (likely(fcb != NULL)) {
 			lstatus |= BD_LFLAG(TXBD_TOE);
+			gfar_tx_vlan(skb, fcb);
 		}
-
-		gfar_tx_vlan(skb, fcb);
 	}
 
 	/* setup the TxBD length and buffer pointer for the first BD */