[2/3] net: Use skb_checksum_start_offset()

diff --git a/drivers/net/atlx/atl1.c b/drivers/net/atlx/atl1.c
index 5336310..def8df8 100644
--- a/drivers/net/atlx/atl1.c
+++ b/drivers/net/atlx/atl1.c
@@ -2174,7 +2174,7 @@  static int atl1_tx_csum(struct atl1_adapter *adapter, struct sk_buff *skb,
 	u8 css, cso;
 
 	if (likely(skb->ip_summed == CHECKSUM_PARTIAL)) {
-		css = (u8) (skb->csum_start - skb_headroom(skb));
+		css = skb_checksum_start_offset(skb);
 		cso = css + (u8) skb->csum_offset;
 		if (unlikely(css & 0x1)) {
 			/* L1 hardware requires an even number here */
diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
index 4256727..21845af 100644
--- a/drivers/net/macvtap.c
+++ b/drivers/net/macvtap.c
@@ -504,8 +504,7 @@  static int macvtap_skb_to_vnet_hdr(const struct sk_buff *skb,
 
 	if (skb->ip_summed == CHECKSUM_PARTIAL) {
 		vnet_hdr->flags = VIRTIO_NET_HDR_F_NEEDS_CSUM;
-		vnet_hdr->csum_start = skb->csum_start -
-					skb_headroom(skb);
+		vnet_hdr->csum_start = skb_checksum_start_offset(skb);
 		vnet_hdr->csum_offset = skb->csum_offset;
 	} /* else everything is zero */
 
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 55f3a3e..7599c45 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -757,7 +757,7 @@  static __inline__ ssize_t tun_put_user(struct tun_struct *tun,
 
 		if (skb->ip_summed == CHECKSUM_PARTIAL) {
 			gso.flags = VIRTIO_NET_HDR_F_NEEDS_CSUM;
-			gso.csum_start = skb->csum_start - skb_headroom(skb);
+			gso.csum_start = skb_checksum_start_offset(skb);
 			gso.csum_offset = skb->csum_offset;
 		} /* else everything is zero */
 
diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
index 65cb1ab..bc86f4b 100644
--- a/drivers/net/usb/smsc95xx.c
+++ b/drivers/net/usb/smsc95xx.c
@@ -1163,9 +1163,8 @@  static int smsc95xx_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
 
 static u32 smsc95xx_calc_csum_preamble(struct sk_buff *skb)
 {
-	int len = skb->data - skb->head;
-	u16 high_16 = (u16)(skb->csum_offset + skb->csum_start - len);
-	u16 low_16 = (u16)(skb->csum_start - len);
+	u16 low_16 = (u16)skb_checksum_start_offset(skb);
+	u16 high_16 = low_16 + skb->csum_offset;
 	return (high_16 << 16) | low_16;
 }
 
@@ -1193,7 +1192,7 @@  static struct sk_buff *smsc95xx_tx_fixup(struct usbnet *dev,
 		if (skb->len <= 45) {
 			/* workaround - hardware tx checksum does not work
 			 * properly with extremely small packets */
-			long csstart = skb->csum_start - skb_headroom(skb);
+			long csstart = skb_checksum_start_offset(skb);
 			__wsum calc = csum_partial(skb->data + csstart,
 				skb->len - csstart, 0);
 			*((__sum16 *)(skb->data + csstart
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index b6d4028..90a23e4 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -519,7 +519,7 @@  static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
 
 	if (skb->ip_summed == CHECKSUM_PARTIAL) {
 		hdr->hdr.flags = VIRTIO_NET_HDR_F_NEEDS_CSUM;
-		hdr->hdr.csum_start = skb->csum_start - skb_headroom(skb);
+		hdr->hdr.csum_start = skb_checksum_start_offset(skb);
 		hdr->hdr.csum_offset = skb->csum_offset;
 	} else {
 		hdr->hdr.flags = 0;
diff --git a/net/core/dev.c b/net/core/dev.c
index d28b3a0..f937726 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1761,7 +1761,7 @@  int skb_checksum_help(struct sk_buff *skb)
 		goto out_set_summed;
 	}
 
-	offset = skb->csum_start - skb_headroom(skb);
+	offset = skb_checksum_start_offset(skb);
 	BUG_ON(offset >= skb_headlen(skb));
 	csum = skb_checksum(skb, offset, skb->len - offset, 0);
 
@@ -2058,8 +2058,8 @@  int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
 			 * checksumming here.
 			 */
 			if (skb->ip_summed == CHECKSUM_PARTIAL) {
-				skb_set_transport_header(skb, skb->csum_start -
-					      skb_headroom(skb));
+				skb_set_transport_header(skb,
+					skb_checksum_start_offset(skb));
 				if (!dev_can_checksum(dev, skb) &&
 				     skb_checksum_help(skb))
 					goto out_kfree_skb;
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 8814a9a..19d6c21 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1824,7 +1824,7 @@  void skb_copy_and_csum_dev(const struct sk_buff *skb, u8 *to)
 	long csstart;
 
 	if (skb->ip_summed == CHECKSUM_PARTIAL)
-		csstart = skb->csum_start - skb_headroom(skb);
+		csstart = skb_checksum_start_offset(skb);
 	else
 		csstart = skb_headlen(skb);
 
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index b37181d..1198adf 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -2226,7 +2226,7 @@  struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb, int features)
 	/* Do software UFO. Complete and fill in the UDP checksum as HW cannot
 	 * do checksum of UDP packets sent as multiple IP fragments.
 	 */
-	offset = skb->csum_start - skb_headroom(skb);
+	offset = skb_checksum_start_offset(skb);
 	csum = skb_checksum(skb, offset, skb->len - offset, 0);
 	offset += skb->csum_offset;
 	*(__sum16 *)(skb->data + offset) = csum_fold(csum);
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index e79efaf..91cb1d7 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1650,8 +1650,7 @@  static int packet_recvmsg(struct kiocb *iocb, struct socket *sock,
 
 		if (skb->ip_summed == CHECKSUM_PARTIAL) {
 			vnet_hdr.flags = VIRTIO_NET_HDR_F_NEEDS_CSUM;
-			vnet_hdr.csum_start = skb->csum_start -
-							skb_headroom(skb);
+			vnet_hdr.csum_start = skb_checksum_start_offset(skb);
 			vnet_hdr.csum_offset = skb->csum_offset;
 		} /* else everything is zero */