ieee1394: eth1394: allow MTU bigger than 1500

RFC 2734 says: "IP-capable nodes may operate with an MTU size larger
than the default [1500 octets], but the means by which a larger MTU is
configured are beyond the scope of this document."

Allow users to set an MTU bigger than 1500.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/ieee1394/eth1394.c

@@ -136,9 +136,6 @@ static const int hdr_type_len[] = {
 	sizeof(struct eth1394_sf_hdr)
 };
 
-/* For now, this needs to be 1500, so that XP works with us */
-#define ETH1394_DATA_LEN	ETH_DATA_LEN
-
 static const u16 eth1394_speedto_maxpayload[] = {
 /*     S100, S200, S400, S800, S1600, S3200 */
 	512, 1024, 2048, 4096,  4096,  4096
@@ -262,17 +259,27 @@ static void ether1394_tx_timeout(struct net_device *dev)
 	ether1394_host_reset(host);
 }
 
+static inline int ether1394_max_mtu(struct hpsb_host* host)
+{
+	return (1 << (host->csr.max_rec + 1))
+			- sizeof(union eth1394_hdr) - ETHER1394_GASP_OVERHEAD;
+}
+
 static int ether1394_change_mtu(struct net_device *dev, int new_mtu)
 {
-	int max_rec =
-		((struct eth1394_priv *)netdev_priv(dev))->host->csr.max_rec;
+	int max_mtu;
 
-	if (new_mtu < 68 ||
-	    new_mtu > ETH1394_DATA_LEN ||
-	    new_mtu > (1 << (max_rec + 1)) - sizeof(union eth1394_hdr) -
-		      ETHER1394_GASP_OVERHEAD)
+	if (new_mtu < 68)
 		return -EINVAL;
 
+	max_mtu = ether1394_max_mtu(
+			((struct eth1394_priv *)netdev_priv(dev))->host);
+	if (new_mtu > max_mtu) {
+		ETH1394_PRINT(KERN_INFO, dev->name,
+			      "Local node constrains MTU to %d\n", max_mtu);
+		return -ERANGE;
+	}
+
 	dev->mtu = new_mtu;
 	return 0;
 }
@@ -476,13 +483,10 @@ static void ether1394_reset_priv(struct net_device *dev, int set_mtu)
 			max_speed = host->speed[i];
 	priv->bc_sspd = max_speed;
 
-	/* We'll use our maximum payload as the default MTU */
 	if (set_mtu) {
-		int max_payload = 1 << (host->csr.max_rec + 1);
-
-		dev->mtu = min(ETH1394_DATA_LEN,
-			       (int)(max_payload - sizeof(union eth1394_hdr) -
-				     ETHER1394_GASP_OVERHEAD));
+		/* Use the RFC 2734 default 1500 octets or the maximum payload
+		 * as initial MTU */
+		dev->mtu = min(1500, ether1394_max_mtu(host));
 
 		/* Set our hardware address while we're at it */
 		memcpy(dev->dev_addr, &guid, sizeof(u64));