[netdevice] Add maximum packet length as a net device property

Currently this length is set at device allocation time, and is never
changed.

src/include/gpxe/ethernet.h

@@ -9,6 +9,7 @@
 
 #include <stdint.h>
 #include <gpxe/netdevice.h>
+#include <gpxe/if_ether.h>
 
 extern struct ll_protocol ethernet_protocol;
 
@@ -26,6 +27,7 @@ static inline struct net_device * alloc_etherdev ( size_t priv_size ) {
 	netdev = alloc_netdev ( priv_size );
 	if ( netdev ) {
 		netdev->ll_protocol = &ethernet_protocol;
+		netdev->max_pkt_len = ETH_FRAME_LEN;
 	}
 	return netdev;
 }

src/include/gpxe/ipoib.h

@@ -8,6 +8,9 @@
 
 #include <gpxe/infiniband.h>
 
+/** IPoIB packet length */
+#define IPOIB_PKT_LEN 2048
+
 /** IPoIB MAC address length */
 #define IPOIB_ALEN 20
 
@@ -68,6 +71,7 @@ static inline struct net_device * alloc_ipoibdev ( size_t priv_size ) {
 	netdev = alloc_netdev ( priv_size );
 	if ( netdev ) {
 		netdev->ll_protocol = &ipoib_protocol;
+		netdev->max_pkt_len = IPOIB_PKT_LEN;
 	}
 	return netdev;
 }

src/include/gpxe/netdevice.h

@@ -241,6 +241,11 @@ struct net_device {
 	 * This is the bitwise-OR of zero or more NETDEV_XXX constants.
 	 */
 	unsigned int state;
+	/** Maximum packet length
+	 *
+	 * This length includes any link-layer headers.
+	 */
+	size_t max_pkt_len;
 	/** TX packet queue */
 	struct list_head tx_queue;
 	/** RX packet queue */