[netdevice] Add vlan_tag() to get the VLAN tag of a network device

The iBFT has a VLAN field that should be filled in.  Add the
vlan_tag() function to extract the VLAN tag of a network device.

Since VLAN support is optional, define a weak function that returns 0
when iPXE is built without VLAN support.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/vlan.h

 
 extern struct net_device * vlan_find ( struct net_device *trunk,
 				       unsigned int tag );
+extern unsigned int vlan_tag ( struct net_device *netdev );
 extern int vlan_can_be_trunk ( struct net_device *trunk );
 extern int vlan_create ( struct net_device *trunk, unsigned int tag,
 			 unsigned int priority );

src/net/netdevice.c

 #include <ipxe/init.h>
 #include <ipxe/device.h>
 #include <ipxe/errortab.h>
+#include <ipxe/vlan.h>
 #include <ipxe/netdevice.h>
 
 /** @file
 	net_poll();
 }
 
+/**
+ * Get the VLAN tag (when VLAN support is not present)
+ *
+ * @v netdev		Network device
+ * @ret tag		0, indicating that device is not a VLAN device
+ */
+__weak unsigned int vlan_tag ( struct net_device *netdev __unused ) {
+	return 0;
+}
+
 /** Networking stack process */
 PERMANENT_PROCESS ( net_process, net_step );

src/net/vlan.c

 	.rx = vlan_rx,
 };
 
+/**
+ * Get the VLAN tag
+ *
+ * @v netdev		Network device
+ * @ret tag		VLAN tag, or 0 if device is not a VLAN device
+ */
+unsigned int vlan_tag ( struct net_device *netdev ) {
+	struct vlan_device *vlan;
+
+	if ( netdev->op == &vlan_operations ) {
+		vlan = netdev->priv;
+		return vlan->tag;
+	} else {
+		return 0;
+	}
+}
+
 /**
  * Check if network device can be used as a VLAN trunk device
  *