[netdevice] Allow non-completion TX errors to be recorded

Allow TX errors to be recorded against a network device even when the
packet didn't make it as far as netdev_tx().

Inspired-by: Dominik Russenberger <dominik.russenberger@terreactive.ch>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/netdevice.h

 extern void netdev_link_err ( struct net_device *netdev, int rc );
 extern void netdev_link_down ( struct net_device *netdev );
 extern int netdev_tx ( struct net_device *netdev, struct io_buffer *iobuf );
+extern void netdev_tx_err ( struct net_device *netdev,
+			    struct io_buffer *iobuf, int rc );
 extern void netdev_tx_complete_err ( struct net_device *netdev,
 				 struct io_buffer *iobuf, int rc );
 extern void netdev_tx_complete_next_err ( struct net_device *netdev, int rc );

src/net/netdevice.c

 }
 
 /**
- * Complete network transmission
+ * Discard transmitted packet
  *
  * @v netdev		Network device
- * @v iobuf		I/O buffer
+ * @v iobuf		I/O buffer, or NULL
  * @v rc		Packet status code
  *
- * The packet must currently be in the network device's TX queue.
+ * The packet is discarded and a TX error is recorded.  This function
+ * takes ownership of the I/O buffer.
  */
-void netdev_tx_complete_err ( struct net_device *netdev,
-			      struct io_buffer *iobuf, int rc ) {
+void netdev_tx_err ( struct net_device *netdev,
+		     struct io_buffer *iobuf, int rc ) {
 
 	/* Update statistics counter */
 	netdev_record_stat ( &netdev->tx_stats, rc );
 		       netdev->name, iobuf, strerror ( rc ) );
 	}
 
+	/* Discard packet */
+	free_iob ( iobuf );
+}
+
+/**
+ * Complete network transmission
+ *
+ * @v netdev		Network device
+ * @v iobuf		I/O buffer
+ * @v rc		Packet status code
+ *
+ * The packet must currently be in the network device's TX queue.
+ */
+void netdev_tx_complete_err ( struct net_device *netdev,
+			      struct io_buffer *iobuf, int rc ) {
+
 	/* Catch data corruption as early as possible */
 	list_check_contains ( iobuf, &netdev->tx_queue, list );
 
 	/* Dequeue and free I/O buffer */
 	list_del ( &iobuf->list );
-	free_iob ( iobuf );
+	netdev_tx_err ( netdev, iobuf, rc );
 }
 
 /**
 	/* Add link-layer header */
 	if ( ( rc = ll_protocol->push ( netdev, iobuf, ll_dest, ll_source,
 					net_protocol->net_proto ) ) != 0 ) {
-		free_iob ( iobuf );
+		/* Record error for diagnosis */
+		netdev_tx_err ( netdev, iobuf, rc );
 		return rc;
 	}