-#define NET_RX_SUCCESS 0 /* keep 'em coming, baby */
-#define NET_RX_DROP 1 /* packet dropped */
+#define NET_RX_SUCCESS 0 /* keep 'em coming, baby */
+#define NET_RX_DROP 1 /* packet dropped */
+
+/*
+ * Transmit return codes: transmit return codes originate from three different
+ * namespaces:
+ *
+ * - qdisc return codes
+ * - driver transmit return codes
+ * - errno values
+ *
+ * Drivers are allowed to return any one of those in their hard_start_xmit()
+ * function. Real network devices commonly used with qdiscs should only return
+ * the driver transmit return codes though - when qdiscs are used, the actual
+ * transmission happens asynchronously, so the value is not propagated to
+ * higher layers. Virtual network devices transmit synchronously, in this case
+ * the driver transmit return codes are consumed by dev_queue_xmit(), all
+ * others are propagated to higher layers.
+ */
+
+/* qdisc ->enqueue() return codes. */
+#define NET_XMIT_SUCCESS 0x00
+#define NET_XMIT_DROP 0x01 /* skb dropped */
+#define NET_XMIT_CN 0x02 /* congestion notification */
+#define NET_XMIT_POLICED 0x03 /* skb is shot by police */
+#define NET_XMIT_MASK 0x0f /* qdisc flags in net/sch_generic.h */