+ * if no packet is ready to be received.
+ *
+ * May return -EOPNOTSUPP if a network device does not implement packet
+ * reception through this interface. This function may be set to null if
+ * it would always return -EOPNOTSUPP anyhow. (This will disable the OVS
+ * integrated DHCP client and OpenFlow controller discovery, and prevent
+ * the network device from being usefully used by the netdev-based
+ * "userspace datapath".) */