@@ -45,6 +45,36 @@ drop, truncate, or pass up oversize pack
packets is preferred.
+Most network devices have an operational state that the device
+monitors. The Linux kernel uses the name "carrier" for this flag which
+is a historical reference to old modems. Carrier is reported to
+userspace via the IFF_RUNNING flag from SIOCGIFFLAGS ioctl.
+Carrier is controlled in the device driver
+by the functions netif_carrier_on and netif_carrier_off. These
+functions trigger the necessary netlink and userspace API changes;
+device drivers must not change netdevice->flags directly.
+The carrier defaults to ON when the device is created and registered.
+Simple devices (such as dummy) do not need to do anything.
+Ethernet style devices should:
+ * alloc_etherdev in probe routine
+ * call netif_carrier_off
+ * register network device
+ * start auto negotiation with phy in open routine
+ * call netif_carrier_on when link is up
+More complex RFC2863 style operational state is also possible
+but not required (see operstates.txt).
+The monitoring of link state is the responsibility of the network
+device driver. It can be done by polling, interrupt, or any other
+mechanism. netif_carrier_on/netif_carrier_off are atomic and can
+safely be called by an interrupt routine. Carrier events are
+managed by the linkwatch work queue and limited to one per second
+to avoid overwhelming management applications.
struct net_device synchronization rules