Linux Device Drivers, Second Edition

Changes in Link State

Network connections, by definition, deal with the world outside the local system. They are thus often affected by outside events, and they can be transient things. The networking subsystem needs to know when network links go up or down, and it provides a few functions that the driver may use to convey that information.

Most networking technologies involving an actual, physical connection provide a carrier state; the presence of the carrier means that the hardware is present and ready to function. Ethernet adapters, for example, sense the carrier signal on the wire; when a user trips over the cable, that carrier vanishes, and the link goes down. By default, network devices are assumed to have a carrier signal present. The driver can change that state explicitly, however, with these functions:

void netif_carrier_off(struct net_device *dev);
void netif_carrier_on(struct net_device *dev);

If your driver detects a lack of carrier on one of its devices, it should call netif_carrier_off to inform the kernel of this change. When the carrier returns, netif_carrier_on should be called. Some drivers also call netif_carrier_off when making major configuration changes (such as media type); once the adapter has finished resetting itself, the new carrier will be detected and traffic can resume.

An integer function also exsists:

int netif_carrier_ok(struct net_device *dev);

This can be used to test the current carrier state (as reflected in the device structure). ...