Linux Device Drivers, Second Edition

Opening and Closing

Our driver can probe for the interface at module load time or at kernel boot. Before the interface can carry packets, however, the kernel must open it and assign an address to it. The kernel will open or close an interface in response to the ifconfig command.

When ifconfig is used to assign an address to the interface, it performs two tasks. First, it assigns the address by means of ioctl(SIOCSIFADDR) (Socket I/O Control Set Interface Address). Then it sets the IFF_UP bit in dev->flag by means of ioctl(SIOCSIFFLAGS) (Socket I/O Control Set Interface Flags) to turn the interface on.

As far as the device is concerned, ioctl(SIOCSIFADDR) does nothing. No driver function is invoked—the task is device independent, and the kernel performs it. The latter command (ioctl(SIOCSIFFLAGS)), though, calls the open method for the device.

Similarly, when the interface is shut down, ifconfig uses ioctl(SIOCSIFFLAGS) to clear IFF_UP, and the stop method is called.

Both device methods return 0 in case of success and the usual negative value in case of error.

As far as the actual code is concerned, the driver has to perform many of the same tasks as the char and block drivers do. open requests any system resources it needs and tells the interface to come up; stop shuts down the interface and releases system resources. There are a couple of additional steps to be performed, however.

First, the hardware address needs to be copied from the hardware device to dev->dev_addr ...