Running Linux, Fourth Edition

Loadable Device Drivers

Traditionally, device drivers have been included as part of the kernel. There are several reasons for this. First of all, nearly all device drivers require the special hardware access provided by being part of the kernel code. Such hardware access can’t be obtained easily through a user program. Also, device drivers are much easier to implement as part of the kernel; such drivers would have complete access to the data structures and other routines in the kernel and could call them freely.

A conglomerate kernel containing all drivers in this manner presents several problems. First of all, it requires the system administrator to rebuild the kernel in order to selectively include device drivers, as we saw in the previous section. Also, this mechanism lends itself to sloppy programming on the part of the driver writers: there’s nothing stopping a programmer from writing code that is not completely modular — code which, for example, directly accesses data private to other parts of the kernel. The cooperative nature of the Linux kernel development compounds this problem, and not all parts of the code are as neatly contained as they should be. This can make it more difficult to maintain and debug the code.

In an effort to move away from this paradigm, the Linux kernel supports loadable device drivers — device drivers that are added to or removed from memory at runtime, with a series of commands. Such drivers are still part of the kernel, but they are compiled ...