Linux Device Drivers

Partitionable Devices

If you try to create partitions with fdisk, you’ll find out that there’s something wrong with them. The fdisk program calls the partitions /dev/sbull01, /dev/sbull02, and so on, but those names don’t exist on the filesystem. Indeed, the base sbull device is a byte array with no entry points to provide access to subregions of the data area, so partitioning sbull doesn’t work.

In order to be able to partition a device, we must assign several minor numbers to each physical device. One number is used to access the whole device (for example, /dev/hda), and the others are used to access the various partitions (such as /dev/hda1). Since fdisk creates partition names by adding a numerical suffix to the whole-disk device name, we’ll follow the same naming convention in our next block driver.

The device I’m going to introduce in this section is called spull, because it is a ``Simple Partitionable Utility.'' The device resides in the spull directory and is completely detached from sbull, even though they share a lot of code.

In the char driver scull, different minor numbers were able to implement different behaviors, so that a single driver could show several different implementations. Differentiating according to the minor number is not possible with block devices, and that’s why sbull and spull are kept separate. The inability to differentiate devices according to the minor number is a basic feature of block drivers, as several of the data structures and macros ...