Linux Device Drivers, Second Edition

Chapter 12. Loading Block Drivers

Our discussion thus far has been limited to char drivers. As we have already mentioned, however, char drivers are not the only type of driver used in Linux systems. Here we turn our attention to block drivers. Block drivers provide access to block-oriented devices—those that transfer data in randomly accessible, fixed-size blocks. The classic block device is a disk drive, though others exist as well.

The char driver interface is relatively clean and easy to use; the block interface, unfortunately, is a little messier. Kernel developers like to complain about it. There are two reasons for this state of affairs. The first is simple history—the block interface has been at the core of every version of Linux since the first, and it has proved hard to change. The other reason is performance. A slow char driver is an undesirable thing, but a slow block driver is a drag on the entire system. As a result, the design of the block interface has often been influenced by the need for speed.

The block driver interface has evolved significantly over time. As with the rest of the book, we cover the 2.4 interface in this chapter, with a discussion of the changes at the end. The example drivers work on all kernels between 2.0 and 2.4, however.

This chapter explores the creation of block drivers with two new example drivers. The first, sbull (Simple Block Utility for Loading Localities) implements a block device using system memory—a RAM-disk driver, essentially. ...