Understanding the Linux Kernel, 3rd Edition

Character Device Drivers

Handling a character device is relatively easy, because usually sophisticated buffering strategies are not needed and disk caches are not involved. Of course, character devices differ in their requirements: some of them must implement a sophisticated communication protocol to drive the hardware device, while others just have to read a few values from a couple of I/O ports of the hardware devices. For instance, the device driver of a multiport serial card device (a hardware device offering many serial ports) is much more complicated than the device driver of a bus mouse.

Block device drivers, on the other hand, are inherently more complex than character device drivers . In fact, applications are entitled to ask repeatedly to read or write the same block of data. Furthermore, accesses to these devices are usually very slow. These peculiarities have a profound impact on the structure of the disk drivers. As we ‘ll see in the next chapters, however, the kernel provides sophisticated components—such as the page cache and the block I/O subsystem—to handle them. In the rest of this chapter we focus our attention on the character device drivers.

A character device driver is described by a cdev structure, whose fields are listed in Table 13-8.

Table 13-8. The fields of the cdev structure

Type	Field	Description
struct kobject	kobj	Embedded kobject
struct module *	owner	Pointer to the module implementing the driver, if any
struct file_operations *	ops	Pointer ...