Understanding the Linux Kernel, Second Edition

Device Drivers

A device driver is a software layer that makes a hardware device respond to a well-defined programming interface. We are already familiar with this kind of interface; it consists of the canonical set of VFS functions (open, read, lseek, ioctl, and so forth) that control a device. The actual implementation of all these functions is delegated to the device driver. Since each device has a unique I/O controller, and thus unique commands and unique state information, most I/O devices have their own drivers.

There are many types of device drivers. They mainly differ in the level of support that they offer to the User Mode applications, as well as in their buffering strategies for the data collected from the hardware devices. Since these choices greatly influence the internal structure of a device driver, we discuss them in Section 13.3.1 and Section 13.3.2.

A device driver does not consist only of the functions that implement the device file operations. Before using a device driver, two activities must have taken place: registering the device driver and initializing it. Finally, when the device driver is performing a data transfer, it must also monitor the I/O operation. We see how all this is done in Section 13.3.3, Section 13.3.4, and Section 13.3.5.

Levels of Kernel Support

The Linux kernel does not fully support all possible existing I/O devices. Generally speaking, in fact, there are three possible kinds of support for a hardware device:

No support at all

The application ...