Device Drivers

If you choose to write a device driver, you must take everything written here as a guide, and no more. I cannot guarantee that this chapter will be free of errors, and I cannot guarantee that you will not damage your computer, even if you follow these instructions exactly. It is highly unlikely that you will damage it, but I cannot guarantee against it. There is only one “infallible” direction I can give you: Back up! Back up before you test your new device driver, or you may regret it later.

What is a Device Driver?
What is this “device driver” stuff anyway? Here’s a very short introduction to the concept.
User-space device drivers
It’s not always necessary to write a “real” device driver. Sometimes you just need to know how to write code that runs as a normal user process and still accesses hardware.
Device Driver Basics
Assuming that you need to write a “real” device driver, there are some things that you need to know regardless of what type of driver you are writing. In fact, you may need to learn what type of driver you ought to write…
Character Device Drivers
This section includes details specific to character device drivers, and assumes that you know everything in the previous section.
TTY drivers
This section hasn’t been written yet. TTY drivers are character devices that interface with the kernel’s generic TTY support, and they require more than just a standard character device interface. I’d appreciate it if someone would write up how to attach a character device driver to the generic TTY layer and submit it to me for inclusion in this guide.
Block Device Drivers
This section includes details specific to block device drivers (suprise!)
Writing a SCSI Device Driver
This is a technical paper written by Rik Faith at the University of North Carolina.
Network Device Drivers
Alan Cox gives an introduction to the network layer, including device drivers.
Supporting Functions
Many functions are useful to all sorts of drivers. Here is a summary of quite a few of them.
Translating Addresses in Kernel Space
An edited version of a post of Linus Torvalds to the linux-kernel mailing list about how to correctly deal with translating memory references when writing kernel source code such as device drivers.
Kernel-Level Exception Handling
An edited version of a post of Joerg Pommnitz to the linux-kernel mailing list about how the new exception mechanism works.

intoduction of divice driver:-

One of the many advantages of free operating systems, as typified by Linux, is that their internals are open for all to view. The operating system, once a dark and mysterious area whose code was restricted to a small number of programmers, can now be readily examined, understood, and modified by anybody with the requisite skills. Linux has helped to democratize operating systems. The Linux kernel remains a large and complex body of code, however, and would-be kernel hackers need an entry point where they can approach the code without being overwhelmed by complexity. Often, device drivers provide that gateway.

Device drivers take on a special role in the Linux kernel. They are distinct “black boxes” that make a particular piece of hardware respond to a well-defined internal programming interface; they hide completely the details of how the device works. User activities are performed by means of a set of standardized calls that are independent of the specific driver; mapping those calls to device-specific operations that act on real hardware is then the role of the device driver. This programming interface is such that drivers can be built separately from the rest of the kernel and “plugged in” at runtime when needed. This modularity makes Linux drivers easy to write, to the point that there are now hundreds of them available.

There are a number of reasons to be interested in the writing of Linux device drivers. The rate at which new hardware becomes available (and obsolete!) alone guarantees that driver writers will be busy for the foreseeable future. Individuals may need to know about drivers in order to gain access to a particular device that is of interest to them. Hardware vendors, by making a Linux driver available for their products, can add the large and growing Linux user base to their potential markets. And the open source nature of the Linux system means that if the driver writer wishes, the source to a driver can be quickly disseminated to millions of users.

This book teaches you how to write your own drivers and how to hack around in related parts of the kernel. We have taken a device-independent approach; the programming techniques and interfaces are presented, whenever possible, without being tied to any specific device. Each driver is different; as a driver writer, you need to understand your specific device well. But most of the principles and basic techniques are the same for all drivers. This book cannot teach you about your device, but it gives you a handle on the background you need to make your device work.

As you learn to write drivers, you find out a lot about the Linux kernel in general; this may help you understand how your machine works and why things aren’t always as fast as you expect or don’t do quite what you want. We introduce new ideas gradually, starting off with very simple drivers and building on them; every new concept is accompanied by sample code that doesn’t need special hardware to be tested.

This chapter doesn’t actually get into writing code. However, we introduce some background concepts about the Linux kernel that you’ll be glad you know later, when we do launch into programming.