As the popularity of the Linux system continues to grow, the interest in writing Linux device drivers steadily increases. Most of Linux is independent of the hardware it runs on, and most users can be (happily) unaware of hardware issues. But, for each piece of hardware supported by Linux, somebody somewhere has written a driver to make it work with the system. Without device drivers, there is no functioning system.
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 will teach 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 will 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 will give you a handle on the background you need to make your device work.
As you learn to write drivers, you will 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’ll introduce new ideas gradually, starting off with very simple drivers and building upon them; every new concept will be 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.
As a programmer, you will be able to make your own choices about your driver, choosing an acceptable trade-off between the programming time required and the flexibility of the result. Though it may appear strange to say that a driver is “flexible,” we like this word because it emphasizes that the role of a device driver is providing mechanism, not policy.
The distinction between mechanism and policy is one of the best ideas behind the Unix design. Most programming problems can indeed be split into two parts: “what capabilities are to be provided” (the mechanism) and “how those capabilities can be used” (the policy). If the two issues are addressed by different parts of the program, or even by different programs altogether, the software package is much easier to develop and to adapt to particular needs.
For example, Unix management of the graphic display is split between the X server, which knows the hardware and offers a unified interface to user programs, and the window and session managers, which implement a particular policy without knowing anything about the hardware. People can use the same window manager on different hardware, and different users can run different configurations on the same workstation. Even completely different desktop environments, such as KDE and GNOME, can coexist on the same system. Another example is the layered structure of TCP/IP networking: the operating system offers the socket abstraction, which implements no policy regarding the data to be transferred, while different servers are in charge of the services (and their associated policies). Moreover, a server like ftpd provides the file transfer mechanism, while users can use whatever client they prefer; both command-line and graphic clients exist, and anyone can write a new user interface to transfer files.
Where drivers are concerned, the same separation of mechanism and policy applies. The floppy driver is policy free—its role is only to show the diskette as a continuous array of data blocks. Higher levels of the system provide policies, such as who may access the floppy drive, whether the drive is accessed directly or via a filesystem, and whether users may mount filesystems on the drive. Since different environments usually need to use hardware in different ways, it’s important to be as policy free as possible.
When writing drivers, a programmer should pay particular attention to this fundamental concept: write kernel code to access the hardware, but don’t force particular policies on the user, since different users have different needs. The driver should deal with making the hardware available, leaving all the issues about how to use the hardware to the applications. A driver, then, is flexible if it offers access to the hardware capabilities without adding constraints. Sometimes, however, some policy decisions must be made. For example, a digital I/O driver may only offer byte-wide access to the hardware in order to avoid the extra code needed to handle individual bits.
You can also look at your driver from a different perspective: it is a software layer that lies between the applications and the actual device. This privileged role of the driver allows the driver programmer to choose exactly how the device should appear: different drivers can offer different capabilities, even for the same device. The actual driver design should be a balance between many different considerations. For instance, a single device may be used concurrently by different programs, and the driver programmer has complete freedom to determine how to handle concurrency. You could implement memory mapping on the device independently of its hardware capabilities, or you could provide a user library to help application programmers implement new policies on top of the available primitives, and so forth. One major consideration is the trade-off between the desire to present the user with as many options as possible and the time in which you have to do the writing as well as the need to keep things simple so that errors don’t creep in.
Policy-free drivers have a number of typical characteristics. These include support for both synchronous and asynchronous operation, the ability to be opened multiple times, the ability to exploit the full capabilities of the hardware, and the lack of software layers to “simplify things” or provide policy-related operations. Drivers of this sort not only work better for their end users, but also turn out to be easier to write and maintain as well. Being policy free is actually a common target for software designers.
Many device drivers, indeed, are released together with user programs to help with configuration and access to the target device. Those programs can range from simple utilities to complete graphical applications. Examples include the tunelp program, which adjusts how the parallel port printer driver operates, and the graphical cardctl utility that is part of the PCMCIA driver package. Often a client library is provided as well, which provides capabilities that do not need to be implemented as part of the driver itself.
The scope of this book is the kernel, so we’ll try not to deal with policy issues, or with application programs or support libraries. Sometimes we’ll talk about different policies and how to support them, but we won’t go into much detail about programs using the device or the policies they enforce. You should understand, however, that user programs are an integral part of a software package and that even policy-free packages are distributed with configuration files that apply a default behavior to the underlying mechanisms.