Chapter5.InputOutput

Chapter 5. Input/Output

In this chapter

• 5.1 How Hardware Does It: Busses, Bridges, Ports, and Interfaces 255

• 5.2 Devices 260

• Summary 281

• Project: Building a Parallel Port Driver 281

• Exercises 293

The Linux kernel is a collection of code that runs on one or more processors. The processors' interface to the rest of the system is through the supporting hardware. At its lowest machine-dependent layer, the kernel communicates with these devices with simple assembly-language instructions. This chapter explores the relationship of the kernel to the surrounding hardware, focusing on file I/O and hardware devices. We illustrate how the Linux kernel ties together software and hardware by discussing how we go from the highest level of a virtual filesystem down to the lowest level of writing bits to physical media.

This chapter starts with an overview of just how the core of a computer, the processor, connects to the rest of the system. The concept of busses is also discussed, including how they connect the processor to other elements of the system (such as memory). We also introduce devices and controllers that make up the chipsets used in most x86 and PowerPC systems.

By having a basic understanding of the components of a system and their interconnection, we can begin to analyze the layers of software from an application to the operating system, to the specific block device used for storagethe hard drive and its controller. Although the concept of the filesystem is not covered until the next chapter, we discuss enough of the components to get us down to the generic block device layer and the most important method of communication for the block device; the request queue.

The important relationship between a mechanical device (the hard drive) and the system software is discussed when we introduce the concept of scheduling I/O. By understanding the physical geometry of a hard drive and how the operating system partitions the drive, we can begin to understand the timing between software and the underlying hardware.

Moving closer to the hardware, we see how the generic block driver interfaces to the specific block driver, which allows us to have common software control over various hardware devices. Finally, in our journey from the application level to the I/O level, we touch on the hardware I/O needed for a disc controller and point you to other examples of I/O and device drivers in this book.

We then discuss the other major device typethe character deviceand how it differs from the block device and the network device. The importance of other devicesthe DMA controller, the clock, and terminal devicesare also contrasted with these.