Kernel device drivers are the mechanism through which the underlying hardware is exposed to the rest of the system. As a developer of embedded systems, you need to know how device drivers fit into the overall architecture and how to access them from user space programs. Your system will probably have some novel pieces of hardware and you will have to work out a way of accessing them. In many cases, you will find that there are device drivers provided for you and you can achieve everything you want without writing any kernel code. For example, you can manipulate GPIO pins and LEDs using files in sysfs, and there are libraries to access serial buses, including SPI and I2C.
There are many places to find out how to write a device driver, but few to tell you why you would want to and the choices you have in doing so. That is what I want to cover here. However, remember that this is not a book dedicated to writing kernel device drivers and that the information given here is to help you navigate the territory but not necessarily to set up home there. There are many good books and articles that will help you to write device drivers, some of which are listed at the end of this chapter.
As mentioned in Chapter 4, Porting and Configuring the Kernel, one of the functions of the kernel is to encapsulate the many hardware interfaces of a computer system and present them in a consistent manner to user-space programs. There are frameworks designed to make it easy to write the interface logic for a device in the kernel and you can integrate it with the kernel: this is a device driver, the piece of code that mediates between the kernel above it and the hardware below. A device driver is a piece of software that controls physical devices such as a UART or an MMC controller, or virtual devices such as the null device (/dev/null) or a ramdisk. One driver may control multiple devices of the same kind.
Kernel device driver code runs at a high privilege level, as does the rest of the kernel. It has full access to the processor address space and hardware registers. It can handle interrupts and DMA transfers. It can make use of the sophisticated kernel infrastructure for synchronization and memory management. There is a downside to this, which is that if something goes wrong in a buggy driver, it can go really wrong and bring the system down. Consequently, there is a principle that device drivers should be as simple as possible, just providing information to applications where the real decisions are made. You often hear this being expressed as no policy in the kernel.
In Linux, there are three main types of device driver:
There is also a fourth type that presents itself as a group of files in one of the pseudo filesystems. For example, you might access the GPIO driver through a group of files in /sys/class/gpio, as I will describe later on in this chapter. Let's begin by looking in more detail at the three basic device types.