The goal of this chapter is to write a complete char device driver. We’ll develop a character driver because this class is suitable for most simple hardware devices. Char drivers are also easier to understand than, for example, block drivers or network drivers. Our ultimate aim is to write a modularized char driver, but we won’t talk about modularization issues in this chapter.
Throughout the chapter, we’ll present code fragments extracted from a real device driver: scull, short for Simple Character Utility for Loading Localities. scull is a char driver that acts on a memory area as though it were a device. A side effect of this behavior is that, as far as scull is concerned, the word device can be used interchangeably with “the memory area used by scull.”
The advantage of scull is that it isn’t hardware dependent, since every computer has memory. scull just acts on some memory, allocated using kmalloc. Anyone can compile and run scull, and scull is portable across the computer architectures on which Linux runs. On the other hand, the device doesn’t do anything “useful” other than demonstrating the interface between the kernel and char drivers and allowing the user to run some tests.
The first step of driver writing is defining the capabilities (the mechanism) the driver will offer to user programs. Since our “device” is part of the computer’s memory, we’re free to do what we want with it. It can be a sequential or random-access device, one device or many, and so on.
To make scull be useful as a template for writing real drivers for real devices, we’ll show you how to implement several device abstractions on top of the computer memory, each with a different personality.
The scull source implements the following devices. Each kind of device implemented by the module is referred to as a type:
Four devices each consisting of a memory area that is both global and persistent. Global means that if the device is opened multiple times, the data contained within the device is shared by all the file descriptors that opened it. Persistent means that if the device is closed and reopened, data isn’t lost. This device can be fun to work with, because it can be accessed and tested using conventional commands such as cp, cat, and shell I/O redirection; we’ll examine its internals in this chapter.
Four FIFO (first-in-first-out) devices, which act like pipes. One process reads what another process writes. If multiple processes read the same device, they contend for data. The internals of scullpipe will show how blocking and nonblocking read and write can be implemented without having to resort to interrupts. Although real drivers synchronize with their devices using hardware interrupts, the topic of blocking and nonblocking operations is an important one and is separate from interrupt handling (covered in Chapter 9).
These devices are similar to scull0, but with some limitations on when an open is permitted. The first (scullsingle) allows only one process at a time to use the driver, whereas scullpriv is private to each virtual console (or X terminal session) because processes on each console/terminal will get a different memory area from processes on other consoles. sculluid and scullwuid can be opened multiple times, but only by one user at a time; the former returns an error of “Device Busy” if another user is locking the device, whereas the latter implements blocking open. These variations of scull add more “policy” than “mechanism;” this kind of behavior is interesting to look at anyway, because some devices require types of management like the ones shown in these scull variations as part of their mechanism.
Each of the scull devices demonstrates different features of a driver and presents different difficulties. This chapter covers the internals of scull0 to skull3; the more advanced devices are covered in Chapter 5: scullpipe is described in Section 5.2.5 and the others in Section 5.6.