Writing a Linux Driver

The main goal of this article is to learn what a driver is, how to implement a driver for Linux and how to integrate it into the operating system—an article for the experienced C programmer.

The concept of an operating system (OS) must be well understood before any attempt to navigate inside it is made. Several definitions are available for an OS:

  1. An OS is the set of manual and automatic procedures which allow a set of users to share a computing system in an efficient manner.

  2. The dictionary defines an OS as a program or set of programs which manage the processes of a computing system and allow the normal execution of the other jobs.

  3. The definition from the Tanenbaum book (see Resources): An operating system is [the program] which controls all the resources of the computer and offers the support where users can develop application programs.

It is also very important to clearly distinguish a program from a process. A program is a block of data plus instructions, which is stored in a file on disk and is ready to be executed. On the other hand, a process is an image in memory of the program which is being executed. This difference is highly important, because usually the processes are running under OS control. Here, our program is the OS, so we cannot speak about processes.

We will use the term kernel to refer to the main body of the OS, which is a program written in the C language. The program file may be named vmlinuz, vmlinux or zImage, and has some things in common with the MS-DOS files COMMAND.COM, MSDOS.SYS and IO.SYS, although their functionality is different. When we discuss compilation of the kernel, we mean that we will edit the source files in order to generate a new kernel.

Peripheral or internal devices allow users to communicate with the computer. Examples of devices are: keyboards, monitors, floppy and hard disks, CD-ROMs, printers, mice (serial/parallel), networks, modems, etc. A driver is the part of the OS that manages communication with devices; thus, they are usually called device drivers.

What is a Driver?

Figure 1. Software/Hardware Scheme

Figure 1 shows the relation between user programs, the OS and the devices. Differences between software and hardware are clearly specified in this scheme. At the left side, user programs may interact with the devices (for example, a hard disk) through a set of high-level library functions. For example, we can open and write to a file of the hard disk calling the C library functions fopen, fprintf and close:

FILE *fid=fopen("filename", "w");
fprintf(fid, "Hello, world!");

The user can also write to a file (or to another device such as a printer) from the OS shell, using commands such as:

echo "Hello, world!" >
echo "Hello, world!" > /dev/lp
To execute this command, both the shell and the library functions perform a call to a low level function of the OS, e.g., open(), write() or close():
fid = open("/dev/lp", O_WRONLY);
write(fid, "Hello, world!");
Each device can be referred to as a special file named /dev/*. Internally, the OS is composed of a set of drivers, which are pieces of software that perform the low-level communication with each device. At this execute level, the kernel calls driver functions such as lp_open() or lp_write().

On the right side of Figure 1, the hardware is composed of the device (a video display or an Ethernet link) plus an interface (a VGA card or a network card). Finally, the device driver is the physical interface between the software and the hardware. The driver reads from and writes to the hardware through ports (memory addresses where the hardware links physically), using the internal functions out_p and in_p:

out_p(0x3a, 0x1f);
data = in_p(0x3b);

Note that these functions are not available to the user. Since the Linux kernel runs in protected mode, the low memory addresses, where the ports addresses reside, are not user accessible. Functions equivalent to the low-level functions in and out do not exist in the high-level library, as in other operating systems such as MS-DOS.



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I like this one

Anonymous's picture

Good introduction, I think, for those who is not familiar with Linux drivers at all.
Thanks a lot to the author. I enjoyed the article.

This article twice uses the

Anonymous's picture

This article twice uses the term "protected mode" where it should be using the term "supervisor mode".

Protected mode is a mode of the Intel x86 processor which provides various protection features, such as memory protection and the ability to disable privileged instructions. Under Linux, all software runs in protected mode, but user applications run at a different privilege level to the kernel.

Of course this article is quite out of date (though surprisingly much of it is still relevant) but on this point it was wrong even back when it was written.

sidebar & port table ?

ARJUN's picture

where is the sidebar & port table that were discussed in this article..?

links and sidebars

Keith Daniels's picture

This is not the magazine so there are no "sidebars" Check the links in the articles they have what you are looking for.

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