Writing a Linux Driver

The main features of a driver are:

There are four types of drivers: character drivers, block drivers, terminal drivers and streams. Character drivers transmit information from the user to the device (or vice versa) byte per byte (see Figure 2). Two examples are the printer, /dev/lp, and the memory (yes, the memory is also a device), /dev/mem.

Figure 2. Character Drivers

Block drivers (see Figure 3) transmit information block per block. This means that the incoming data (from the user or from the device) are stored in a buffer until the buffer is full. When this occurs, the buffer content is physically sent to the device or to the user. This is the reason why all the printed messages do not appear in the screen when a user program crashes (the messages in the buffer were lost), or the floppy drive light does not always turn on when the user writes to a file. The clearest examples of this type of driver are disks: floppy disks (/dev/fd0), IDE hard disks (/dev/hda) and SCSI hard disks (/dev/sd1).

Figure 3. Block Drivers

Terminal drivers (see Figure 4) constitute a special set of character drivers for user communication. For example, command tools in an open windows environment, an X terminal or a console, are devices which require special functions, e.g., the up and down arrows for a command buffer manager or tabbing in the bash shell. Examples of block drivers are /dev/tty0 or /dev/ttya (a serial port). In both cases the kernel includes special routines, and the driver special procedures, to cope with all particular features.

Figure 4. Terminal Drivers

Streams are the youngest drivers (see Figure 5) and are designed for very high speed data flows. Both the kernel and the driver include several protocol layers. The best example of this type is a network driver.

Figure 5. Stream Drivers

As we have said, a driver is a piece of a program. It is composed of a set of C functions, some of which are mandatory. For example, for a printer device, some typical functions only called by the kernel, may be:

Some additional functions are available for particular applications, like *_lseek(), *_readdir(), *_select() and *_mmap(). You may find more information about them in Michael Johnson's Hacker's Guide (see Resources).

There are several reasons for writing our own device driver:

Conversely, there are also several reasons for not writing our own driver:

In order to understand the following explanation, you must know the C programming language, the basic I/O procedures, a minimum about the internal architecture of a PC and have some experience in the development of software applications for Unix systems.

Finally, we must add that writing our own device driver is only necessary when the device manufacturer does not supply a driver for our OS or when we wish to add extra functionality to the one we have.