Writing a Linux Driver

The task of integrating the driver into the kernel includes several steps:

The insertion of the OS call to mrv4_init() is done in the /usr/src/linux/kernel/mem.c file. The other driver function calls (open, read, write, ioctl, release, etc.) are user transparent. They are carried out through the file_operations structure. A driver major number must be added to the list located at /usr/include/linux/major.h. Search for a free driver number; for example, if number 62 is free, you must add one or both of the following lines to the file, depending on the Linux release:

/dev/mrv4 62
#define MRV4_MAJOR 62

Each device is referenced by one major and one minor number. The major number represents the number of the driver. The minor number distinguishes between several devices which are controlled by the same device (e.g., several hard disks controlled by the same IDE driver: hd0, hd1, hd2).

The next step is to create a logical device to access the driver. You must use the command mknod in this way:

mknod -m og+rw /dev/mrv4 c 62 0

where 62 is the major, 0 the minor (only one physical device) and c indicates a character device. Set the permissions as necessary, although you can modify them later with the command chmod. For example, enable rw if you want to allow all users to access the device:

crw-rw-rw-2 bin bin 62, 0 Mar 12 1997 /dev/mrv4

To allow driver compilation within the kernel, the following lines must be added to the script file /usr/src/linux/arch/i386/config.in:

comment 'MRV 4'
bool 'MRV 4 card support' CONFIG_MRV4

and the following lines to /usr/src/linux/drivers/char/Makefile:

ifdef CONFIG_MRV4
  L_OBJS += mrv4.o
  endif

cd /usr/src/linux/drivers/char
gcc -c mrv4.c -Wall -D__KERNEL__

rm -f mrv4.o

cd /usr/src/linux
make config

make zdisk    # generate a bootable
# floppy disk
dev -R /dev/fd0 1   # disable writes to<\n>
# floppy

If the kernel seems to work, you might test the driver by writing one or more user programs, i.e., mrv4test.c which call the driver functions:

fid = open("/dev/mrv4", ...);
read(fid, ...);
write(fid, ...);
ioctl(fid, ...);
close(fid);

You can obtain privileged documentation at sunsite.unc.edu (see Resources). But of course, you will never be able to write your own driver using only the general guidelines of this article. To facilitate this task, we supply the source files for a dummy driver for Linux 2.0.24, which is a model for character driver development. It simulates the equation y = a x and includes an example of interrupt management (which does not work since it is not associated with any hardware). Its name is foo, since Linux already has a driver called dummy. These files are:

You can obtain these files via anonymous FTP at ftp://ftp.linuxjournal.com/pub/lj/listings/issue48/2476.tgz.

Guidelines

Resources