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Learning Linux Kernel Device Drivers

Last update: 2015.03.11 (Mi) 12:15:02 (UTC +0100 CET)

What is this about ?

This is a way to learn how to write/programm device drivers for the Linux Kernel. The primary goal is to learn to use the available API (3.7.x or newer) and the pertinent concepts and write correct code and document it, in order to reduce the learning curve. Optimizations may be included/use if appropriate. Correctness comes first.

This is a working in progress, and therefore bugs/errors may be found. Suggestions, critiques, contributions are welcome. :)

Pre-requisites

Before starting, it is import to have already experience with C programming. C is the Linux Kernel "Lingua Franca". Almost 97% the source code is written C and a small part in Assembly. A good knowledge about hardware standards and how it works is strongly recommended, specially for device drivers, as well the operating system theory and concetps.

Coding Style

The Linux Kernel coding style is followed. You can find it here:

Linux Kernel Coding Style

Getting the Linux Kernel Code

The Linux Stable tree is used in this project. Here is how to get the code:

mkdir ~/src
cd ~/src
git clone git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git

I suggest to use a tagged version instead of the master tree, because you know where you are working on.

cd linux-stable
git co v3.14

One can replace the tag (in this case v3.14) by any other one. Then I suggest to make a symbolic link to your git tree. It makes your life easier if you need to change to other trees and one keeps a conssitent name:

cd ~/src
ln -s linux-stable linux

Compiling the kernel

On the llkdd wiki there is a howto to compile and install the Linux kernel on the X86_64 archtecture.

Compile the llkdd code

The default path for the current Kernel source code points to $HOME/src/linux. Put the kernel source code in this direcory or create a symlink to it. The next step is change to one of the driver's directory and then execute make:

cd helloworld
make

and the corresponding device driver will be compiled.

To use the compiled driver:

sudo insmod drivername.ko

And it is loaded in memory. To remove it just type:

sudo  rmmod drivername.ko

In order to see the driver's messages in the kernel execute in a terminal:

journalctl -f _TRANSPORT=kernel

if you use systemd or:

tail -f /var/log/kern.log

if you use syslog-ng.

Install the llkdd udev rules file

Copy the udev rules file, as root, to the udev configuration directory:

sudo cp -v 10-llkdd.rules /etc/udev/rules.d

and update the udev configuration by running:

sudo udevadm control --reload-rules

Current Status

Here is a short summary about the current drivers beeing developed and its status.

driver code documentation
helloworld 100% 100%
one 100% 100%
intn 100% 50%
intn2 100% 10%
procfs 100% 0%
usbstick 20% 0%
keylogger 60% 10%

The code status columns means how many of the planned features were implemented, it does not mean the code has no bugs !

See more infos at the project wiki page.