Cross-compiling kernel modules for Raspbian

This is a follow-up post to my post about compiling out-of-tree kernel modules for Raspbian / Raspberry Pi from last week. In that post I talked about compiling a kernel module on the Raspberry Pi itself. This time, I did the same thing, but I did it on my i386 Ubuntu workstation, using a cross-compiler.

For this to work, you need the following software:

• The kernel source
• A cross-compiler, and it has to be the same version that your actual Raspbian-kernel was compiled with
• A Module.symvers file, or the kernel and all modules built

The easiest thing is to create a directory that you keep all the Raspberry Pi stuff in. If you haven’t downloaded the kernel source yet, clone the Git repository; it allows you to stay up to date. Also download the pre-built compiler toolchain using Git:

[code]
mkdir ~/raspberrypi
cd ~/raspberrypi
git clone https://github.com/raspberrypi/linux.git
git clone https://github.com/raspberrypi/tools.git
[/code]

Just for fun & exercise, instead of using the downloaded Modules.symvers, build the kernel. See http://elinux.org/RPi_Kernel_Compilation#compiling for more detailed information.

[code]
cd linux
zcat /proc/config.gz > .config
make -j 6 ARCH=arm CROSS_COMPILE=$HOME/raspberrypi/tools/arm-bcm2708/arm-bcm2708-linux-gnueabi/bin/arm-bcm2708-linux-gnueabi- oldconfig
make -j 6 ARCH=arm CROSS_COMPILE=$HOME/raspberrypi/tools/arm-bcm2708/arm-bcm2708-linux-gnueabi/bin/arm-bcm2708-linux-gnueabi-
make -j 6 ARCH=arm CROSS_COMPILE=$HOME/raspberrypi/tools/arm-bcm2708/arm-bcm2708-linux-gnueabi/bin/arm-bcm2708-linux-gnueabi- modules
[/code]

Please note the ‘-j 6’, which sets the number of concurrent jobs to 6, which may be less than ideal if you have a slow (single/dual core) machine. If your computer has a quad-core processor or better, -j 6 should be fine. It will make your build go a whole lot faster.

Note: if you don’t feel like waiting for a kernel build to complete, you can also use the provided Module.symvers file, prepare and configure the kernel tree and build your module from there. Please see my previous post on how to do that; just add the cross-compilation flags.

Now, you are ready to cross-compile your out-of-tree kernel module:

[code]
cd /path/to/module/source
make -C $HOME/raspberrypi/linux ARCH=arm \
CROSS_COMPILE=$HOME/raspberrypi/tools/arm-bcm2708/arm-bcm2708-linux-gnueabi/bin/arm-bcm2708-linux-gnueabi- \
SUBDIRS=$PWD modules
[/code]

Compiling kernel modules for Raspbian / Raspberry Pi

I needed to compile an out-of-tree (vendor supplied) driver for a USB device, to use it on my Raspberry Pi. Since the process was less than obvious, I’ll document it here. Maybe it will be of use to anyone else.

Normally, for compiling kernel modules, you need the kernel headers available. They have to match the version of the kernel you are targeting. In case of the Raspberry Pi running the 2012-07-15 image of Raspbian, that would be 3.1.9+. On plain Debian you would install a ‘linux-headers-x.y.z-flavour’ package, but that doesn’t seem to be available on Raspbian (at least not the right version), so we use the full kernel source from Github.

The main problem with that, is the absence of a file named Module.symvers, that contains the symbol versions of the running kernel and all of its modules. That file would normally come in the linux-headers package, that I mentioned above. To get that file, and be able to build ‘good’ modules for the kernel, you would need to build the entire kernel and all its modules yourself. At the end of the build, the file will be created. However, if you want to do that on the Pi itself, make sure you have plenty of time, because a complete kernel build will take quite a few hours.

To spare you the trouble, you can download it here: Module.symvers. Please note: this file is for the 3.1.9 kernel from Raspbian-2012-07-15, and it will not work for other kernel versions.

Assuming you have downloaded the file to your home directory, you can now download the Linux kernel source and prepare it for building out-of-tree modules like this:

cd /usr/src
sudo wget -O raspberrypi-linux-3.1.9.tar.gz https://github.com/raspberrypi/linux/tarball/rpi-patches
sudo tar xzf raspberrypi-linux-3.1.9.tar.gz
sudo ln -s /usr/src/raspberrypi-linux-eab45cb /lib/modules/`uname -r`/build
cd /usr/src/raspberrypi-linux-eab45cb
sudo sh -c 'zcat /proc/config.gz  > .config'
sudo sed -i 's/EXTRAVERSION =.*/EXTRAVERSION = +/' Makefile
sudo cp ~/Module.symvers .
sudo make oldconfig
sudo make modules_prepare

A few notes:

• The name of the source directory is probably different in your case, because it contains the ID of the latest commit in the Git repository and commits occur regularly.
• The sed command on line 7 is to add a ‘+’ to the version in the source. The version should be 3.1.9+, while the version in the repository is just 3.1.9.

Now most well-maintained drivers should ship a proper Makefile for building them, but the usual command line for building such a module would be something like this:

[code]
make -C /lib/modules/`uname -r`/build SUBDIRS=$PWD modules
[/code]