How to build out of tree Kernel Module on Debian

debian-kmod-howto.md

How to build out of tree Kernel Module on Debian

Install Kernel Development files

First get the general build tools for compiling kernel modules: sudo apt install build-essential

Also install the kernel development headers for the currently running kernel:

debian@sr-imx8:~$ uname -a
Linux sr-imx8 4.19.72-imx8-sr #1 SMP PREEMPT Thu Apr 30 03:44:47 UTC 2020 aarch64 GNU/Linux
debian@sr-imx8:~$ apt-cache search imx8-sr
linux-4.19.y-imx8-sr-dev - kernel API development files
linux-headers-4.19.y-imx8-sr - kernel development files
linux-headers-4.19.y-imx8-sr-dbgsym - debug symbols for linux-headers-4.19.y-imx8-sr
linux-image-4.19.y-imx8-sr - LONGNAME kernel
debian@sr-imx8:~$ sudo apt install linux-headers-4.19.y-imx8-sr

Compile the Driver

Unpack sources:

sudo apt install unzip
unzip ~/MMA8652FC-ccaeb5d75dc2.zip
cd MMA8X5X

It appears this driver comes without a Makefile ...

debian@sr-imx8:~/MMA8X5X$ ls
mma8x5x.c  readme.txt

and without a Kbuild file :(

As per https://www.kernel.org/doc/html/latest/kbuild/modules.html the simplified steps are:

1. Create simple Kbuild file:

   obj-m := mma8x5x.o
   

2. invoke system-wide kernel Makefile

   debian@sr-imx8:~/MMA8X5X$ uname -r
   4.19.72-imx8-sr
   make -C /lib/modules/4.19.72-imx8-sr/source M=$PWD
   

3. have fun fixing all the build errors 😅

4. load the final module

   insmod mma8x5x.ko
   

Per https://www.kernel.org/doc/html/latest/kbuild/modules.html it should be the build directory, not soruce:

$ make -C /lib/modules/`uname -r`/build M=$PWD

Debian actually installs the kernel build system in a non-standard location, you need to pass e.g. /usr/src/linux-headers-6.1.0-25-common/ as the argument to -C.

The build may also complain about missing build scripts like so:

make: Entering directory '/usr/src/linux-headers-6.1.0-25-common'
Makefile:1103: scripts/Makefile.extrawarn: No such file or directory
make: *** No rule to make target 'scripts/Makefile.extrawarn'.  Stop.
make: Leaving directory '/usr/src/linux-headers-6.1.0-25-common'

In that case, you need to install the respective linux-kbuild package (e.g. linux-kbuild-6.1).