Compiling kernel modules for the ClearFog CX LX2

NXP LX2160
1

Hello,
How do we compile kernel modules (a camera device driver in our case) for the ClearFog CX LX2? /lib/modules/$(uname -r)/build is empty after installing a pre-built image. Are the kernel headers and binaries distributed anywhere?
I tried to build an image, and headers, myself by following this process:

  1. Following the steps here, built an image with docker on an x86-64 machine, based on commit bc46e34: GitHub - SolidRun/lx2160a_build: Scripts to build basic images for LX2160A COM express type 7 modules
  2. Flash the ClearFog board with the created image file.
  3. Copied the lx2160a_build/build/linux directory to the ClearFog board.
  4. Unfortunately, copying the contents of the linux directory into /lib/modules/$(uname -r)/build and then compiling the kernel module did not work. This is because the docker container used the host compiler rather than the cross-compiler when compiling the contents of build/linux/scripts, as well as some other files. It’s apparently quite common for kernel cross-compilation setups to let the host compiler leak through, according to some online searches.
  5. Run make scripts -j16 LOCALVERSION="-00007-g9c7b74fdbb19" and make modules -j16 LOCALVERSION="-00007-g9c7b74fdbb19" inside the linux directory, on the ClearFog, to build the kernel. I had to specify the LOCALVERSION, otherwise the kernel’s release string will would simply be 5.4.47-generic (If I recall correctly), rather than the system’s actual uname -r output of 5.4.47-00007-g9c7b74fdbb19. Kernel module compilation would fail if they were mismatched. I don’t know why this is necessary, as I can’t see where the docker container sets any version string.
  6. Copy the linux directory’s contents into /lib/modules/$(uname -r)/build and then build the kernel module. It successfully built and the module was then loaded. Unfortunately, the camera API produced a non-descriptive error when I attempted to connect to the camera, using this driver.

I can think of a few issues with my process:

  • In step 5, compiling the kernel headers with the ClearFog used the system’s default GCC version. However, the actual kernel itself was compiled with a specific version of GCC Linaro as defined in the Docker container. ABI issues, perhaps?
  • In step 5, certain config was lost when building the kernel headers. I don’t understand where the Docker container produces the 5.4.47-00007-g9c7b74fdbb19 string to the kernel name, and why I need to manually specify it when building on the ClearFog. Other settings may have been lost.
  • The driver compiled fine, but it’s an issue with the camera itself, or the way the camera interfaces with this particular ARM board.

Is there a simpler way to build the kernel headers, or be provided them prebuilt?

2

If you are building your own image, then I would recommend just tweaking the kernel config file to include the kernel modules you need for your hardware.

3

It’s an out-of-tree module. Would it make sense for me to add the kernel’s source code to linux/drivers in the container, tweaking the makefiles and Kconfig files so it’s part of the build?

4

that is probably easiest. Newer distributions should have the hardware supported in kernels 5.14 and newer with device-tree if you want to try a packaged kernel.

5

It’s probably easiest to cross-compile the module on the host:

$ cd <module_dir>
$ export PATH=$ROOTDIR/build/toolchain/gcc-linaro-7.4.1-2019.02-x86_64_aarch64-linux-gnu/bin:$PATH
$ export CROSS_COMPILE=aarch64-linux-gnu-
$ export ARCH=arm64
$ make -C <path_to_kernel_src> M=$PWD
<copy .ko to the target>

6

that will also work for a one off module.