c - How to add a Linux kernel driver module as a Buildroot package?

Question

I am currently building an Embedded Linux for my Zybo Board from Xilinx. For this I use Buildroot. Now I want to add a driver, written in C, which can be used by a user program to write to some specific registers, enabling it to control some LEDs. When I checked the manual, it basically says the first thing to do is create a Config.in in a new package folder, where you write some text explaining the driver. Okay, I did that. But now the makefile: I don't quite understand what needs to be in there. Is it just a compile command like gcc -o ledcontrol hellofunc.c? Is there something else I need to do apart from the Config.in and Makefile?

Answer 1

Fully automated out-of-tree QEMU example

https://github.com/cirosantilli/linux-kernel-module-cheat/tree/753cbe68ff50bea0982e1791c8a2178a999d8377/buildroot_packages/kernel_modules

Source tree:

• buildroot/: Buildroot 2017.02, ideally as a git submodule
• kernel_module/: external package with some modules
  • Config.in
  • external.mk
  • Makefile
  • hello.c: hello world module

kernel_module/Config.in

config BR2_PACKAGE_KERNEL_MODULE
        bool "kernel_module"
        depends on BR2_LINUX_KERNEL
        help
                Linux Kernel Module Cheat.

kernel_module/external.mk

KERNEL_MODULE_VERSION = 1.0
KERNEL_MODULE_SITE = $(BR2_EXTERNAL_KERNEL_MODULE_PATH)
KERNEL_MODULE_SITE_METHOD = local
$(eval $(kernel-module))
$(eval $(generic-package))

kernel_module/Makefile

obj-m += $(addsuffix .o, $(notdir $(basename $(wildcard $(BR2_EXTERNAL_KERNEL_MODULE_PATH)/*.c))))
ccflags-y := -DDEBUG -g -std=gnu99 -Wno-declaration-after-statement

.PHONY: all clean

all:
    $(MAKE) -C '$(LINUX_DIR)' M='$(PWD)' modules

clean:
    $(MAKE) -C '$(LINUX_DIR)' M='$(PWD)' clean

kernel_module/hello.c

#include <linux/module.h>
#include <linux/kernel.h>

MODULE_LICENSE("GPL");

static int myinit(void)
{
    printk(KERN_INFO "hello init
");
    return 0;
}

static void myexit(void)
{
    printk(KERN_INFO "hello exit
");
}

module_init(myinit)
module_exit(myexit)

Usage:

cd buildroot
make BR2_EXTERNAL="$(pwd)/../kernel_module" qemu_x86_64_defconfig
echo 'BR2_PACKAGE_KERNEL_MODULE=y' >> .config
make BR2_JLEVEL="$(($(nproc) - 2))" all
qemu-system-x86_64 -M pc -kernel output/images/bzImage -drive file=output/images/rootfs.ext2,if=virtio,format=raw -append root=/dev/vda -net nic,model=virtio -net user

QEMU opens up, then run:

root
modprobe hello
modprobe -r hello

dmesg shows:

hello init
hello exit

The key line is $(eval $(kernel-module)) in external.mk, which sets everything up for us, and installs the modules where modprobe will find them (/lib/modules/*/extra/hello.ko), including modules.dep for inter-module dependencies: How to call exported kernel module functions from another module?

How to GDB step debug the kernel modules: How to debug Linux kernel modules with QEMU?

To autoload modules at startup, use BR2_ROOTFS_OVERLAY="../rootfs_overlay" and a rootfs_overlay/etc/init.d/S99modules file that does the modprobe.

In-tree example: https://github.com/cirosantilli/buildroot/tree/9580078b98f885ca94e4dfc896265a8a491f6ae1 It is less clean, but also works.

Tested on a Ubuntu 16.04 host.