How do I configure the Linux kernel within Buildroot?

Question

I'm trying to build a rootfs for an x86 target, which is all simple enough. However I can't figure out how I configure the kernel that buildroot produces. The first run through came up with menuconfig, but it's cached the .config since then and I can't see where to change it.

~650MB of kernel modules don't do good things to an embedded target :P

Is there an easy way to configure the kernel within buildroot? Something like the uclibc-menuconfig target would be perfect.

Answer 1

I always do the following:

1. configure Linux kernel: make linux-menuconfig
2. After leaving menuconfig your configuration will be stored in file: output/build/linux-XYZ/.config where XYZ is your kernel version.
3. After that you can copy file output/build/linux-*XYZ*/.config to board/your_kernel_config/.config
4. later in Buildroot menuconfig you can under kernel settings configure to use custom kernel config file and enter path: board/your_kernel_config/.config

Answer 2

Do not forget to set also defconfig to i386 in menuconfig:

  Kernel  —>
  [*] Linux Kernel
  (i386) Defconfig name 

Answer 3

BR2_LINUX_KERNEL_CONFIG_FRAGMENT_FILES

Adds extra configs to your existing one.

E.g., if you are using buildroot as a submodule, the directory tree looks like:

.git/
buildroot/
.gitmodules
kernel-config-frag

E.g. to turn on CONFIG_DEBUG_FS, do:

echo 'CONFIG_DEBUG_FS=y' > kernel-config-frag

and then configure buildroot with:

cd buildroot
make qemu_x86_64_defconfig
echo 'BR2_LINUX_KERNEL_CONFIG_FRAGMENT_FILES=../kernel-config-frag' >> buildroot/.config
make

This way you can git track just a diff between qemu_x86_64_defconfig and your extra configs.

I believe this uses scripts/kconfig/merge_config.sh form the kernel as mentioned at: How do you non-interactively turn on features in a Linux kernel .config file?

After you change the config fragment, just remember to do:

rm -rf buildroot/output/build/linux-*.*.*/

before the next build.

Minimal runnable example at: https://github.com/cirosantilli/linux-kernel-module-cheat/blob/bb8f4eb79565c9771356c80e0964c8fefc163e11/kernel-config-frag

BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE

Selects the full .config to be used.

For some reason I have to nuke the kernel's .config for this to take effect? Why when I change BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE and run make linux-reconfigure the kernel .config does not change?