Changing Fedora kernel configuration options

Fedora aims to provide a kernel with as many configuration options enabled as possible. Sometimes users may want to change those options for testing or for a feature Fedora doesn’t support. This is a brief guide to how kernel configurations are generated and how to best make changes for a custom kernel.

Finding the configuration files

Fedora generates kernel configurations using a hierarchy of files. Kernel options common to all architectures and configurations are listed in individual files under baseconfig. Subdirectories under baseconfig can override the settings as needed for architectures. As an example:

$ find baseconfig -name CONFIG_SPI
$ cat baseconfig/CONFIG_SPI
# CONFIG_SPI is not set
$ cat baseconfig/x86/CONFIG_SPI
$ cat baseconfig/arm/CONFIG_SPI

As shown above, CONFIG_SPI is initially turned off for all architectures but x86 and arm enable it.

The directory debugconfig contains options that get enabled in kernel debug builds. The file config_generation lists the order in which directories are combined and overridden to make configs. After you change a setting in one of the individual files, you must run the script to combine the individual files into configuration files. These exist in kernel-$flavor.config.

When rebuilding a custom kernel, the easiest way to change kernel configuration options is to put them in kernel-local. This file is merged automatically when building the kernel for all configuration options. You can set options to be disabled (# CONFIG_FOO is not set), enabled (CONFIG_FOO=y), or modular (CONFIG_FOO=M) in kernel-local.

Catching and fixing errors in your configuration files

The Fedora kernel build process does some basic checks on configuration files to help catch errors. By default, the Fedora kernel requires that all kernel options are  explicitly set. One common error happens when enabling one kernel option exposes another option that needs to be set. This produces errors related to .newoptions, as an example:

+ Arch=x86_64
+ grep -E '^CONFIG_'
+ make ARCH=x86_64 listnewconfig
+ '[' -s .newoptions ']'
+ cat .newoptions
+ exit 1
error: Bad exit status from /var/tmp/rpm-tmp.6BXufs (%prep)

RPM build errors:
 Bad exit status from /var/tmp/rpm-tmp.6BXufs (%prep)

To fix this error, explicitly set the options (CONFIG_R8188EU in this case) in kernel-local as well.

Another common mistake is setting an option incorrectly. The kernel Kconfig dependency checker silently changes configuration options that are not what it expects. This commonly happens when one option selects another option, or has a dependency that isn’t satisfied. Fedora attempts a basic sanity check that the options specified in tree match what the kernel configuration engine expects. This may produce errors related to mismatches:

+ ./check_configs.awk configs/kernel-4.13.0-i686-PAE.config temp-kernel-4.13.0-i686-PAE.config
+ '[' -s .mismatches ']'
+ echo 'Error: Mismatches found in configuration files'
Error: Mismatches found in configuration files
+ cat .mismatches
Found CONFIG_I2C_DESIGNWARE_CORE=y  after generation, had CONFIG_I2C_DESIGNWARE_CORE=m in Fedora tree
+ exit 1

In this example, the Fedora configuration specified CONFIG_I2C_DESIGNWARE_CORE=m, but the kernel configuration engine set it to CONFIG_I2C_DESIGNWARE_CORE=y. The kernel configuration engine is ultimately what gets used, so the solution is either to change the option to what the kernel expects (CONFIG_I2C_DESIGNWARE_CORE=y in this case) or to further investigate what is causing the unexpected configuration setting.

Once the kernel configuration options are set to your liking, you can follow standard kernel build procedures to build your custom kernel.

Fedora Project community


  1. Costa A.

    I’m not an expert but, is this related to modprobe command, or changing parameters during boot?

    • Sebastiaan Franken

      Neither. This sets kernel options. Modprobe enables/disables kernelmodules on the fly, and setting a parameter during boot isn’t permanent (unless you change it in /etc/default/grub or something).

      This is for making a config change. Let’s say you want to enable Thunderbolt 3 USB charging, but the Fedora default kernel doesn’t enable this. You use kerneloptions to change this, thus making a new version of the kernel for your machine.

  2. fred

    Where to start ?
    find: ‘baseconfig’: No such file or directory

    are we supposed to download something first

    • @fred: Yes, you are supposed to have downloaded the kernel SRPM, installed it, and are working from those sources. The article doesn’t attempt to go over all the steps for using a kernel, so check out (this wiki page)[] for more information.

  3. Cody

    I’m rather surprised; here you’re actually documenting what Fedora has always frowned upon or rather has said is unsupported. So does this mean Fedora will start supporting it? From my experience in the older days I would find it rather … fascinating … if Fedora would now support all the customised kernels.

    So what is the purpose here? That is to say: why are you documenting how to do something Fedora doesn’t support? Maybe there should be a big warning on this? Because anyone who doesn’t know how to do this probably won’t be able to solve the problems on their own (maybe some of the problems but certainly not all of the problems). The comments already suggest this exactly.

  4. Always , my big problem with custom kernels was that always it brakes the update procedure .

    is there any way or how to create a custom kernel and implement it on hardware and to not brake this part of the distro that it important for the OS ?

    Thank you for any answer 🙂

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