Translations:Kernel/Gentoo Kernel Configuration Guide/17/en

For other parts of the configuration, built-in is an absolute requirement. For example, if the root partition was an btrfs filesystem the system would not boot if btrfs was built as a module. The system would have to look on the root partition to find the btrfs module (since modules are stored in the root partition), but it cannot look on the root partition unless it already has btrfs support loaded! If btrfs has not been built-in then the init process will fail to find the root device.