reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/arnd/playground.git/commit/?id=ec6a6b690caeb0e6a10757890c166bc226b799f5
        git remote add arnd-playground https://git.kernel.org/pub/scm/linux/kernel/git/arnd/playground.git
        git fetch --no-tags arnd-playground to-build
        git checkout ec6a6b690caeb0e6a10757890c166bc226b799f5
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 olddefconfig
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash drivers/mfd/