reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/lee/mfd.git/commit/?id=690f145503fe1e4d31a1f0250122b3e04876ee90
        git remote add lee-mfd https://git.kernel.org/pub/scm/linux/kernel/git/lee/mfd.git
        git fetch --no-tags lee-mfd for-mfd-next
        git checkout 690f145503fe1e4d31a1f0250122b3e04876ee90
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=um olddefconfig
        make W=1 O=build_dir ARCH=um SHELL=/bin/bash drivers/mfd/