reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e4d4371253029528c02bfb43a46c252e1c3d035f
        git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
        git fetch --no-tags linus master
        git checkout e4d4371253029528c02bfb43a46c252e1c3d035f
        # 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/mux/