reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git/commit/?id=20acf4dd46e4c0905676ad2a1d9a32041469893f
        git remote add char-misc https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git
        git fetch --no-tags char-misc char-misc-testing
        git checkout 20acf4dd46e4c0905676ad2a1d9a32041469893f
        # save the config file
        make W=1 ARCH=um olddefconfig
        make W=1 ARCH=um