reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/sandeen/linux.git/commit/?id=4a72a2292eafad1371bc73defec4fdea64ea6de7
        git remote add sandeen https://git.kernel.org/pub/scm/linux/kernel/git/sandeen/linux.git
        git fetch --no-tags sandeen convert-9p
        git checkout 4a72a2292eafad1371bc73defec4fdea64ea6de7
        # 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/gpu/drm/ fs/9p/