reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git/commit/?id=3cc16ae096f164ae0c6b98416c25a01db5f3a529
        git remote add tty https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
        git fetch --no-tags tty tty-testing
        git checkout 3cc16ae096f164ae0c6b98416c25a01db5f3a529
        # 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/tty/serial/