reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git/commit/?id=5a91e16ba44d9850088278ebe209b5c533f87cb8
        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 5a91e16ba44d9850088278ebe209b5c533f87cb8

        # 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/