reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/bcain/linux.git/commit/?id=520783c1a795b1b4bd4c87483c2300e9f8e866c6
        git remote add bcain https://git.kernel.org/pub/scm/linux/kernel/git/bcain/linux.git
        git fetch --no-tags bcain bcain/qemu_boot
        git checkout 520783c1a795b1b4bd4c87483c2300e9f8e866c6

        # 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/