reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git/commit/?id=20ffe4b3330a8bde9e933e9ba2323d5e9386caa5
        git remote add tty https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
        git fetch --no-tags tty tty-next
        git checkout 20ffe4b3330a8bde9e933e9ba2323d5e9386caa5
        # apt-get install sparse
        # sparse: v0.6.5-rc1

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=um olddefconfig
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=um SHELL=/bin/bash drivers/tty/serial/