reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?id=9c7dc1dd897a1cdcade9566ea4664b03fbabf4a4
        git remote add tip https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git
        git fetch --no-tags tip objtool/core
        git checkout 9c7dc1dd897a1cdcade9566ea4664b03fbabf4a4

        git clone https://github.com/intel/lkp-tests.git ~/lkp-tests
        # apt-get install sparse
        # sparse version: v0.6.4-71-g610d841d

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-10.5.0 ~/lkp-tests/kbuild/make.cross W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=powerpc olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-10.5.0 ~/lkp-tests/kbuild/make.cross W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=powerpc SHELL=/bin/bash drivers/tty/serial/ fs/overlayfs/