reproduce (this is a W=1 build):
        # https://github.com/avpatel/linux/commit/b85d9f545481fd4c956f5bc45ec5c6e2cb3d72ab
        git remote add avpatel https://github.com/avpatel/linux.git
        git fetch --no-tags avpatel riscv_sbi_dbtr_v3
        git checkout b85d9f545481fd4c956f5bc45ec5c6e2cb3d72ab

        git clone https://github.com/intel/lkp-tests.git ~/lkp-tests
        # apt-get install sparse
        # sparse: v0.6.5-rc1

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-8.5.0 ~/lkp-tests/kbuild/make.cross W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=riscv olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-8.5.0 ~/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=riscv SHELL=/bin/bash arch/riscv/kernel/