reproduce (this is a W=1 build):
        git clone https://github.com/intel/lkp-tests.git ~/lkp-tests
        # install riscv cross compiling tool for clang build
        # apt-get install binutils-riscv64-linux-gnu
        # clang-21 might not be officially released. Try a lower version if installation fails.
        # https://github.com/avpatel/linux/commit/fe469c80e363d4ba72ebe2689227b6fc00895cf3
        git remote add avpatel https://github.com/avpatel/linux.git
        git fetch --no-tags avpatel riscv_sbi_mpxy_mailbox_v4
        git checkout fe469c80e363d4ba72ebe2689227b6fc00895cf3
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang-21 ~/lkp-tests/kbuild/make.cross W=1 O=build_dir ARCH=riscv olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang-21 ~/lkp-tests/kbuild/make.cross W=1 O=build_dir ARCH=riscv SHELL=/bin/bash