reproduce (this is a W=1 build):
        # https://github.com/morimoto/linux/commit/ef32c3846d431426fb87413b018477aad6c13d65
        git remote add morimoto https://github.com/morimoto/linux
        git fetch --no-tags morimoto renesas-lts/v6.18-rc3-2025-10-28-x5h-ironhide
        git checkout ef32c3846d431426fb87413b018477aad6c13d65

        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=$HOME/0day COMPILER=gcc-15.1.0 ~/lkp-tests/kbuild/make.cross W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=arm64 olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-15.1.0 ~/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=arm64 SHELL=/bin/bash drivers/mailbox/