reproduce:
        git remote add opencloudos https://gitee.com/OpenCloudOS/OpenCloudOS-Kernel.git
        git fetch --no-tags opencloudos linux-5.4/lts/5.4.119-20.0009
        git checkout af2ddee074010b709289319d0b9c4cc8cf5744f6
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 1.7.9
        make htmldocs