reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/arnd/playground.git/commit/?id=6b2a645daeb0f325ab9601d75e52d17334c3535d
        git remote add arnd-playground https://git.kernel.org/pub/scm/linux/kernel/git/arnd/playground.git
        git fetch --no-tags arnd-playground config-gpio-legacy-7.1
        git checkout 6b2a645daeb0f325ab9601d75e52d17334c3535d
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # pip install -r Documentation/sphinx/requirements.txt
        make htmldocs