reproduce:
        # https://github.com/pdp7/linux/commit/0c2865dfa6aa9fb67ef9b12cf723cce6fa46e959
        git remote add pdp7 https://github.com/pdp7/linux
        git fetch --no-tags pdp7 b4/th1520-dwmac
        git checkout 0c2865dfa6aa9fb67ef9b12cf723cce6fa46e959
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs