reproduce:
        git remote add drm-intel git://anongit.freedesktop.org/drm-intel
        git fetch --no-tags drm-intel drm-intel-next
        git checkout f6757dfcfde722fdeaee371b66f63d7eb61dd7e4
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        make htmldocs