reproduce:
        git remote add sailus-media-tree git://linuxtv.org/sailus/media_tree.git
        git fetch --no-tags sailus-media-tree v4l2-isp
        git checkout 1a426ed271eb152cfed4f942a0caaa37e00ea707
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # pip install -r Documentation/sphinx/requirements.txt
        make htmldocs