reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=3f21de6784a654503cc148169d1f59d89971da13
        git remote add linux-next https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
        git fetch --no-tags linux-next master
        git checkout 3f21de6784a654503cc148169d1f59d89971da13
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # pip install -r Documentation/sphinx/requirements.txt
        make htmldocs