reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=b4a0774bd7fd859c1d243be0919c8df39eac20c0
        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 b4a0774bd7fd859c1d243be0919c8df39eac20c0
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # pip install -r Documentation/sphinx/requirements.txt
        make htmldocs