reproduce:
        # https://github.com/nunojsa/linux/commit/c54362d581209a7421fe0d0caf70ff172289da75
        git remote add nsa https://github.com/nunojsa/linux
        git fetch --no-tags nsa xlnx/fix/buf-mmap-multibuffer
        git checkout c54362d581209a7421fe0d0caf70ff172289da75
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # pip install -r Documentation/sphinx/requirements.txt
        make htmldocs