reproduce:
        git remote add rockchip https://git.kernel.org/pub/scm/linux/kernel/git/mmind/linux-rockchip.git
        git fetch rockchip for-next
        git checkout rockchip/for-next
        b4 shazam https://lore.kernel.org/r/db605935ad21c4780b73db87605b52d30bc936a4.1700132457.git.mehdi.djait@bootlin.com
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs