reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/pinchartl/linux.git/commit/?id=3d28ed2a4e36222cb404c08eca65a1cbae14fa50
        git remote add pinchartl https://git.kernel.org/pub/scm/linux/kernel/git/pinchartl/linux.git
        git fetch --no-tags pinchartl rpi/v6.9/isp/v2
        git checkout 3d28ed2a4e36222cb404c08eca65a1cbae14fa50
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs