reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/mtd/linux.git/commit/?id=ecbba8d91f4bd0fccd722a7e5a57cec385c11ca8
        git remote add mtd https://git.kernel.org/pub/scm/linux/kernel/git/mtd/linux.git
        git fetch --no-tags mtd spi-nor/next
        git checkout ecbba8d91f4bd0fccd722a7e5a57cec385c11ca8
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # pip install -r Documentation/sphinx/requirements.txt
        make htmldocs