reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git/commit/?id=8a8f622c329e2e3690b54826370e4a0af45f66ef
        git remote add riscv https://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git
        git fetch --no-tags riscv for-next
        git checkout 8a8f622c329e2e3690b54826370e4a0af45f66ef
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # pip install -r Documentation/sphinx/requirements.txt
        make htmldocs