Hi Geert,

I love your patch! Yet something to improve:

[auto build test ERROR on robh/for-next]
[also build test ERROR on staging/staging-testing linus/master v5.14-rc1 next-20210714]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Geert-Uytterhoeven/auxdisplay-ht16k33-Add-character-display-support/20210714-231351
base:   https://git.kernel.org/pub/scm/linux/kernel/git/robh/linux.git for-next
config: arm-randconfig-r036-20210714 (attached as .config)
compiler: clang version 13.0.0 (https://github.com/llvm/llvm-project 8d69635ed9ecf36fd0ca85906bfde17949671cbe)
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # install arm cross compiling tool for clang build
        # apt-get install binutils-arm-linux-gnueabi
        # https://github.com/0day-ci/linux/commit/a8d991401542b45f91eb59e1cdf24fd6c419360e
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Geert-Uytterhoeven/auxdisplay-ht16k33-Add-character-display-support/20210714-231351
        git checkout a8d991401542b45f91eb59e1cdf24fd6c419360e
        # save the attached .config to linux build tree
        mkdir build_dir
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross O=build_dir ARCH=arm SHELL=/bin/bash

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

>> ld.lld: error: undefined symbol: devm_led_classdev_register_ext
   >>> referenced by ht16k33.c
   >>>               auxdisplay/ht16k33.o:(ht16k33_probe) in archive drivers/built-in.a

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org