Anyway, my test line was:

ffmpeg -init_hw_device opencl=gpu -filter_hw_device gpu -i ~/20210419_055507A.mp4 -vf "hwupload, boxblur_opencl, hwdownload,format=yuv420p" -f null /dev/null

for different filters format might be yuv420p or nv12 ...

I wonder if anyone here have working OpenCL ffmpeg setup ?

I'll try with llvmpipe/rusticl on desktop .... but it guaranteed to be slow.