How to install openCL for amd graphic driver on ubuntu 18.04

I’ve searched on the Internet, but, it can’t find a suitable way for doing this.
It seems that, openCL amd driver doesn’t works with opensource drivers (and its the case, I cannot create an openCL context for a GPU devide with opensource driver, it fails), so you need to install proprietary driver but proprietary amd drivers doesn’t work for me on ubuntu.
So it seems that it’s impossible for me to use openCL on a gpu device.