I just recently installed Ubuntu 18 LTS. Previously I had Ubuntu 16 LTS working properly. I have been trying to use the latest stable version of Blender (2.79).
For Cycles renderining, my Radeon WX7100 no longer appears as a device in the User Preferences.
It says that v18 comes with AMD drivers, but I did download and install the latest amdgpu-pro drivers. I also installed some possibly missing ocl packages.
I remember last time there was a option for amdgpu-pro... but couldn''t remember. Read the install script for amdgpu install and found what I needed.
amdgpu-pro-install -y --pro
Seems to have fixed the issue.
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