AMDGPU however does NOT support undervolt OR clock control for R9 series cards (try it, the clocks DO NOT MOVE no matter what settings you use)
It works fine for GCN3 (i.e. Tonga).
sudo ./ohgodatool -i $r380 --core-state 5 --core-clock 906
sudo ./ohgodatool -i $r380 --core-state 5 --core-vddc-idx 1
It's not quite as good as Polaris, because the AMDGPU driver seems to use fixed voltage tables with Tonga rather than EVV.
You also need to watch out for the ACPI bug that causes powerplay to fail. It seems BIOS and slot dependent, so you might not encounter it. I had problems with the the 16x slot on H97 anniversary boards, and after trying a couple different BIOS versions I gave up and just use the 5 1x slots.