I recently upgraded a Ubuntu 14.04/fglrx Asrock H97 rig to 16.04.3/AMDGPU-PRO 17.40 and kernel 4.10.5. The cards are all Tonga (R9 380 & 380x).
I encountered issues with powerplay not working where no active clock shows in pp_dpm_*, and I get powerplay "failed to send message" errors.
I tried various kernel versions up to 4.14.5, different module parameters like ppfeaturemask=1, I upgraded the motherboard BIOS, and tried different VGA BIOS versions on the cards.
After 2 days of experimenting, I figured out that powerplay won't work with a card plugged into the 16x motherboard slot. When I plug 1 or more cards into the 1x slots using 16x-1x risers, powerplay works fine.
I also have an Asus z87 motherboard, and Tonga cards work fine in the 16x slots on that board with AMDGPU-PRO 17.40 and kernel 4.10.5.