Finally managed to get opencl support in darktable;
@asn, although I made progress following your tips, ultimately I got stalled on version number of the DRM.
I bit the bullet and installed amdgpu-pro version 17.50 using the install script, which built a new kernel and ramdisk image.
I followed the directions on this page
The speed-up factor in use on darktable is only slight over 2, however, which is considerably less than the factor I achieved using my corei7 CPU laptop with opencl. I don’t know the exact factor, but I do know that I was exporting a jpg for a 4000x6000 image in about 8 seconds. Unfortunately, exports using my new computer are still taking nearly a minute each.
Interestingly my stand-alone video card (yes a pretty low-spec Radeon R5 230 was not detected as a usable device at all. Given it has but two GPU units (versus 8 on the CPU die - I have an AMD A12) means this is a moot point.
Mileage _will vary, but maybe this tale will help someone with similar hardware & OS version (Ubuntu Studio 17.10)