Thanks for doing so much investigating here. I'm sad to hear Arc isn't working for Power yet, but hopefully Intel's driver team will clean up the issues soon. To contribute a little that I've learned so far:
- ReBAR's reportedly a PCIe 3.0 feature, so I'm a little surprised Blackbird and Talos don't support it. The feature is important for maximizing hardware performance on Arc / Alchemist, but I don't know how drastic a difference it would make for the less game-centric 3D use cases of a typical Power9 build.
- The video encoding and decoding are not impacted by ReBAR at all. The A750 in my Windows machine is able to transcode a 1080p MKV from Blu-ray quality to a rough equivalent of x264's Slow quality at north of 325 frames per second. Better still, the video encoder is a constant for the product line - to my knowledge the A380 is just as proficient for encoding and decoding video as an A770. Given Power9's relatively pokey SIMD and the agonizing wait we've endured to see x264 adopt optimized vector code into the codebase, a $140 USD Arc A380 could improve video encoding on the platform by a factor of ten, and more in the case of h.265 and AV1. Even with ReBAR disabled, that card would probably deliver performance north of a Radeon RX 560.
- Arc also benefits from PCIe link state power management - would you need to pass i915.aspm=0 until someone at Intel manages to get that covered as well?
