Christmas Miracle: Intel Mesa Driver Tessellation For Ivy Bridge & Haswell

A few days ago Intel landed OpenGL tessellation support in their open-source driver as required by OpenGL 4. However, this initial implementation was limited to support Intel's Broadwell hardware and newer. With new patches, that is now changing.

Intel's Kenneth Graunke wrote excited today on the Mesa-dev list.

This morning, I woke up and somehow "knew" what was causing my HS GPU hangs on Gen7/7.5. It turns out I was (completely) wrong, but through some miraculous series of illogical leaps, I arrived at a solution anyway.

I don't honestly know how I got it working on Christmas Eve after failing to figure it out for months on end. After exhausting every bit of documentation and every tool available, and finding zero information, somehow randomly flailing in the dark resulted in a solution, today of all days. Honestly, I had pretty much no hope for figuring this out, so I'm relieved to have it working at last...

It turns out that setting interleave on the EOT URB write does bad things. Fixing this fixed all the GPU hangs when releasing inputs one at a time, I then added back the ability to release inputs in pairs, which caused more GPU hangs. It turned out I needed to be more careful and enable both halves.

Everything seems to be working just fine now, so let's turn it on.

This set of patches, assuming the other Mesa developers sign off on it, would allow Ivy Bridge and Haswell to have tessellation support and thus OpenGL 4 access. Hopefully everything is all good and this will get into Mesa Git ASAP.