Open-Source GPU Architecture from the University of Wisconsin-Madison

Isn't University of Wisconsin Milwaukee the folks that have the really good cryptography courses?

the ISA isn't open?

One set you might find interesting, with likely expired patents, are the once-revolutionary SGI systems. Their O2 workstations and InfiniteReality systems specifically. Systems like O2 and Octane were used for movies such as Fight Club plus visualization applications (and Quake). The InfiniteReality technology was used in the Final Fantasy movie albeit with a lot of it. These might be copied in an open effort.

Note: To be clear, these aren't GPGPU systems but straight GPU's. I think MIMD architectures for general-purpose have proven better than GPU's. Just less investment and good one's mostly get acquired. FPGA's can be better, too, with good HLS tools and S-ASIC/ASIC conversion for better performance later.

http://webstaff.itn.liu.se/~matco/TNM053/Papers/p45-kilgard....

https://en.wikipedia.org/wiki/InfiniteReality

Far as internal architecture of regular GPU's, just search for it in Google with terms like that to find it. There should be descriptions of their architectures and plenty that came before them.