A few years ago, an interesting kickstarter projected popped up:
https://www.kickstarter.com/projects/725991125/open-source-graphics-processor-gpu
The goal was to publish source code to a GPU that is register compatible with the late 90's era
Number Nine "Ticket To Ride IV" GPU. Although the project didn't meet its funding goal, the person behind it later published the code on github.
https://github.com/asicguy/gplgpu
Although this is an older design, it has a lots that is worth studying. It's instructive to compare it to the VideoCore GPU that I walked through in a
previous post. While there are some fundamental differences, there are surprising number of similarities, which shows how modern GPUs evolved from earlier ones.