For some AMD cards, the documentation is fairly complete. I think the main problem is that end-to-end support (OpenGL -> X server integration -> kernel module)

For some AMD cards, the documentation is fairly complete. I think the main problem is that end-to-end support (OpenGL -> X server integration -> kernel module) is simply a huge task these days, and the Mesa codebase in particular is massive. I'm considering helping out in that effort, as my background in working with game console GPUs might be an advantage. Depends how much time and money I can realistically invest in it.