Linux and open-source in general seems to have a problem with lacking high-quality developer documentation, but for the open-source GPU drivers when it comes to DRM, X.Org, and Mesa/Gallium3D it's even worse. There isn't much documentation at all so developers are really just forced to dive into the code, figure out the APIs, and hope it all works out. Or at least base their driver off a simple driver to get started.
Laurent Pinchart while writing a Renesas DRM driver ended up writing a fairly thorough DRM framework guide. It doesn't provide 100% coverage of the kernel's Direct Rendering Manager, but it appears better than some of the limited documentation that has been written in the past.
To ensure the documentation doesn't get out of date, he proposes that any patch from a developer that changes the API must be accompanied by updated documentation or else it won't be merged.
Those wishing to look at the documentation can find it on the mailing list. While this document was published on Wednesday, two days later no other upstream DRM developers have commented on it...