Frequently it's come up in our forums of how to get involved with the open-source ATI graphics driver or how exactly the Linux drivers work, but AMD's Alex Deucher has begun writing a series of blog posts that cover this topic quite in-depth -- down to describing things on a per-function basis.
Alex, who is responsible for a great deal of the open-source ATI driver work, began by publishing a blog post entitled Understanding GPUs From The Ground-Up. This nearly 2,000 word post begins covering how the Radeon driver works and where all the magic happens beginning with the driver's initialization process for bringing up the card.
Immediately following that post was the second part, which covers Notes On The Radeon Display Hardware. This covers the way that displays are handled by the ATI graphics cards. This series of posts is focusing upon the ATI Evergreen (Radeon HD 5000 series) support upbringing that is currently in progress, but much of the information applies to previous ATI Radeon graphics card generations too.
Once Alex continues in this series of posts, we'll be sure to provide an update. If you have any questions about the content, Alex is one of the AMD members of the Phoronix Forums. If you want some more reading, you may be interested in this DRM documentation written last year by Intel's Jesse Barnes that covers newer Linux DRM technologies like TTM/GEM/KMS.