David Airlie began the PRIME code quite a while ago for the Direct Rendering Manager code in the Linux kernel to allow for GPU offloading in notebook/desktop multi-GPU configurations. The proof-of-concept code was published in March of 2010, but was only actively worked on for a short period of time. (See that earlier article for more details on Airlie's PRIME implementation.)
The good news to report now is that Airlie is back to working on the code. See this Git repository branch for those interested.