It's going to be a long process. What I have to work with to start is basically sample code. Talking to the engineers who wrote the code drops we've seen released from Broadcom so far, they're happy to tell me about the clever things they did (their IR is pretty cool for the target subset of their architecture they chose, and it makes instruction scheduling and register allocation *really* easy), but I've had universal encouragement so far to throw it all away and start over.
So far, I'm just beginning. I'm still working on getting a useful development environment set up and building my first bits of stub DRM code. There are a lot of open questions still as to how we'll manage the transition from having most of the graphics hardware communication managed by the VPU to having it run on the ARM (since the VPU code is a firmware blob currently, we have to be careful to figure out when it will stomp on various bits of hardware as I incrementally take over things that used to be its job).
I'll have repos up as soon as I have some code that does anything.