We are barely figuring this our ourselves. First off, SoC vendors usually don’t have this documentation in the first place, and we have to reverse engineer their SDK to understand how they put the system together. SoC vendors also almost never follow Debian policies so we have to do a lot of hacking to make the system working. There is also no guaranteed stability between SDK releases, so you cannot expect your hack will work forever.
You are welcome to check our hacks to figure out how we did those. It’s all public:
We have no involvement to the development of SystemReady BIOS so we cannot answer any of those questions. We should have the source code eventually and that’s when we can make adjustment.