It's registers are documented yes, but from the point of how it actually works. How data flows, how it is buffered internally, how it is timed etc it is absolute mystery. Yes, you can configure various parameters, but you can't for example skip functional blocks and get data quicker. It is unknown if it is all done with hardcoded static logic blocks, some sort of purpose made vector cpu and firmware. No one knows(or those that do don't say).

I worked with rk3566, but I believe rk3399 has the earlier version of the exact same isp. The version in rk3566 is 1.1. There are also versions 1.0 and just 1. All are documented in TRMs in the same way.