That doesn't really cover the processor under discussion here, only the shader cores called the QPUs.

This work and the processor under discussion is another core called the VPU that unfortunately gets confused because with the default firmware it runs the majority of the OpenGL driver stack, controlling the QPUs.

> Turns out there is some pretty good architecture documentation: https://docs.broadcom.com/doc/12358545 .. but that only describes the 3D capabilities, not the whole register map.

This documentation is for the VideoCore IV. The BCM2711 SOC that is used in the Raspberry Pi 4 has a VideoCore VI.