The open source driver doesn't use VMCS, it instead puts aside a fixed block of memory (typically 256 MiB, which I think is also a limit due to some hardware bugs) for use through the Linux Contiguous Memory Allocator (CMA) that it then draws from.

VMCS only comes into the picture if you use video decode, but I think Dave Stevenson from the foundation hacked the firmware side to support importing Linux allocated memory blocks into VMCS so that you can do zero-copy decode and import into EGL (or more likely HVS, the EGL support for the formats is pretty limited).

(I really liked the design of the HVS - having pretty much scripted planes is a fresh approach over similar hardware blocks that have a fixed number of planes each with own idiosyncracies and limitations)