If I was to decide, I would suggest:

>2. Add an extra step to the drivers using shaders to split the frame up into >separate fields in vlVdpVideoSurfaceGallium
>(src/gallium/state_trackers/vdpau/surface.c) for chips that don't support field >based decode.  This adds an extra copy and someone needs to write the code.  With >that done, the extension can be enabled on all asics.

Reason:
- No application needs to be changed and all methods would be supported by this
hardware, too. 
- Our workarounds + extra code to handle the env would also easily be 50+ lines
:-) and that per application (kodi, mpv, ...)

So @Christian happy for pointers on how to get it going.