Ah, thanks for this one. However I already got it from Firefly's forum. I didn't know about rockchip.fr though. Nice find ! That said, libmpp implementation seems to define 100+ registers for JPEG decoding, while this doc seems to document at most ~10 registers.
I'll try to do something with this, if I see nobody documenting this in a few weeks, after the kernel 4.10 release. There's currently a lot of people interested by the VPU, so Rockchip might provide more documentation before that.