I know both Ryzen and vega are both designed for HBM, using infinity fabric/GMI links. I also think an APU is coming soon that uses all of those technologies, if it does then it will use 2GB of HBM2, with significantly less than 200gb/s bandwidth, HBCC will ensure up to 4GB of graphics memory can be used with very little performance penalty vs 4GB vram.
This is technically possible and cost effective to be doing this with Raven Ridge in terms of BOM/die cost.
My only concern is the engineering man hours required to put together this ambitious architecture might be beyond AMDs budget this year, with Ryzen, ryzen 2, Naples, Vega 10 11?, polaris refresh, software optimisations and likely manymore projects ive missed, thats alot of engineering there with a finite amount of engineers to cover them, whilst keeping quality control and time constraints in check.
We have already seen evidence of this difficulty with the rushed rx480 and ryzen launches.
My argument is not that it is not feasible or that AMD dont have the intention to use HBM with APUs, its i dont think its a priority over many other projects.
AMD will still have a graphics lead in APUs without it.