The current RPi source tree includes a hwcomposer impl. It's not used for compositing because we haven't extended it with support for ColorLayers. However, even if we did, it's implemented using the RPi's 3d unit so probably isn't going to help power or rendering efficiency much. The RPi's chip includes a video scaler unit (HVS) that should be just right to target for a hwcomposer/composer2d implementation. I don't know for sure that it's possible to implement color layers using HVS but I'm pretty sure we could. One small issue is that the ARM core can only talk to HVS by using the vchiq interface (AFAIK anyway), since HVS is on the VideoCore silicon. We'd need to ensure that gralloc surfaces are allocated in such a way that HVS can read them.