I used to work on mobile graphics and the android HWC stack. The scanout-time hardware was often less useful that you might think - only in dynamic scenes wher

I used to work on mobile graphics and the android HWC stack. The scanout-time hardware was often less useful that you might think - only in dynamic scenes where the GPU is otherwise idle (like playing video possibly with a static UI overlay was the premier use case). For static scenes it's more efficient to render out to a buffer (using the GPU as the scanout overlay pipes often had limited feedback capability) and just output that using overlays disabled. It didn't take many frames for that to be worth it. For apps that were animating or otherwise updating it's window, most UI toolkits used the GPU for widget rendering. And often the scanout pipes didn't hook into the (relatively large) system caches like the GPU did, so there were times it was again faster to composite the screen on the GPU to a single scanout buffer than flush already cached data, the get the scanout hardware to read it back from the memory bus. And there weren't as cheap as people thought - one stat I remember was that the total area of the GPU on the omap4 platform was smaller than the display pipes. Though that is now a pretty old chip, and always had a bit of focus on "multimedia".