Comment 5 for bug 1109963

There are quite a lot of moving parts to this, ranging in difficulty from an estimated ‘pretty easy’ to ‘blocked until we're running a Mir session compositor under a Mir system compositor’.

The first step would be for the system compositor to not composite when there's no need to - which should be all the time when there's not an active transition occurring. That's the easy one.

Then we have the X server copy-to-Mir overhead. We can't reasonably remove this¹ because X's rendering model is persistent. Once we're running under a Mir session compositor we can run XMir without a root window and have each X window backed by its own Mir surface. For GL clients this would allow us to pass the Mir buffer all the way through DRI2, eliminating X's buffer copy.

¹: Although we could optimise it somewhat by Mir telling the client how old the buffer its receiving is. Then XMir could track damage between frames, and only copy the diff.