EXA for radeon experimental patch

[Benjamin Herrenschmidt]

> I'n not taking about migrating the whole framebuffer. Usually the composite 
> operation is restricted to a window that quite a bit smaller, and all you 
> need is a temporary pixmap in main memory that contains a copy of the window. 
> DownloadFromScreen makes exactly this cheap.

It won't do it in many cases afaik. BTW. Somebody knows if radeon
HOSTDATABLIT can be used to blit to host memory or only from host
memory ?