render improvements

[Alan Cox]

On Maw, 2005-04-19 at 16:01, Owen Taylor wrote:
> Well, no, it's just mildly bad, assuming that you have enough free
> scratch space to copy the pixmap into video ram immediately before
> compositing it... if compositing is just limited by system => video
> bandwidth, that still is pretty good.

You might be better off tiling for large operations anyway. The loop
then becomes close to maximally efficient when using DMA and DRI to
retrieve tiles from the video ram

	kick DMA off for tile n+1
	Render tile n
	polled wait DMA complete n+1

overlapping full speed bus transfers with software rendering.