render improvements

[Alan Cox]

> The speedup using a recent x.org server is less, we currently suspect that 
> framebuffer reads are the limiting factor now. We're not sure why they are so 
> slow, but I'm hope on the list can enlighten us. My current suspicion is that 
> they are not cached in the CPU at all and always go directly to graphics 
> memory.

Correct for PCI/AGP

> We can try to reduce framebuffer reads to speed this up (I have some ideas how 
> to do this), but we wonder if there are ways to speed up the reads from the 
> framebuffer, at least from offscreen memory?

Use DRI hooks and DMA. That will give you full PCI/AGP bandwidth copying
of tiles from the card to the CPU and will be faster for larger blocks.
For small blocks the best you can do is ensure you do maximal sized
transfers on natural alignment with an x86. 

Soreen did some timing work on this btw

Alan