[Linux-fbdev-devel] Re: radeon, apertures & memory mapping

[Jon Smirl]

On Sun, 13 Mar 2005 19:47:14 +0200, Ville Syrjälä <syrjala at sci.fi> wrote:
> I don't understand why we have "GART memory" anyway. It's just main memory
> and I don't see any point going through the GART to access it with the
> CPU. Only the graphics card needs to use the GART.

I see no need to for the CPU to go through the GART either. The main
CPU page tables can provide the same rearranging that the GART does.

We do need specially marked GART memory because of caching issues. If
the CPU writes to GART RAM the write may still be on the CPU chip in a
cache. We have to make sure it gets pushed into physical memory so
that the GPU can see it.

The best model would be to chuck the AGP/PCI Express interface on the
board and have a hyperchannel instead. Hyperchannel provides full
cache consistency without all of these flushing problems. The GPU
really is another specialized CPU, give it a CPU class memory
interface.

-- 
Jon Smirl
jonsmirl at gmail.com