radeon, apertures & memory mapping

[Benjamin Herrenschmidt]

> I don't think "normal" drivers do them at all. I did experiment with 
> DirectFB at one point and had it place all offscreen surfaces to AGP 
> memory. It worked really well on my hardware (G400 + VIA KT133 
> northbridge). I also tried it with PCI transfers and that too worked but 
> was naturally slower. I'd like to make DirectFB use AGP again since 32MB 
> of video memory isn't always enough.
> 
> > (Which is interesting, that means that if we want to copy something out
> > of video memory, we can't write it to AGP memory and then read it, we
> > need to actually do the blit from the CPU, good to know for our memory
> > manager. That also means that we have a problem if the video memory
> > isn't entirely accessible by the CPU ...)
> 
> What about PCI master writes? Are there bridges that don't support even 
> those?

Usually, those work. I have a list of errata of Apple bridges at least
in Darwin, some do not support PCI master writes, but it's only
apparently some never-released revisions :)

> > That's something we should probably think about doing properly: Have a
> > list of AGP "issues" (errata ?) bits that are communicated by the AGP
> > host driver to the DRM.
> > 
> > At least all the early Apple AGP bridges don't do writes, and I remember
> > we have trouble with a few x86 ones as well. There are also issues when
> > a single AGP burst crosses a page boundary, and other things like that.
> 
> :(
> 
-- 
Benjamin Herrenschmidt <benh at kernel.crashing.org>