radeon, apertures & memory mapping

[Ville Syrjälä]

On Mon, Mar 14, 2005 at 10:48:19AM +1100, Benjamin Herrenschmidt wrote:
> 
> > > That shouldn't matter the page brought in would be for a speculative
> > > read and never accessed. It should just fall out of the cache and not
> > > be written back. There is only one cachable mapping. In this model
> > > writes are always followed by a flush before telling the GPU to access
> > > the memory that has just been written.
> > 
> > What about this scenario?
> > 
> > Speculative read -> AGP master writes new data -> CPU has invalid data in 
> > cache :(
> 
> First, we must be very careful with AGP master writes. I don't know if
> we do a lot of them currently, but I know a collection of north bridges
> that do not support them.

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?

> 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.

:(

-- 
Ville Syrjälä
syrjala at sci.fi
http://www.sci.fi/~syrjala/