radeon, apertures & memory mapping

[Jon Smirl]

On Mon, 14 Mar 2005 10:48:19 +1100, Benjamin Herrenschmidt
<benh at kernel.crashing.org> 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 :(
> 

You need to reverse the cache flush process if you are going to read
data written by the GPU.

1) Make sure GPU is finished writing
2) flush your cache
3) read AGP memory like normal RAM.

-- 
Jon Smirl
jonsmirl at gmail.com