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 [EMAIL PROTECTED] http://www.sci.fi/~syrjala/ ------------------------------------------------------- SF email is sponsored by - The IT Product Guide Read honest & candid reviews on hundreds of IT Products from real users. Discover which products truly live up to the hype. Start reading now. http://ads.osdn.com/?ad_ide95&alloc_id396&op=click -- _______________________________________________ Dri-devel mailing list Dri-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/dri-devel