Alan Cox wrote:
On Iau, 2004-10-07 at 15:40, Ville Syrjl wrote:
Why can't we make AGP memory cached? Wouldn't it be enought to flush the
caches at some critical points?
Possibly although it is not trivial to see how we get that right,
especially with the 4Mb kernel maps. The x86 processor cannot handle a
page being mapped both cached and uncached at once. Even more excitingly
this includes implicit suprise caching by the CPU (speculative and
hardware prefetch). This is more a TLB than cache issue.
I was playing around with DirectFB and AGP some years ago and enabling
write-back caching didn't seem to have any side effects. Without caching
AGP is almost as bad as video memory for sw fallbacks.
It would be nice to get cacheable AGP but that means some fairly hairy
things especially on SMP systems. Pulling a page out of AGP doing a TLB
shootdown for it, and then putting it back after wbinvd might work.
One to talk to the processor people about.
I think the traditional path is to pull pages into out of the agp table.
So, blit to agp, then remove those pages from the gart table and (somehow)
make them cacheable and allow the client to access them directly.
Keith
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