Re: Not coherent cache DMA for G3/G4 CPUs: clarification needed
On Fri, Apr 21, 2006 at 07:06:13AM +1000, Benjamin Herrenschmidt wrote:
> Unfortunately, he has to do things a bit differently. He can't afford to
> have the kernel BAT mapping cover his non-cacheable pages. Thus he needs
> a reserved pool. Last I looked at our coherent code, it didn't reserve
> memory at all, just address space, thus assuming the CPU can handle
> having both a caheable and a non-cacheable mapping of the same pages...
> (On 6xx this is deadly even if you don't access those cacheable pages
> because the CPU prefetch may do it for you).
Ben, is this _real_ problem on 6xx or just a theory? Does 6xx actually
prefetch beyond page boundary?
So far, all "prefetching" I saw which broke non-coherent DMA was not
due to the CPU doing prefetching, but _software_ prefetching being
too aggressive.
--
Eugene
Reply to: