On Mon, May 11, 2020 at 4:25 PM Rui Salvaterra rsalvaterra@gmail.com wrote:
A segunda, 11/05/2020, 21:21, Alex Deucher alexdeucher@gmail.com escreveu:
Note there is no loss of functionality here, at least on radeon hardware. It just comes down to which MMU gets used for access to system memory, the AGP MMU on the chipset or the MMU built into the GPU. On powerpc hardware, AGP has been particularly unstable, and IIRC, AGP has been disabled by default on radeon on powerpc for a while.
So this basically just drops support for the AGP GART? What happens to the AGP signalling rates (beyond the base rate)?
I don't remember enough of the details, but I strongly doubt it was related to which MMU was used per se. On r1xx/r2xx parts, AGP was effectively the non-snooped route to memory and the internal MMU only provided snooped (coherent) access to memory. That and the limited TLB space are probably want limited performance in that case. I don't recall what sort of TLBs the chipset GART tables provided. On r3xx and newer the, on-chip MMU supported both snooped and unsnooped transactions and had more TLB space so the difference wasn't significant IIRC.
Alex