Created attachment 123045 [details]
dmesg

Full dmesg attached. I have a Power Mac G5 (late 2005, PCIe) with a Radeon HD
6450 (CAICOS, x86 BIOS) installed on the PCIe x16 slot, and I get this error on
boot. I also have a GeForce 6600 (Open Firmware) on a PCIe x8 slot, which works
fine, minus the usual nouveau caveats. I have no idea if this is related at
all, but the x16 slot is directly connected to the U4 northbridge, and is
capable of bypassing the DART IOMMU for 64-bit DMA capable devices (like the
Radeon). The other slots are connected to a PCIe bridge on the HyperTransport
bus, and DMA always goes through the DART. If needed, I can provide ssh access
to the machine.