On Wed, Apr 25, 2018 at 2:41 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > On Wed, Apr 25, 2018 at 02:24:36AM -0400, Alex Deucher wrote: >> > It has a non-coherent transaction mode (which the chipset can opt to >> > not implement and still flush), to make sure the AGP horror show >> > doesn't happen again and GPU folks are happy with PCIe. That's at >> > least my understanding from digging around in amd the last time we had >> > coherency issues between intel and amd gpus. GPUs have some bits >> > somewhere (in the pagetables, or in the buffer object description >> > table created by userspace) to control that stuff. >> >> Right. We have a bit in the GPU page table entries that determines >> whether we snoop the CPU's cache or not. > > I can see how that works with the GPU on the same SOC or SOC set as the > CPU. But how is that going to work for a GPU that is a plain old PCIe > card? The cache snooping in that case is happening in the PCIe root > complex. I'm not a pci expert, but as far as I know, the device sends either a snooped or non-snooped transaction on the bus. I think the transaction descriptor supports a no snoop attribute. Our GPUs have supported this feature for probably 20 years if not more, going back to PCI. Using non-snooped transactions have lower latency and faster throughput compared to snooped transactions. Alex
