Re: [Linaro-mm-sig] [PATCH 4/8] dma-buf: add peer2peer flag

[Alex Deucher]

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