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

Lucas Stach
2018-05-04T12:45:37Z

Am Mittwoch, den 25.04.2018, 13:44 -0400 schrieb Alex Deucher:
> On Wed, Apr 25, 2018 at 2:41 AM, Christoph Hellwig <hch@infradead.org
> > 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.

PCI-X (as in the thing which as all the rage before PCIe) added a
attribute phase to each transaction where the requestor can enable
relaxed ordering and/or no-snoop on a transaction basis. As those are
strictly performance optimizations the root-complex isn't required to
honor those attributes, but implementations that care about performance
usually will.

Regards,
Lucas