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From:   Alex Deucher <alexdeucher@gmail.com>
Date:   Wed, 25 Apr 2018 13:44:33 -0400
Message-ID: <CADnq5_O5t8aSJE6SFzyjw-6Pmba7B2aYMztVMOOQCF3drBHR5Q@mail.gmail.com>
Subject: Re: [Linaro-mm-sig] [PATCH 4/8] dma-buf: add peer2peer flag
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Daniel Vetter <daniel@ffwll.ch>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@lists.linaro.org>,
        Jerome Glisse <jglisse@redhat.com>,
        amd-gfx list <amd-gfx@lists.freedesktop.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
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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.

Alex