Re: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory

[Dan Williams]

On Wed, Mar 14, 2018 at 12:03 PM, Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote:
>
>
> On 14/03/18 12:51 PM, Bjorn Helgaas wrote:
>> You are focused on PCIe systems, and in those systems, most topologies
>> do have an upstream switch, which means two upstream bridges.  I'm
>> trying to remove that assumption because I don't think there's a
>> requirement for it in the spec.  Enforcing this assumption complicates
>> the code and makes it harder to understand because the reader says
>> "huh, I know peer-to-peer DMA should work inside any PCI hierarchy*,
>> so why do we need these two bridges?"
>
> Yes, as I've said, we focused on being behind a single PCIe Switch
> because it's easier and vaguely safer (we *know* switches will work but
> other types of topology we have to assume will work based on the spec).
> Also, I have my doubts that anyone will ever have a use for this with
> non-PCIe devices.

P2P over PCI/PCI-X is quite common in devices like raid controllers.
It would be useful if those configurations were not left behind so
that Linux could feasibly deploy offload code to a controller in the
PCI domain.