Re: [PATCH] PCI/P2PDMA: Add AMD Zen 2 root complex to the list of allowed bridges

[Logan Gunthorpe <logang@deltatee.com>]

On 2020-07-23 1:57 p.m., Bjorn Helgaas wrote:
> [+cc Andrew, Armen, hpa]
> 
> On Thu, Jul 23, 2020 at 02:01:17PM -0400, Alex Deucher wrote:
>> On Thu, Jul 23, 2020 at 1:43 PM Logan Gunthorpe <logang@deltatee.com> wrote:
>>>
>>> The AMD Zen 2 root complex (Starship/Matisse) was tested for P2PDMA
>>> transactions between root ports and found to work. Therefore add it
>>> to the list.
>>>
>>> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
>>> Cc: Bjorn Helgaas <bhelgaas@google.com>
>>> Cc: Christian König <christian.koenig@amd.com>
>>> Cc: Huang Rui <ray.huang@amd.com>
>>> Cc: Alex Deucher <alexdeucher@gmail.com>
>>
>> Starting with Zen, all AMD platforms support P2P for reads and writes.
> 
> What's the plan for getting out of the cycle of "update this list for
> every new chip"?  Any new _DSMs planned, for instance?

Well there was an effort to add capabilities in the PCI spec to describe
this but, as far as I know, they never got anywhere, and hardware still
doesn't self describe with this.

> A continuous trickle of updates like this is not really appealing.  So
> far we have:
> 
>   7d5b10fcb81e ("PCI/P2PDMA: Add AMD Zen Raven and Renoir Root Ports to whitelist")
>   7b94b53db34f ("PCI/P2PDMA: Add Intel Sky Lake-E Root Ports B, C, D to the whitelist")
>   bc123a515cb7 ("PCI/P2PDMA: Add Intel SkyLake-E to the whitelist")
>   494d63b0d5d0 ("PCI/P2PDMA: Whitelist some Intel host bridges")
>   0f97da831026 ("PCI/P2PDMA: Allow P2P DMA between any devices under AMD ZEN Root Complex")
> 
> And that's just from the last year, not including this patch.

Yes, it's not ideal. But most of these are adding old devices as people
test and care about running on those platforms -- a lot of this is
bootstrapping the list. I'd expect this to slow down a bit as by now we
have hopefully got a lot of the existing platforms people care about.
But we'd still probably expect to be adding a new Intel and AMD devices
about once a year as they produce new hardware designs.

Unless, the Intel and AMD folks know of a way to detect this, or even to
query if a root complex is newer than a certain generation, I'm not sure
what else we can do here.

Logan