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

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Date:   Fri, 24 Jul 2020 09:56:39 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Alex Deucher <alexdeucher@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Christian König <christian.koenig@....com>,
        Huang Rui <ray.huang@....com>,
        Andrew Maier <andrew.maier@...eticom.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Jonathan Cameron <jonathan.cameron@...wei.com>
Subject: Re: [PATCH] PCI/P2PDMA: Add AMD Zen 2 root complex to the list of
 allowed bridges

[+cc Jonathan]

On 2020-07-24 9:06 a.m., Bjorn Helgaas wrote:
> On Thu, Jul 23, 2020 at 02:10:52PM -0600, Logan Gunthorpe wrote:
>> On 2020-07-23 1:57 p.m., Bjorn Helgaas wrote:
>>> 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@...tatee.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@...tatee.com>
>>>>> Cc: Bjorn Helgaas <bhelgaas@...gle.com>
>>>>> Cc: Christian König <christian.koenig@....com>
>>>>> Cc: Huang Rui <ray.huang@....com>
>>>>> Cc: Alex Deucher <alexdeucher@...il.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.
> 
> Any idea what happened?  Is there hope for the future?  I'm really not
> happy about signing up for open-ended device-specific patches like
> this.  It's certainly not in the plug and play spirit that has made
> PCI successful.  I know, preaching to the choir here.

Agreed, though I'm not really hooked into the PCI SIG. The last email I
got about this was an RFC from Jonathan Cameron in late 2018. I've CC'd
him here, maybe he'll have a bit more insight.

Logan