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

[Alex Deucher]

On Thu, Jul 23, 2020 at 4:11 PM Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote:
>
>
>
> 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@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> >>> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> >>> Cc: Christian KÃnig <christian.koenig@xxxxxxx>
> >>> Cc: Huang Rui <ray.huang@xxxxxxx>
> >>> Cc: Alex Deucher <alexdeucher@xxxxxxxxx>
> >>
> >> 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.

I started a thread internally to see if I can find a way.  FWIW,
pre-ZEN parts also support p2p DMA, but only for writes.  If I can get
a definitive list, maybe we could switch to a blacklist for the old
ones?

Alex

>
> Logan