Re: [PATCH v3 5/7] PCI: ACPI: Detect PCIe root ports that are used for tunneling

[Mika Westerberg <mika.westerberg@linux.intel.com>]

Hi Mario,

On Wed, Nov 15, 2023 at 11:08:43AM -0600, Mario Limonciello wrote:
> On 11/15/2023 04:40, Mika Westerberg wrote:
> > Hi Mario,
> > 
> > On Tue, Nov 14, 2023 at 02:07:53PM -0600, Mario Limonciello wrote:
> > > USB4 routers support a feature called "PCIe tunneling". This
> > > allows PCIe traffic to be transmitted over USB4 fabric.
> > > 
> > > PCIe root ports that are used in this fashion can be discovered
> > > by device specific data that specifies the USB4 router they are
> > > connected to. For the PCI core, the specific connection information
> > > doesn't matter, but it's interesting to know that this root port is
> > > used for tunneling traffic. This will allow other decisions to be
> > > made based upon it.
> > > 
> > > Detect the `usb4-host-interface` _DSD and if it's found save it
> > > into a new `is_virtual_link` bit in `struct pci_device`.
> > 
> > While this is fine for the "first" tunneled link, this does not take
> > into account possible other "virtual" links that lead to the endpoint in
> > question. Typically for eGPU it only makes sense to plug it directly to
> > the host but say there is a USB4 hub (with PCIe tunneling capabilities)
> > in the middle. Now the link from the hub to the eGPU that is also
> > "virtual" is not marked as such and the bandwidth calculations may not
> > get what is expected.
> 
> Right; you mentioned the DVSEC available for hubs in this case.  As I don't
> have one of these to validate it works properly I was thinking that should
> be a follow up.
> 
> If you think it should be part of the same series I'll add it, but I'd ask
> if you can please check I did it right on one that reports the DVSEC?

I don't think it should be part of this series. I just checked and DVSEC
is only required for hosts so kind of hardware equivalent for the _DSD
property you are using here. For hubs there is no such luxury
unfortunately.

I think I do have hardware here with the DVSEC in place so if you
decide to add it, I should be able to try it.