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. It should be possible to map the PCIe ports that go over USB4 links through router port operation "Get PCIe Downstream Entry Mapping" and for the Thunderbolt 3 there is the DROM entries (I believe Lukas has patches for this part already) but I guess it is outside of the scope of this series. Out of curiosity I tried to look in Windows documentation if there is such interface for GPUs as we have in Linux but could not find (which does not mean it does not exist, though).
