On Sat, Aug 24, 2024 at 12:20 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: > > On Sat, Aug 24, 2024 at 07:26:35AM +0300, Mika Westerberg wrote: > > On Fri, Aug 23, 2024 at 04:12:54PM -0500, Bjorn Helgaas wrote: > > > On Fri, Aug 23, 2024 at 04:53:16PM +0000, Esther Shimanovich wrote: > > > > Some computers with CPUs that lack Thunderbolt features use discrete > > > > Thunderbolt chips to add Thunderbolt functionality. These Thunderbolt > > > > chips are located within the chassis; between the root port labeled > > > > ExternalFacingPort and the USB-C port. > > > > > > Is this a firmware defect? I asked this before, and I interpret your > > > answer of "ExternalFacingPort is not 100% accurate all of the time" as > > > "yes, this is a firmware defect." That should be part of the commit > > > log and code comments. > > > I like how Lukas explained it here: https://lore.kernel.org/all/ZstGP0EgttNAxjp2@xxxxxxxxx/ It's a bit unclear whether this is firmware implemented incorrectly or the spec not being specific enough. Being that I see this interpretation of the spec on all devices with discrete Thunderbolt chips (across different manufacturers)--that makes me think that this is an ambiguity on the spec's part. Given that, how do you suggest I modify the commit log and code comments? > > > > 2) If a root port does not have integrated Thunderbolt capabilities, but > > > > has the ExternalFacingPort ACPI property, that means the manufacturer > > > > has opted to use a discrete Thunderbolt host controller that is > > > > built into the computer. > > > > > > Unconvincing. If a Root Port has an external connector, is it > > > impossible to plug in a Thunderbolt device to that connector? I > > > assume the wires from a Root Port could be traces on a PCB to a > > > soldered-down Thunderbolt controller, OR could be wires to a connector > > > where a Thunderbolt controller could be plugged in. How could we tell > > > the difference? > > We may assume this both because of how the spec is worded, and how I've seen it implemented in the case of a discrete Thunderbolt controller, across all cases. https://learn.microsoft.com/en-us/windows-hardware/drivers/pci/dsd-for-pcie-root-ports#mapping-native-protocols-pcie-displayport-tunneled-through-usb4-to-usb4-host-routers "ExternalFacingPort" is only used when there is an externally exposed PCIe hierarchy. (Never otherwise) I don't know any other examples of externally exposed PCIe hierarchies other than Thunderbolt. Therefore I assume, that if there is "ExternalFacingPort", that means the device has Thunderbolt capabilities. If we have confirmed that the device has no integrated thunderbolt capabilities, that means the device MUST expect a discrete thunderbolt chip outside of its ExternalFacing root port. That is my understanding of this. Also let me know how I can reword here as well.
