[PATCH v2 7/9] PCI: ACPI: Detect PCIe root ports that are used for tunneling

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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 the `is_tunneling` bit in `struct pci_device`.

Link: https://www.usb.org/document-library/usb4r-specification-v20
      USB4 V2 with Errata and ECN through June 2023
Link: https://learn.microsoft.com/en-us/windows-hardware/design/component-guidelines/usb4-acpi-requirements#port-mapping-_dsd-for-usb-3x-and-pcie
Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx>
 drivers/pci/pci-acpi.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c
index 05b7357bd258..81dbfd335f34 100644
--- a/drivers/pci/pci-acpi.c
+++ b/drivers/pci/pci-acpi.c
@@ -1414,12 +1414,28 @@ static void pci_acpi_set_external_facing(struct pci_dev *dev)
 		dev->external_facing = 1;
+static void pci_acpi_set_tunneling(struct pci_dev *dev)
+	if (!pci_is_pcie(dev))
+		return;
+	switch (pci_pcie_type(dev)) {
+		dev->is_tunneled = device_property_present(&dev->dev, "usb4-host-interface");
+		break;
+	default:
+		return;
+	}
 void pci_acpi_setup(struct device *dev, struct acpi_device *adev)
 	struct pci_dev *pci_dev = to_pci_dev(dev);
 	pci_acpi_optimize_delay(pci_dev, adev->handle);
+	pci_acpi_set_tunneling(pci_dev);
 	pci_acpi_add_pm_notifier(adev, pci_dev);

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