[PATCH v3 7/7] PCI: Exclude PCIe ports used for virtual links in pcie_bandwidth_available()

[Mario Limonciello]

The USB4 spec specifies that PCIe ports that are used for tunneling
PCIe traffic over USB4 fabric will be hardcoded to advertise 2.5GT/s and
behave as a PCIe Gen1 device. The actual performance of these ports is
controlled by the fabric implementation.

Callers for pcie_bandwidth_available() will always find the PCIe ports
used for tunneling as a limiting factor potentially leading to incorrect
performance decisions.

To prevent such problems check explicitly for ports that are marked as
virtual links or as thunderbolt controllers and skip them when looking
for bandwidth limitations of the hierarchy. If the only device connected
is a port used for tunneling then report that device.

Callers to pcie_bandwidth_available() could make this change on their
own as well but then they wouldn't be able to detect other potential
speed bottlenecks from the hierarchy without duplicating
pcie_bandwidth_available() logic.

Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2925#note_2145860
Link: https://www.usb.org/document-library/usb4r-specification-v20
      USB4 V2 with Errata and ECN through June 2023
      Section 11.2.1
Signed-off-by: Mario Limonciello <mario.limonciello at amd.com>
---
v2->v3:
 * Split from previous patch version
 * Look for thunderbolt or virtual link
---
 drivers/pci/pci.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 0ff7883cc774..b1fb2258b211 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -6269,11 +6269,20 @@ static u32 pcie_calc_bw_limits(struct pci_dev *dev, u32 bw,
  * limiting_dev, speed, and width pointers are supplied) information about
  * that point.  The bandwidth returned is in Mb/s, i.e., megabits/second of
  * raw bandwidth.
+ *
+ * This excludes the bandwidth calculation that has been returned from a
+ * PCIe device that is used for transmitting tunneled PCIe traffic over a virtual
+ * link part of larger hierarchy. Examples include Thunderbolt3 and USB4 links.
+ * The calculation is excluded because the USB4 specification specifies that the
+ * max speed returned from PCIe configuration registers for the tunneling link is
+ * always PCI 1x 2.5 GT/s.  When only tunneled devices are present, the bandwidth
+ * returned is the bandwidth available from the first tunneled device.
  */
 u32 pcie_bandwidth_available(struct pci_dev *dev, struct pci_dev **limiting_dev,
 			     enum pci_bus_speed *speed,
 			     enum pcie_link_width *width)
 {
+	struct pci_dev *vdev = NULL;
 	u32 bw = 0;
 
 	if (speed)
@@ -6282,10 +6291,20 @@ u32 pcie_bandwidth_available(struct pci_dev *dev, struct pci_dev **limiting_dev,
 		*width = PCIE_LNK_WIDTH_UNKNOWN;
 
 	while (dev) {
+		if (dev->is_virtual_link || dev->is_thunderbolt) {
+			if (!vdev)
+				vdev = dev;
+			goto skip;
+		}
 		bw = pcie_calc_bw_limits(dev, bw, limiting_dev, speed, width);
+skip:
 		dev = pci_upstream_bridge(dev);
 	}
 
+	/* If nothing "faster" found on hierarchy, limit to first virtual link */
+	if (vdev && !bw)
+		bw = pcie_calc_bw_limits(vdev, bw, limiting_dev, speed, width);
+
 	return bw;
 }
 EXPORT_SYMBOL(pcie_bandwidth_available);
-- 
2.34.1