linux-kernel - [PATCH 2/2] PCI: Ignore PCIe ports used for tunneling in pcie_bandwidth

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Message-ID: <20231031133438.5299-2-mario.limonciello@amd.com>
Date:   Tue, 31 Oct 2023 08:34:38 -0500
From:   Mario Limonciello <mario.limonciello@....com>
To:     <bhelgaas@...gle.com>, <mika.westerberg@...ux.intel.com>
CC:     <andreas.noever@...il.com>, <michael.jamet@...el.com>,
        <YehezkelShB@...il.com>, <linux-pci@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-usb@...r.kernel.org>,
        <Alexander.Deucher@....com>,
        Mario Limonciello <mario.limonciello@....com>
Subject: [PATCH 2/2] PCI: Ignore PCIe ports used for tunneling in pcie_bandwidth_available()

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.

In reality these ports speed is controlled by the fabric implementation.

Downstream drivers such as amdgpu which utilize pcie_bandwidth_available()
to program the device will always find the PCIe ports used for
tunneling as a limiting factor and may make incorrect decisions.

To prevent problems in downstream drivers check explicitly for ports
being used for PCIe tunneling and skip them when looking for bandwidth
limitations.

2 types of devices are detected:
1) PCIe root port used for PCIe tunneling
2) Intel Thunderbolt 3 bridge

Downstream drivers could make this change on their own but then they
wouldn't be able to detect other potential speed bottlenecks.

Link: https://lore.kernel.org/linux-pci/7ad4b2ce-4ee4-429d-b5db-3dfc360f4c3e@amd.com/
Link: https://www.usb.org/document-library/usb4r-specification-v20
      USB4 V2 with Errata and ECN through June 2023 - CLEAN p710
Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2925
Signed-off-by: Mario Limonciello <mario.limonciello@....com>
---
 drivers/pci/pci.c | 41 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 59c01d68c6d5..4a7dc9c2b8f4 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -6223,6 +6223,40 @@ int pcie_set_mps(struct pci_dev *dev, int mps)
 }
 EXPORT_SYMBOL(pcie_set_mps);
 
+/**
+ * pcie_is_tunneling_port - Check if a PCI device is used for TBT3/USB4 tunneling
+ * @dev: PCI device to check
+ *
+ * Returns true if the device is used for PCIe tunneling, false otherwise.
+ */
+static bool
+pcie_is_tunneling_port(struct pci_dev *pdev)
+{
+	struct device_link *link;
+	struct pci_dev *supplier;
+
+	/* Intel TBT3 bridge */
+	if (pdev->is_thunderbolt)
+		return true;
+
+	if (!pci_is_pcie(pdev))
+		return false;
+
+	if (pci_pcie_type(pdev) != PCI_EXP_TYPE_ROOT_PORT)
+		return false;
+
+	/* PCIe root port used for tunneling linked to USB4 router */
+	list_for_each_entry(link, &pdev->dev.links.suppliers, c_node) {
+		supplier = to_pci_dev(link->supplier);
+		if (!supplier)
+			continue;
+		if (supplier->class == PCI_CLASS_SERIAL_USB_USB4)
+			return true;
+	}
+
+	return false;
+}
+
 /**
  * pcie_bandwidth_available - determine minimum link settings of a PCIe
  *			      device and its bandwidth limitation
@@ -6236,6 +6270,8 @@ EXPORT_SYMBOL(pcie_set_mps);
  * 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 function excludes root ports and bridges used for USB4 and TBT3 tunneling.
  */
 u32 pcie_bandwidth_available(struct pci_dev *dev, struct pci_dev **limiting_dev,
 			     enum pci_bus_speed *speed,
@@ -6254,6 +6290,10 @@ u32 pcie_bandwidth_available(struct pci_dev *dev, struct pci_dev **limiting_dev,
 	bw = 0;
 
 	while (dev) {
+		/* skip root ports and bridges used for tunneling */
+		if (pcie_is_tunneling_port(dev))
+			goto skip;
+
 		pcie_capability_read_word(dev, PCI_EXP_LNKSTA, &lnksta);
 
 		next_speed = pcie_link_speed[lnksta & PCI_EXP_LNKSTA_CLS];
@@ -6274,6 +6314,7 @@ u32 pcie_bandwidth_available(struct pci_dev *dev, struct pci_dev **limiting_dev,
 				*width = next_width;
 		}
 
+skip:
 		dev = pci_upstream_bridge(dev);
 	}
 
-- 
2.34.1