[PATCH v4] PCI: Check for PCIe downtraining conditions

[Alexandru Gagniuc <mr.nuke.me@gmail.com>]

PCIe downtraining happens when both the device and PCIe port are
capable of a larger bus width or higher speed than negotiated.
Downtraining might be indicative of other problems in the system, and
identifying this from userspace is neither intuitive, nor straigh
forward.

The easiest way to detect this is with pcie_print_link_status(),
since the bottleneck is usually the link that is downtrained. It's not
a perfect solution, but it works extremely well in most cases.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
---

Changes since v3:
 - Remove extra newline and parentheses.
 - Make sure v4.18-rc1 is still happy with the change (it is).

Changes since v2:
 - Check dev->is_virtfn flag

Changes since v1:
 - Use pcie_print_link_status() instead of reimplementing logic

 drivers/pci/probe.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index ac876e32de4b..5e0d0a34fe8c 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2205,6 +2205,24 @@ static struct pci_dev *pci_scan_device(struct pci_bus *bus, int devfn)
 	return dev;
 }
 
+static void pcie_check_upstream_link(struct pci_dev *dev)
+{
+	if (!pci_is_pcie(dev))
+		return;
+
+	/* Look from the device up to avoid downstream ports with no devices. */
+	if ((pci_pcie_type(dev) != PCI_EXP_TYPE_ENDPOINT) &&
+	    (pci_pcie_type(dev) != PCI_EXP_TYPE_LEG_END) &&
+	    (pci_pcie_type(dev) != PCI_EXP_TYPE_UPSTREAM))
+		return;
+
+	/* Multi-function PCIe share the same link/status. */
+	if (PCI_FUNC(dev->devfn) != 0 || dev->is_virtfn)
+		return;
+
+	pcie_print_link_status(dev);
+}
+
 static void pci_init_capabilities(struct pci_dev *dev)
 {
 	/* Enhanced Allocation */
@@ -2240,6 +2258,9 @@ static void pci_init_capabilities(struct pci_dev *dev)
 	/* Advanced Error Reporting */
 	pci_aer_init(dev);
 
+	/* Check link and detect downtrain errors */
+	pcie_check_upstream_link(dev);
+
 	if (pci_probe_reset_function(dev) == 0)
 		dev->reset_fn = 1;
 }
-- 
2.14.3