This now fails unconditionally and will be always optimised away, but 
provides for quirks to implement recovery for failed links detected in 
device probing and device hot plug events.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
---
New change in v9, factored out from 7/7:

- Rename `pcie_downstream_link_retrain' to `pcie_failed_link_retrain'.

- Add stub implementation in "pci.h".
---
 drivers/pci/pci.c   |    2 ++
 drivers/pci/pci.h   |    4 ++++
 drivers/pci/probe.c |    2 ++
 3 files changed, 8 insertions(+)

linux-pcie-failed-link-retrain.diff
Index: linux-macro/drivers/pci/pci.c
===================================================================

--- linux-macro.orig/drivers/pci/pci.c
+++ linux-macro/drivers/pci/pci.c

@@ -4912,6 +4912,8 @@ static bool pcie_wait_for_link_delay(str
 	if (active)
 		msleep(20);
 	ret = pcie_wait_for_link_status(pdev, false, active);
+	if (active && !ret)
+		ret = pcie_failed_link_retrain(pdev);
 	if (active && ret)
 		msleep(delay);
 

Index: linux-macro/drivers/pci/pci.h
===================================================================

--- linux-macro.orig/drivers/pci/pci.h
+++ linux-macro/drivers/pci/pci.h

@@ -554,6 +554,10 @@ static inline int pci_dev_specific_disab
 	return -ENOTTY;
 }
 #endif
+static inline bool pcie_failed_link_retrain(struct pci_dev *dev)
+{
+	return false;
+}
 
 /* PCI error reporting and recovery */
 pci_ers_result_t pcie_do_recovery(struct pci_dev *dev,

Index: linux-macro/drivers/pci/probe.c
===================================================================

--- linux-macro.orig/drivers/pci/probe.c
+++ linux-macro/drivers/pci/probe.c

@@ -2549,6 +2549,8 @@ void pci_device_add(struct pci_dev *dev,
 	dma_set_max_seg_size(&dev->dev, 65536);
 	dma_set_seg_boundary(&dev->dev, 0xffffffff);
 
+	pcie_failed_link_retrain(dev);
+
 	/* Fix up broken headers */
 	pci_fixup_device(pci_fixup_header, dev);