diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 62c3a8bc83b3d..f599d321c881e 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -4951,6 +4951,8 @@ static bool pcie_wait_for_link_delay(struct pci_dev *pdev, bool active,
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);
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index 6c257acbae90a..e3a468a58cd29 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -543,6 +543,7 @@ void pci_acs_init(struct pci_dev *dev);
int pci_dev_specific_acs_enabled(struct pci_dev *dev, u16 acs_flags);
int pci_dev_specific_enable_acs(struct pci_dev *dev);
int pci_dev_specific_disable_acs_redir(struct pci_dev *dev);
+bool pcie_failed_link_retrain(struct pci_dev *dev);
#else
static inline int pci_dev_specific_acs_enabled(struct pci_dev *dev,
u16 acs_flags)
@@ -557,6 +558,10 @@ static inline int pci_dev_specific_disable_acs_redir(struct pci_dev *dev)
{
return -ENOTTY;
}
+static inline bool pcie_failed_link_retrain(struct pci_dev *dev)
+{
+ return false;
+}
#endif
/* PCI error reporting and recovery */
diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 782925bac64ab..f547db0a728fc 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2550,6 +2550,8 @@ void pci_device_add(struct pci_dev *dev, struct pci_bus *bus)
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);
diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
index 5c0a438f7e64c..a46678563b336 100644
--- a/drivers/pci/quirks.c
+++ b/drivers/pci/quirks.c
@@ -33,6 +33,99 @@
#include <linux/switchtec.h>
#include "pci.h"
+/*
+ * Retrain the link of a downstream PCIe port by hand if necessary.
+ *
+ * This is needed at least where a downstream port of the ASMedia ASM2824
+ * Gen 3 switch is wired to the upstream port of the Pericom PI7C9X2G304
+ * Gen 2 switch, and observed with the Delock Riser Card PCI Express x1 >
+ * 2 x PCIe x1 device, P/N 41433, plugged into the SiFive HiFive Unmatched
+ * board.
+ *
+ * In such a configuration the switches are supposed to negotiate the link
+ * speed of preferably 5.0GT/s, falling back to 2.5GT/s. However the link
+ * continues switching between the two speeds indefinitely and the data
+ * link layer never reaches the active state, with link training reported
+ * repeatedly active ~84% of the time. Forcing the target link speed to
+ * 2.5GT/s with the upstream ASM2824 device makes the two switches talk to
+ * each other correctly however. And more interestingly retraining with a
+ * higher target link speed afterwards lets the two successfully negotiate
+ * 5.0GT/s.
+ *
+ * With the ASM2824 we can rely on the otherwise optional Data Link Layer
+ * Link Active status bit and in the failed link training scenario it will
+ * be off along with the Link Bandwidth Management Status indicating that
+ * hardware has changed the link speed or width in an attempt to correct
+ * unreliable link operation. For a port that has been left unconnected
+ * both bits will be clear. So use this information to detect the problem
+ * rather than polling the Link Training bit and watching out for flips or
+ * at least the active status.
+ *
+ * Since the exact nature of the problem isn't known and in principle this
+ * could trigger where an ASM2824 device is downstream rather upstream,
+ * apply this erratum workaround to any downstream ports as long as they
+ * support Link Active reporting and have the Link Control 2 register.
+ * Restrict the speed to 2.5GT/s then with the Target Link Speed field,
+ * request a retrain and wait 200ms for the data link to go up.
+ *
+ * If this turns out successful and we know by the Vendor:Device ID it is
+ * safe to do so, then lift the restriction, letting the devices negotiate
+ * a higher speed. Also check for a similar 2.5GT/s speed restriction the
+ * firmware may have already arranged and lift it with ports that already
+ * report their data link being up.
+ *
+ * Return TRUE if the link has been successfully retrained, otherwise FALSE.
+ */
+bool pcie_failed_link_retrain(struct pci_dev *dev)
+{
+ static const struct pci_device_id ids[] = {
+ { PCI_VDEVICE(ASMEDIA, 0x2824) }, /* ASMedia ASM2824 */
+ {}
+ };
+ u16 lnksta, lnkctl2;
+
+ if (!pci_is_pcie(dev) || !pcie_downstream_port(dev) ||
+ !pcie_cap_has_lnkctl2(dev) || !dev->link_active_reporting)
+ return false;
+
+ pcie_capability_read_word(dev, PCI_EXP_LNKCTL2, &lnkctl2);
+ pcie_capability_read_word(dev, PCI_EXP_LNKSTA, &lnksta);
+ if ((lnksta & (PCI_EXP_LNKSTA_LBMS | PCI_EXP_LNKSTA_DLLLA)) ==
+ PCI_EXP_LNKSTA_LBMS) {
+ pci_info(dev, "broken device, retraining non-functional downstream link at 2.5GT/s\n");
+
+ lnkctl2 &= ~PCI_EXP_LNKCTL2_TLS;
+ lnkctl2 |= PCI_EXP_LNKCTL2_TLS_2_5GT;
+ pcie_capability_write_word(dev, PCI_EXP_LNKCTL2, lnkctl2);
+
+ if (!pcie_retrain_link(dev, false)) {
+ pci_info(dev, "retraining failed\n");
+ return false;
+ }
+
+ pcie_capability_read_word(dev, PCI_EXP_LNKSTA, &lnksta);
+ }
+
+ if ((lnksta & PCI_EXP_LNKSTA_DLLLA) &&
+ (lnkctl2 & PCI_EXP_LNKCTL2_TLS) == PCI_EXP_LNKCTL2_TLS_2_5GT &&
+ pci_match_id(ids, dev)) {
+ u32 lnkcap;
+
+ pci_info(dev, "removing 2.5GT/s downstream link speed restriction\n");
+ pcie_capability_read_dword(dev, PCI_EXP_LNKCAP, &lnkcap);
+ lnkctl2 &= ~PCI_EXP_LNKCTL2_TLS;
+ lnkctl2 |= lnkcap & PCI_EXP_LNKCAP_SLS;
+ pcie_capability_write_word(dev, PCI_EXP_LNKCTL2, lnkctl2);
+
+ if (!pcie_retrain_link(dev, false)) {
+ pci_info(dev, "retraining failed\n");
+ return false;
+ }
+ }
+
+ return true;
+}
+
static ktime_t fixup_debug_start(struct pci_dev *dev,
void (*fn)(struct pci_dev *dev))
{