[PATCH v2] PCI: Re-enable downstream port LTR if it was previously enabled

[Mika Westerberg <mika.westerberg@linux.intel.com>]

PCIe r5.0, sec 7.5.3.16 says that the downstream ports must reset the
LTR enable bit if the link goes down (port goes DL_Down status). Now, if
we had LTR previously enabled and the PCIe endpoint gets hot-removed and
then hot-added back the ->ltr_path of the downstream port is still set
but the port now does not have the LTR enable bit set anymore.

For this reason check if the bridge upstream had LTR enabled previously
and re-enable it before enabling LTR for the endpoint.

Reported-by: Utkarsh H Patel <utkarsh.h.patel@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
---
Previous version can be found here:

  https://lore.kernel.org/linux-pci/20210114134724.79511-1-mika.westerberg@linux.intel.com/

Changes from the previous version:

  * Corrected typos in the commit message
  * No need to call pcie_downstream_port()

 drivers/pci/probe.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 0eb68b47354f..a4a8c0305fb9 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2153,7 +2153,7 @@ static void pci_configure_ltr(struct pci_dev *dev)
 {
 #ifdef CONFIG_PCIEASPM
 	struct pci_host_bridge *host = pci_find_host_bridge(dev->bus);
-	struct pci_dev *bridge;
+	struct pci_dev *bridge = NULL;
 	u32 cap, ctl;
 
 	if (!pci_is_pcie(dev))
@@ -2191,6 +2191,21 @@ static void pci_configure_ltr(struct pci_dev *dev)
 	if (pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT ||
 	    ((bridge = pci_upstream_bridge(dev)) &&
 	      bridge->ltr_path)) {
+		/*
+		 * Downstream ports reset the LTR enable bit when the
+		 * link goes down (e.g on hot-remove) so re-enable the
+		 * bit here if not set anymore.
+		 * PCIe r5.0, sec 7.5.3.16.
+		 */
+		if (bridge) {
+			pcie_capability_read_dword(bridge, PCI_EXP_DEVCTL2, &ctl);
+			if (!(ctl & PCI_EXP_DEVCTL2_LTR_EN)) {
+				pci_dbg(bridge, "re-enabling LTR\n");
+				pcie_capability_set_word(bridge, PCI_EXP_DEVCTL2,
+							 PCI_EXP_DEVCTL2_LTR_EN);
+			}
+		}
+		pci_dbg(dev, "enabling LTR\n");
 		pcie_capability_set_word(dev, PCI_EXP_DEVCTL2,
 					 PCI_EXP_DEVCTL2_LTR_EN);
 		dev->ltr_path = 1;
-- 
2.29.2