[v5] PCI: Avoid unsync of LTR mechanism configuration

[mingchuang.qiao at mediatek.com]

From: Mingchuang Qiao <mingchuang.qiao at mediatek.com>

In bus scan flow, the "LTR Mechanism Enable" bit of DEVCTL2 register is
configured in pci_configure_ltr(). If device and bridge both support LTR
mechanism, the "LTR Mechanism Enable" bit of device and bridge will be
enabled in DEVCTL2 register. And pci_dev->ltr_path will be set as 1.

If PCIe link goes down, the "LTR Mechanism Enable" bit of bridge will 
change to 0 according to PCIe r5.0, sec 7.5.3.16. However, the ->ltr_path 
of bridge is still set.

Following shows two scenarios of this LTR issue:

-scenario of device restore
 -- bridge LTR enabled
 -- device LTR enabled
 -- reset device
 -- link goes down, bridge disables LTR
 -- link comes back up, LTR disabled in both bridge and device
 -- restore device state, including LTR enable
 -- device sends LTR message
 -- bridge reports Unsupported Request

-scenario of device hot-remove/hot-add
 -- bridge LTR enabled
 -- device LTR enabled
 -- hot-remove device
 -- link goes down, bridge disables LTR
 -- hot-add device and link comes back up
 -- scan device, set LTR enable bit of device
 -- device sends LTR message
 -- bridge reports Unsupported Request

This issue was noticed by AER log as following shows:
pcieport 0000:00:1d.0: AER: Uncorrected (Non-Fatal) error received: id=00e8
pcieport 0000:00:1d.0: PCIe Bus Error: severity=Uncorrected (Non-Fatal), 
type=Transaction Layer, id=00e8(Requester ID)
pcieport 0000:00:1d.0:   device [8086:9d18] error 
status/mask=00100000/00010000
pcieport 0000:00:1d.0:    [20] Unsupported Request    (First)

It was also noticed when PCIe devices (thunderbolt docks) were hot removed 
from chromebooks, and then hot-plugged back again. Once hotplugged back, 
the newer Intel chromebooks fail to go into S0ix low power state because 
of this LTR issue.(https://patchwork.kernel.org/project/linux-pci/patch/
20210204095125.9212-1-mingchuang.qiao at mediatek.com/)

To resolve this issue, check and re-configure "LTR Mechanism Enable" bit
of bridge to make "LTR Mechanism Enable" bit match ltr_path value in
following conditions.
   -before configuring device's LTR for hot-remove/hot-add
   -before restoring device's DEVCTL2 register when restore device state

Reviewed-by: Mika Westerberg <mika.westerberg at linux.intel.com>
Signed-off-by: Mingchuang Qiao <mingchuang.qiao at mediatek.com>
---
changes of v5
 -add more details in commit message
changes of v4
 -fix typo of commit message
 -rename: pci_reconfigure_bridge_ltr()->pci_bridge_reconfigure_ltr()
changes of v3
 -call pci_reconfigure_bridge_ltr() in probe.c
changes of v2
 -modify patch description
 -reconfigure bridge's LTR before restoring device DEVCTL2 register
---
 drivers/pci/pci.c   | 25 +++++++++++++++++++++++++
 drivers/pci/pci.h   |  1 +
 drivers/pci/probe.c | 13 ++++++++++---
 3 files changed, 36 insertions(+), 3 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index b9fecc25d213..6bf65d295331 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1437,6 +1437,24 @@ static int pci_save_pcie_state(struct pci_dev *dev)
 	return 0;
 }
 
+void pci_bridge_reconfigure_ltr(struct pci_dev *dev)
+{
+#ifdef CONFIG_PCIEASPM
+	struct pci_dev *bridge;
+	u32 ctl;
+
+	bridge = pci_upstream_bridge(dev);
+	if (bridge && bridge->ltr_path) {
+		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);
+		}
+	}
+#endif
+}
+
 static void pci_restore_pcie_state(struct pci_dev *dev)
 {
 	int i = 0;
@@ -1447,6 +1465,13 @@ static void pci_restore_pcie_state(struct pci_dev *dev)
 	if (!save_state)
 		return;
 
+	/*
+	 * Downstream ports reset the LTR enable bit when link goes down.
+	 * Check and re-configure the bit here before restoring device.
+	 * PCIe r5.0, sec 7.5.3.16.
+	 */
+	pci_bridge_reconfigure_ltr(dev);
+
 	cap = (u16 *)&save_state->cap.data[0];
 	pcie_capability_write_word(dev, PCI_EXP_DEVCTL, cap[i++]);
 	pcie_capability_write_word(dev, PCI_EXP_LNKCTL, cap[i++]);
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index 5c59365092fa..b3a5e5287cb7 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -111,6 +111,7 @@ void pci_free_cap_save_buffers(struct pci_dev *dev);
 bool pci_bridge_d3_possible(struct pci_dev *dev);
 void pci_bridge_d3_update(struct pci_dev *dev);
 void pci_bridge_wait_for_secondary_bus(struct pci_dev *dev);
+void pci_bridge_reconfigure_ltr(struct pci_dev *dev);
 
 static inline void pci_wakeup_event(struct pci_dev *dev)
 {
diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 953f15abc850..ade055e9fb58 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2132,9 +2132,16 @@ static void pci_configure_ltr(struct pci_dev *dev)
 	 * Complex and all intermediate Switches indicate support for LTR.
 	 * PCIe r4.0, sec 6.18.
 	 */
-	if (pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT ||
-	    ((bridge = pci_upstream_bridge(dev)) &&
-	      bridge->ltr_path)) {
+	if (pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT) {
+		pcie_capability_set_word(dev, PCI_EXP_DEVCTL2,
+					 PCI_EXP_DEVCTL2_LTR_EN);
+		dev->ltr_path = 1;
+		return;
+	}
+
+	bridge = pci_upstream_bridge(dev);
+	if (bridge && bridge->ltr_path) {
+		pci_bridge_reconfigure_ltr(dev);
 		pcie_capability_set_word(dev, PCI_EXP_DEVCTL2,
 					 PCI_EXP_DEVCTL2_LTR_EN);
 		dev->ltr_path = 1;
-- 
2.18.0