[PATCH 1/2] PCI: Write RCB only when it changes

[Stefan Roese <stefan.roese@mailbox.org>]

pci_configure_rcb() does a read-modify-write of the Link Control
register of every endpoint at enumeration, even when RCB already has
the right value. Some devices react to any write of this register.

The Renesas uPD720201 xHCI (1912:0014) comes out of reset with ASPM L0s
and L1 enabled in Link Control. On a link whose Root Port supports no
ASPM, nothing else writes that register before the driver loads, and
the chip clears ASPM Control itself during the firmware download. After
a host write, even of the unchanged value 0x0003, it no longer does
so. ASPM stays enabled, and the first access to the xHCI BAR runs into
PCIe completion timeouts that hang the system.

Seen on an AMD Versal board (CPM Root Port without ASPM support): the
hang bisects to this commit, reverting it fixes it, and on a kernel
without it a single setpci write of the unchanged value reproduces it.

Read Link Control first and write it only when RCB has to change.

Fixes: 1a6845aaa6de ("PCI: Initialize RCB from pci_configure_device()")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Stefan Roese <stefan.roese@mailbox.org>
---
 drivers/pci/probe.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 721daf5c5184..35e794df3be4 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2426,7 +2426,7 @@ static void pci_configure_serr(struct pci_dev *dev)
 static void pci_configure_rcb(struct pci_dev *dev)
 {
 	struct pci_dev *rp;
-	u16 rp_lnkctl;
+	u16 rp_lnkctl, lnkctl, rcb;
 
 	/*
 	 * Per PCIe r7.0, sec 7.5.3.7, RCB is only meaningful in Root Ports
@@ -2448,10 +2448,17 @@ static void pci_configure_rcb(struct pci_dev *dev)
 		return;
 
 	pcie_capability_read_word(rp, PCI_EXP_LNKCTL, &rp_lnkctl);
-	pcie_capability_clear_and_set_word(dev, PCI_EXP_LNKCTL,
-					   PCI_EXP_LNKCTL_RCB,
-					   (rp_lnkctl & PCI_EXP_LNKCTL_RCB) ?
-					   PCI_EXP_LNKCTL_RCB : 0);
+	rcb = rp_lnkctl & PCI_EXP_LNKCTL_RCB;
+
+	/*
+	 * Write Link Control only when RCB actually changes. Some devices
+	 * react to any write of this register, even one with an unchanged
+	 * value.
+	 */
+	pcie_capability_read_word(dev, PCI_EXP_LNKCTL, &lnkctl);
+	if ((lnkctl & PCI_EXP_LNKCTL_RCB) != rcb)
+		pcie_capability_clear_and_set_word(dev, PCI_EXP_LNKCTL,
+						   PCI_EXP_LNKCTL_RCB, rcb);
 }
 
 static void pci_configure_device(struct pci_dev *dev)
-- 
2.56.0