PCI: keystone: Add link up check to ks_pcie_other_map_bus()

K2G forwards the error triggered by a link-down state (e.g., no connected
endpoint device) on the system bus for PCI configuration transactions;
these errors are reported as an SError at system level, which is fatal and
hangs the system.

So, apply fix similar to how it was done in the DesignWare Core driver
commit 15b2390 ("PCI: dwc: Add link up check in dw_child_pcie_ops.map_bus()").

Fixes: 10a797c ("PCI: dwc: keystone: Use pci_ops for config space accessors")
Link: https://lore.kernel.org/r/20240524105714.191642-3-s-vadapalli@ti.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
[kwilczynski: commit log, added tag for stable releases]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Cc: stable@vger.kernel.org

Author: kishon

drivers/pci/controller/dwc/pci-keystone.c

@@ -455,6 +455,17 @@ static void __iomem *ks_pcie_other_map_bus(struct pci_bus *bus,
 	struct keystone_pcie *ks_pcie = to_keystone_pcie(pci);
 	u32 reg;
 
+	/*
+	 * Checking whether the link is up here is a last line of defense
+	 * against platforms that forward errors on the system bus as
+	 * SError upon PCI configuration transactions issued when the link
+	 * is down. This check is racy by definition and does not stop
+	 * the system from triggering an SError if the link goes down
+	 * after this check is performed.
+	 */
+	if (!dw_pcie_link_up(pci))
+		return NULL;
+
 	reg = CFG_BUS(bus->number) | CFG_DEVICE(PCI_SLOT(devfn)) |
 		CFG_FUNC(PCI_FUNC(devfn));
 	if (!pci_is_root_bus(bus->parent))