PCI: xgene: Defer probing if the MSI widget driver hasn't probed yet

As a preparatory work to make the XGene MSI driver probe less of
a sorry hack, make the PCI driver check for the availability of
the MSI parent domain, and defer the probing otherwise.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20250708173404.1278635-3-maz@kernel.org

Author: Marc Zyngier

drivers/pci/controller/pci-xgene.c

@@ -12,6 +12,7 @@
 #include <linux/jiffies.h>
 #include <linux/memblock.h>
 #include <linux/init.h>
+#include <linux/irqdomain.h>
 #include <linux/of.h>
 #include <linux/of_address.h>
 #include <linux/of_pci.h>
@@ -594,6 +595,24 @@ static struct pci_ops xgene_pcie_ops = {
 	.write = pci_generic_config_write32,
 };
 
+static bool xgene_check_pcie_msi_ready(void)
+{
+	struct device_node *np;
+	struct irq_domain *d;
+
+	if (!IS_ENABLED(CONFIG_PCI_XGENE_MSI))
+		return true;
+
+	np = of_find_compatible_node(NULL, NULL, "apm,xgene1-msi");
+	if (!np)
+		return true;
+
+	d = irq_find_matching_host(np, DOMAIN_BUS_PCI_MSI);
+	of_node_put(np);
+
+	return d && irq_domain_is_msi_parent(d);
+}
+
 static int xgene_pcie_probe(struct platform_device *pdev)
 {
 	struct device *dev = &pdev->dev;
@@ -602,6 +621,10 @@ static int xgene_pcie_probe(struct platform_device *pdev)
 	struct pci_host_bridge *bridge;
 	int ret;
 
+	if (!xgene_check_pcie_msi_ready())
+		return dev_err_probe(&pdev->dev, -EPROBE_DEFER,
+				     "MSI driver not ready\n");
+
 	bridge = devm_pci_alloc_host_bridge(dev, sizeof(*port));
 	if (!bridge)
 		return -ENOMEM;