diff --git a/drivers/irqchip/irq-gic-v3-its-pci-msi.c b/drivers/irqchip/irq-gic-v3-its-pci-msi.c
index aee1c60..7793121 100644
--- a/drivers/irqchip/irq-gic-v3-its-pci-msi.c
+++ b/drivers/irqchip/irq-gic-v3-its-pci-msi.c
@@ -41,27 +41,22 @@
 	.irq_write_msi_msg	= pci_msi_domain_write_msg,
 };
 
-struct its_pci_alias {
-	struct pci_dev	*pdev;
-	u32		count;
-};
-
-static int its_pci_msi_vec_count(struct pci_dev *pdev)
+static int its_pci_msi_vec_count(struct pci_dev *pdev, void *data)
 {
-	int msi, msix;
+	int msi, msix, *count = data;
 
 	msi = max(pci_msi_vec_count(pdev), 0);
 	msix = max(pci_msix_vec_count(pdev), 0);
+	*count += max(msi, msix);
 
-	return max(msi, msix);
+	return 0;
 }
 
 static int its_get_pci_alias(struct pci_dev *pdev, u16 alias, void *data)
 {
-	struct its_pci_alias *dev_alias = data;
+	struct pci_dev **alias_dev = data;
 
-	if (pdev != dev_alias->pdev)
-		dev_alias->count += its_pci_msi_vec_count(pdev);
+	*alias_dev = pdev;
 
 	return 0;
 }
@@ -69,9 +64,9 @@
 static int its_pci_msi_prepare(struct irq_domain *domain, struct device *dev,
 			       int nvec, msi_alloc_info_t *info)
 {
-	struct pci_dev *pdev;
-	struct its_pci_alias dev_alias;
+	struct pci_dev *pdev, *alias_dev;
 	struct msi_domain_info *msi_info;
+	int alias_count = 0;
 
 	if (!dev_is_pci(dev))
 		return -EINVAL;
@@ -79,16 +74,20 @@
 	msi_info = msi_get_domain_info(domain->parent);
 
 	pdev = to_pci_dev(dev);
-	dev_alias.pdev = pdev;
-	dev_alias.count = nvec;
-
-	pci_for_each_dma_alias(pdev, its_get_pci_alias, &dev_alias);
+	/*
+	 * If pdev is downstream of any aliasing bridges, take an upper
+	 * bound of how many other vectors could map to the same DevID.
+	 */
+	pci_for_each_dma_alias(pdev, its_get_pci_alias, &alias_dev);
+	if (alias_dev != pdev && alias_dev->subordinate)
+		pci_walk_bus(alias_dev->subordinate, its_pci_msi_vec_count,
+			     &alias_count);
 
 	/* ITS specific DeviceID, as the core ITS ignores dev. */
 	info->scratchpad[0].ul = pci_msi_domain_get_msi_rid(domain, pdev);
 
 	return msi_info->ops->msi_prepare(domain->parent,
-					  dev, dev_alias.count, info);
+					  dev, max(nvec, alias_count), info);
 }
 
 static struct msi_domain_ops its_pci_msi_ops = {