x86/PCI: Handle PIRQ routing tables with no router device given

PIRQ routing tables provided by the PCI BIOS usually specify the PCI 
vendor:device ID as well as the bus address of the device implementing 
the PIRQ router, e.g.:

PCI: Interrupt Routing Table found at 0xc00fde10
[...]
PCI: Attempting to find IRQ router for [8086:7000]
pci 0000:00:07.0: PIIX/ICH IRQ router [8086:7000]

however in some cases they do not, in which case we fail to match the 
router handler, e.g.:

PCI: Interrupt Routing Table found at 0xc00fdae0
[...]
PCI: Attempting to find IRQ router for [0000:0000]
PCI: Interrupt router not found at 00:00

This is because we always match the vendor:device ID and the bus address 
literally, even if they are all zeros.

Handle this case then and iterate over all PCI devices until we find a 
matching router handler if the vendor ID given by the routing table is 
the invalid value of zero:

PCI: Attempting to find IRQ router for [0000:0000]
PCI: Trying IRQ router for [1039:0496]
pci 0000:00:05.0: SiS85C497 IRQ router [1039:0496]
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nikolai Zhubr <zhubr.2@gmail.com>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2203302018570.9038@angie.orcam.me.uk

arch/x86/pci/irq.c

@@ -1175,10 +1175,32 @@ static struct pci_dev *pirq_router_dev;
  *	chipset" ?
  */
 
+static bool __init pirq_try_router(struct irq_router *r,
+				   struct irq_routing_table *rt,
+				   struct pci_dev *dev)
+{
+	struct irq_router_handler *h;
+
+	DBG(KERN_DEBUG "PCI: Trying IRQ router for [%04x:%04x]\n",
+	    dev->vendor, dev->device);
+
+	for (h = pirq_routers; h->vendor; h++) {
+		/* First look for a router match */
+		if (rt->rtr_vendor == h->vendor &&
+		    h->probe(r, dev, rt->rtr_device))
+			return true;
+		/* Fall back to a device match */
+		if (dev->vendor == h->vendor &&
+		    h->probe(r, dev, dev->device))
+			return true;
+	}
+	return false;
+}
+
 static void __init pirq_find_router(struct irq_router *r)
 {
 	struct irq_routing_table *rt = pirq_table;
-	struct irq_router_handler *h;
+	struct pci_dev *dev;
 
 #ifdef CONFIG_PCI_BIOS
 	if (!rt->signature) {
@@ -1197,27 +1219,29 @@ static void __init pirq_find_router(struct irq_router *r)
 	DBG(KERN_DEBUG "PCI: Attempting to find IRQ router for [%04x:%04x]\n",
 	    rt->rtr_vendor, rt->rtr_device);
 
-	pirq_router_dev = pci_get_domain_bus_and_slot(0, rt->rtr_bus,
-						      rt->rtr_devfn);
-	if (!pirq_router_dev) {
-		DBG(KERN_DEBUG "PCI: Interrupt router not found at "
-			"%02x:%02x\n", rt->rtr_bus, rt->rtr_devfn);
-		return;
+	/* Use any vendor:device provided by the routing table or try all.  */
+	if (rt->rtr_vendor) {
+		dev = pci_get_domain_bus_and_slot(0, rt->rtr_bus,
+						  rt->rtr_devfn);
+		if (dev && pirq_try_router(r, rt, dev))
+			pirq_router_dev = dev;
+	} else {
+		dev = NULL;
+		for_each_pci_dev(dev) {
+			if (pirq_try_router(r, rt, dev)) {
+				pirq_router_dev = dev;
+				break;
+			}
+		}
 	}
 
-	for (h = pirq_routers; h->vendor; h++) {
-		/* First look for a router match */
-		if (rt->rtr_vendor == h->vendor &&
-			h->probe(r, pirq_router_dev, rt->rtr_device))
-			break;
-		/* Fall back to a device match */
-		if (pirq_router_dev->vendor == h->vendor &&
-			h->probe(r, pirq_router_dev, pirq_router_dev->device))
-			break;
-	}
-	dev_info(&pirq_router_dev->dev, "%s IRQ router [%04x:%04x]\n",
-		 pirq_router.name,
-		 pirq_router_dev->vendor, pirq_router_dev->device);
+	if (pirq_router_dev)
+		dev_info(&pirq_router_dev->dev, "%s IRQ router [%04x:%04x]\n",
+			 pirq_router.name,
+			 pirq_router_dev->vendor, pirq_router_dev->device);
+	else
+		DBG(KERN_DEBUG "PCI: Interrupt router not found at "
+		    "%02x:%02x\n", rt->rtr_bus, rt->rtr_devfn);
 
 	/* The device remains referenced for the kernel lifetime */
 }