[PATCH] fix PCI bridge handling in probe.c

As Ben pointed out, disabling the PCI-to-PCI bridge before scanning
its secondary bus is a serious bug.

It can easily crash even on PCs when booting with "pci=assign-busses"
option. For example, that code completely disconnects the AGP bridge
(along with VGA behind it) from the bus and then calls pci_do_scan_bus()
where printks are quite possible.

drivers/pci/probe.c

@@ -285,13 +285,12 @@ int __devinit pci_scan_bridge(struct pci_bus *bus, struct pci_dev * dev, int max
 	} else {
 		/*
 		 * We need to assign a number to this bus which we always
-		 * do in the second pass. We also keep all address decoders
-		 * on the bridge disabled during scanning.  FIXME: Why?
+		 * do in the second pass.
 		 */
 		if (!pass)
 			return max;
-		pci_read_config_word(dev, PCI_COMMAND, &cr);
-		pci_write_config_word(dev, PCI_COMMAND, 0x0000);
+
+		/* Clear errors */
 		pci_write_config_word(dev, PCI_STATUS, 0xffff);
 
 		child = pci_add_new_bus(bus, dev, ++max);
@@ -319,7 +318,6 @@ int __devinit pci_scan_bridge(struct pci_bus *bus, struct pci_dev * dev, int max
 		 */
 		child->subordinate = max;
 		pci_write_config_byte(dev, PCI_SUBORDINATE_BUS, max);
-		pci_write_config_word(dev, PCI_COMMAND, cr);
 	}
 	sprintf(child->name, (is_cardbus ? "PCI CardBus #%02x" : "PCI Bus #%02x"), child->number);