PCI: generic: Fix address window calculation for non-zero starting bus

Make the offset from the beginning of the "reg" property be from the
starting bus number, rather than zero.  Hoist the invariant size
calculation out of the mapping for loop.

Update host-generic-pci.txt to clarify the semantics of the "reg" property
with respect to non-zero starting bus numbers.
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/pci/host-generic-pci.txt

@@ -34,8 +34,9 @@ Properties of the host controller node:
 - #size-cells    : Must be 2.
 
 - reg            : The Configuration Space base address and size, as accessed
-                   from the parent bus.
-
+                   from the parent bus.  The base address corresponds to
+                   the first bus in the "bus-range" property.  If no
+                   "bus-range" is specified, this will be bus 0 (the default).
 
 Properties of the /chosen node:
 

drivers/pci/host/pci-host-generic.c

@@ -169,6 +169,7 @@ static int gen_pci_parse_map_cfg_windows(struct gen_pci *pci)
 	struct resource *bus_range;
 	struct device *dev = pci->host.dev.parent;
 	struct device_node *np = dev->of_node;
+	u32 sz = 1 << pci->cfg.ops->bus_shift;
 
 	err = of_address_to_resource(np, 0, &pci->cfg.res);
 	if (err) {
@@ -196,10 +197,9 @@ static int gen_pci_parse_map_cfg_windows(struct gen_pci *pci)
 	bus_range = pci->cfg.bus_range;
 	for (busn = bus_range->start; busn <= bus_range->end; ++busn) {
 		u32 idx = busn - bus_range->start;
-		u32 sz = 1 << pci->cfg.ops->bus_shift;
 
 		pci->cfg.win[idx] = devm_ioremap(dev,
-						 pci->cfg.res.start + busn * sz,
+						 pci->cfg.res.start + idx * sz,
 						 sz);
 		if (!pci->cfg.win[idx])
 			return -ENOMEM;