[PATCH v4 4/5] PCI: generic: Correct, and avoid overflow, in bus_max calculation.

[David Daney <ddaney.cavm@gmail.com>]

From: David Daney <david.daney@cavium.com>

There are two problems with the bus_max calculation:

1) The u8 data type can overflow for large config space windows.

2) The calculation is incorrect for a bus range that doesn't start at
   zero.

Since the configuration space is relative to bus zero, make bus_max
just be the size of the config window scaled by bus_shift.  Then clamp
it to a maximum of 255, per PCI.  Use a data type of int to avoid
overflow problems.

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>
---
Change from V3: Add to explanation of "reg" property in
host-generic-pci.txt.  Add error message if "reg" property is too big.

 Documentation/devicetree/bindings/pci/host-generic-pci.txt |  6 +++++-
 drivers/pci/host/pci-host-generic.c                        | 12 +++++++++---
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/Documentation/devicetree/bindings/pci/host-generic-pci.txt b/Documentation/devicetree/bindings/pci/host-generic-pci.txt
index cf3e205..42303bb 100644
--- a/Documentation/devicetree/bindings/pci/host-generic-pci.txt
+++ b/Documentation/devicetree/bindings/pci/host-generic-pci.txt
@@ -34,7 +34,11 @@ 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
+                   bus zero, even though the "bus-range" property may specify
+                   a different starting bus number.  The driver must only map
+                   or access the portion of the Configuration Space that
+                   corresponds to the "bus-range"
 
 
 Properties of the /chosen node:
diff --git a/drivers/pci/host/pci-host-generic.c b/drivers/pci/host/pci-host-generic.c
index 216ded5..5cce837 100644
--- a/drivers/pci/host/pci-host-generic.c
+++ b/drivers/pci/host/pci-host-generic.c
@@ -164,7 +164,7 @@ out_release_res:
 static int gen_pci_parse_map_cfg_windows(struct gen_pci *pci)
 {
 	int err;
-	u8 bus_max;
+	int bus_max;
 	resource_size_t busn;
 	struct resource *bus_range;
 	struct device *dev = pci->host.dev.parent;
@@ -177,8 +177,14 @@ static int gen_pci_parse_map_cfg_windows(struct gen_pci *pci)
 	}
 
 	/* Limit the bus-range to fit within reg */
-	bus_max = pci->cfg.bus_range->start +
-		  (resource_size(&pci->cfg.res) >> pci->cfg.ops->bus_shift) - 1;
+	bus_max = (resource_size(&pci->cfg.res) >> pci->cfg.ops->bus_shift) - 1;
+	if (bus_max > 255) {
+		dev_err(dev,
+			"\"reg\" size corresponds to bus %d, truncating to 255\n",
+			bus_max);
+		bus_max = 255;
+	}
+
 	pci->cfg.bus_range->end = min_t(resource_size_t,
 					pci->cfg.bus_range->end, bus_max);
 
-- 
1.9.1