PCI: Add resource allocation comments

Add comments in the code to match the allocation strategy of 7c671426dfc3
("PCI: Restrict 64-bit prefetchable bridge windows to 64-bit resources").

No functional change.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

1 files changed, 19 insertions, 16 deletions

diff --git a/drivers/pci/setup-res.c b/drivers/pci/setup-res.c
index 3bdac9dc4a88..3da2542eb4df 100644
--- a/drivers/pci/setup-res.c
+++ b/drivers/pci/setup-res.c
@@ -209,20 +209,25 @@ static int __pci_assign_resource(struct pci_bus *bus, struct pci_dev *dev,
 
 	min = (res->flags & IORESOURCE_IO) ? PCIBIOS_MIN_IO : PCIBIOS_MIN_MEM;
 
-	/* First, try exact prefetching match.. */
+	/*
+	 * First, try exact prefetching match.  Even if a 64-bit
+	 * prefetchable bridge window is below 4GB, we can't put a 32-bit
+	 * prefetchable resource in it because pbus_size_mem() assumes a
+	 * 64-bit window will contain no 32-bit resources.  If we assign
+	 * things differently than they were sized, not everything will fit.
+	 */
 	ret = pci_bus_alloc_resource(bus, res, size, align, min,
 				     IORESOURCE_PREFETCH | IORESOURCE_MEM_64,
 				     pcibios_align_resource, dev);
 	if (ret == 0)
 		return 0;
 
+	/*
+	 * If the prefetchable window is only 32 bits wide, we can put
+	 * 64-bit prefetchable resources in it.
+	 */
 	if ((res->flags & (IORESOURCE_PREFETCH | IORESOURCE_MEM_64)) ==
 	     (IORESOURCE_PREFETCH | IORESOURCE_MEM_64)) {
-		/*
-		 * That failed.
-		 *
-		 * Try 32bit pref
-		 */
 		ret = pci_bus_alloc_resource(bus, res, size, align, min,
 					     IORESOURCE_PREFETCH,
 					     pcibios_align_resource, dev);
@@ -230,18 +235,16 @@ static int __pci_assign_resource(struct pci_bus *bus, struct pci_dev *dev,
 			return 0;
 	}
 
-	if (res->flags & (IORESOURCE_PREFETCH | IORESOURCE_MEM_64)) {
-		/*
-		 * That failed.
-		 *
-		 * But a prefetching area can handle a non-prefetching
-		 * window (it will just not perform as well).
-		 *
-		 * Also can put 64bit under 32bit range. (below 4g).
-		 */
+	/*
+	 * If we didn't find a better match, we can put any memory resource
+	 * in a non-prefetchable window.  If this resource is 32 bits and
+	 * non-prefetchable, the first call already tried the only possibility
+	 * so we don't need to try again.
+	 */
+	if (res->flags & (IORESOURCE_PREFETCH | IORESOURCE_MEM_64))
 		ret = pci_bus_alloc_resource(bus, res, size, align, min, 0,
 					     pcibios_align_resource, dev);
-	}
+
 	return ret;
 }