drm: Compare only lower 32 bits of framebuffer map offsets

Drivers using multiple framebuffers got broken by commit
41c2e75e which ignored the framebuffer
(or register) map offset when looking for existing maps. The rationale
was that the kernel-userspace ABI is fixed at a 32-bit offset, so the
real offsets could not always be handed over for comparison.

Instead of ignoring the offset we will compare the lower 32 bit. Drivers
using multiple framebuffers should just make sure that the lower 32 bit
are different. The existing drivers in question are practically limited
to 32-bit systems so that should be fine for them.

It is assumed that current drivers always specify a correct framebuffer
map offset, even if this offset was ignored since above commit. So this
patch should not change anything for drivers using only one framebuffer.

Drivers needing multiple framebuffers with 64-bit map offsets will need
to cook up something, for instance keeping an ID in the lower bit which
is to be aligned away when it comes to using the offset.

All of above applies to _DRM_REGISTERS as well.
Signed-off-by: Tormod Volden <debian.tormod@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_bufs.c

@@ -46,10 +46,11 @@ static struct drm_map_list *drm_find_matching_map(struct drm_device *dev,
 	list_for_each_entry(entry, &dev->maplist, head) {
 		/*
 		 * Because the kernel-userspace ABI is fixed at a 32-bit offset
-		 * while PCI resources may live above that, we ignore the map
-		 * offset for maps of type _DRM_FRAMEBUFFER or _DRM_REGISTERS.
-		 * It is assumed that each driver will have only one resource of
-		 * each type.
+		 * while PCI resources may live above that, we only compare the
+		 * lower 32 bits of the map offset for maps of type
+		 * _DRM_FRAMEBUFFER or _DRM_REGISTERS.
+		 * It is assumed that if a driver have more than one resource
+		 * of each type, the lower 32 bits are different.
 		 */
 		if (!entry->map ||
 		    map->type != entry->map->type ||
@@ -59,9 +60,12 @@ static struct drm_map_list *drm_find_matching_map(struct drm_device *dev,
 		case _DRM_SHM:
 			if (map->flags != _DRM_CONTAINS_LOCK)
 				break;
+			return entry;
 		case _DRM_REGISTERS:
 		case _DRM_FRAME_BUFFER:
-			return entry;
+			if ((entry->map->offset & 0xffffffff) ==
+			    (map->offset & 0xffffffff))
+				return entry;
 		default: /* Make gcc happy */
 			;
 		}