drm/i915: Codify our assumption that the Global GTT is <= 4GiB

Throughout the code base, we use u32 for offsets into the global GTT. If
we ever see any hardware with a larger GGTT, then we run the real risk
of silent corruption. So test for our assumption up front so that we
have a nice reminder should the time come when it fails.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
[Rebased and changed 1ull -> 1ULL, cut 80 char line]
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458290579-27783-1-git-send-email-joonas.lahtinen@linux.intel.com

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -3191,6 +3191,14 @@ int i915_gem_gtt_init(struct drm_device *dev)
 	if (ret)
 		return ret;
 
+	if ((ggtt->base.total - 1) >> 32) {
+		DRM_ERROR("We never expected a Global GTT with more than 32bits"
+			  "of address space! Found %lldM!\n",
+			  ggtt->base.total >> 20);
+		ggtt->base.total = 1ULL << 32;
+		ggtt->mappable_end = min(ggtt->mappable_end, ggtt->base.total);
+	}
+
 	/*
 	 * Initialise stolen early so that we may reserve preallocated
 	 * objects for the BIOS to KMS transition.