drm/i915: fix i915 running as dom0 under Xen

Commit 920cf41949 ("drm/i915: Introduce an internal allocator for
disposable private objects") introduced a regression for the kernel
running as Xen dom0: when switching to graphics mode a GPU HANG
occurred.

Reason seems to be a missing adaption similar to that done in
commit 7453c549f5 ("swiotlb: Export swiotlb_max_segment to users")
to i915_gem_object_get_pages_internal().

So limit the maximum page order to be used according to the maximum
swiotlb segment size instead to the complete swiotlb size.

Fixes: 920cf41949 ("drm/i915: Introduce an internal allocator for disposable private objects")
Signed-off-by: Juergen Gross <jgross@suse.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170202094711.939-1-jgross@suse.com
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: <drm-intel-fixes@lists.freedesktop.org> # v4.10-rc1+
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gem_internal.c

@@ -68,8 +68,16 @@ i915_gem_object_get_pages_internal(struct drm_i915_gem_object *obj)
 
 	max_order = MAX_ORDER;
 #ifdef CONFIG_SWIOTLB
-	if (swiotlb_nr_tbl()) /* minimum max swiotlb size is IO_TLB_SEGSIZE */
-		max_order = min(max_order, ilog2(IO_TLB_SEGPAGES));
+	if (swiotlb_nr_tbl()) {
+		unsigned int max_segment;
+
+		max_segment = swiotlb_max_segment();
+		if (max_segment) {
+			max_segment = max_t(unsigned int, max_segment,
+					    PAGE_SIZE) >> PAGE_SHIFT;
+			max_order = min(max_order, ilog2(max_segment));
+		}
+	}
 #endif
 
 	gfp = GFP_KERNEL | __GFP_HIGHMEM | __GFP_RECLAIMABLE;