[PATCH] drm/i915: fix i915 running as dom0 under Xen

[Juergen Gross <jgross@suse.com>]

Commit 920cf4194954ec ("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 7453c549f5f648 ("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.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
Please consider for 4.10 as otherwise 4.10 will be unusable as Xen dom0
with i915 graphics.
---
 drivers/gpu/drm/i915/i915_gem_internal.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_internal.c b/drivers/gpu/drm/i915/i915_gem_internal.c
index 4b3ff3e..d09c749 100644
--- a/drivers/gpu/drm/i915/i915_gem_internal.c
+++ b/drivers/gpu/drm/i915/i915_gem_internal.c
@@ -66,8 +66,16 @@ 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;
-- 
2.10.2