drm/i915: Recreate internal objects with single page segments if dmar fails

If we fail to dma-map the object, the most common cause is lack of space
inside the SW-IOTLB due to fragmentation. If we recreate the_sg_table
using segments of PAGE_SIZE (and single page allocations), we may succeed
in remapping the scatterlist.

First became a significant problem for the mock selftests after commit
5584f1b1d73e ("drm/i915: fix i915 running as dom0 under Xen") increased
the max_order.

Fixes: 920cf4194954 ("drm/i915: Introduce an internal allocator for disposable private objects")
Fixes: 5584f1b1d73e ("drm/i915: fix i915 running as dom0 under Xen")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170202132721.12711-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: <drm-intel-fixes@lists.freedesktop.org> # v4.10-rc1+

drivers/gpu/drm/i915/i915_gem_internal.c

@@ -48,24 +48,12 @@ static struct sg_table *
 i915_gem_object_get_pages_internal(struct drm_i915_gem_object *obj)
 {
 	struct drm_i915_private *i915 = to_i915(obj->base.dev);
-	unsigned int npages = obj->base.size / PAGE_SIZE;
 	struct sg_table *st;
 	struct scatterlist *sg;
+	unsigned int npages;
 	int max_order;
 	gfp_t gfp;
 
-	st = kmalloc(sizeof(*st), GFP_KERNEL);
-	if (!st)
-		return ERR_PTR(-ENOMEM);
-
-	if (sg_alloc_table(st, npages, GFP_KERNEL)) {
-		kfree(st);
-		return ERR_PTR(-ENOMEM);
-	}
-
-	sg = st->sgl;
-	st->nents = 0;
-
 	max_order = MAX_ORDER;
 #ifdef CONFIG_SWIOTLB
 	if (swiotlb_nr_tbl()) {
@@ -87,6 +75,20 @@ i915_gem_object_get_pages_internal(struct drm_i915_gem_object *obj)
 		gfp |= __GFP_DMA32;
 	}
 
+create_st:
+	st = kmalloc(sizeof(*st), GFP_KERNEL);
+	if (!st)
+		return ERR_PTR(-ENOMEM);
+
+	npages = obj->base.size / PAGE_SIZE;
+	if (sg_alloc_table(st, npages, GFP_KERNEL)) {
+		kfree(st);
+		return ERR_PTR(-ENOMEM);
+	}
+
+	sg = st->sgl;
+	st->nents = 0;
+
 	do {
 		int order = min(fls(npages) - 1, max_order);
 		struct page *page;
@@ -114,8 +116,15 @@ i915_gem_object_get_pages_internal(struct drm_i915_gem_object *obj)
 		sg = __sg_next(sg);
 	} while (1);
 
-	if (i915_gem_gtt_prepare_pages(obj, st))
+	if (i915_gem_gtt_prepare_pages(obj, st)) {
+		/* Failed to dma-map try again with single page sg segments */
+		if (get_order(st->sgl->length)) {
+			internal_free_pages(st);
+			max_order = 0;
+			goto create_st;
+		}
 		goto err;
+	}
 
 	/* Mark the pages as dontneed whilst they are still pinned. As soon
 	 * as they are unpinned they are allowed to be reaped by the shrinker,