drm/i915: Convert 4096 alignment request to 0 for drm_mm allocations

As we always allocate in chunks of 4096 (that being both the PAGE_SIZE
and our own GTT_PAGE_SIZE), we know that all results from the drm_mm are
aligned to at least 4096. The drm_mm allocator itself is optimised for
alignment == 0, and so by converting alignments of 4096 to 0 we can
satisfy our own requirements and still hit the faster path.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470324762-2545-10-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -3072,6 +3072,15 @@ i915_gem_object_insert_into_vm(struct drm_i915_gem_object *obj,
 			alloc_flag = DRM_MM_CREATE_DEFAULT;
 		}
 
+		/* We only allocate in PAGE_SIZE/GTT_PAGE_SIZE (4096) chunks,
+		 * so we know that we always have a minimum alignment of 4096.
+		 * The drm_mm range manager is optimised to return results
+		 * with zero alignment, so where possible use the optimal
+		 * path.
+		 */
+		if (alignment <= 4096)
+			alignment = 0;
+
 search_free:
 		ret = drm_mm_insert_node_in_range_generic(&vm->mm, &vma->node,
 							  size, alignment,