drm/i915: refactor eb_get_batch()

Precursor for fix to secure batch execution. We will need to be able to
retrieve the batch VMA (as well as the batch itself) from the eb list,
so this patch extracts that part of eb_get_batch() into a separate
function, and moves both parts to a more logical place in the file, near
where the eb list is created.

Also, it may not be obvious, but the current execbuffer2 ioctl interface
requires that the buffer object containing the batch-to-be-executed be
the LAST entry in the exec2_list[] array (I expected it to be the
first!).

To clarify this, we can replace the rather obscure construct
	"list_entry(eb->vmas.prev, ...)"
in the old version of eb_get_batch() with the equivalent but more
explicit
	"list_last_entry(&eb->vmas,...)"
in the new eb_get_batch_vma() and of course add an explanatory comment.

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468504324-12690-2-git-send-email-david.s.gordon@intel.com

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -186,6 +186,35 @@ err:
 	return ret;
 }
 
+static inline struct i915_vma *
+eb_get_batch_vma(struct eb_vmas *eb)
+{
+	/* The batch is always the LAST item in the VMA list */
+	struct i915_vma *vma = list_last_entry(&eb->vmas, typeof(*vma), exec_list);
+
+	return vma;
+}
+
+static struct drm_i915_gem_object *
+eb_get_batch(struct eb_vmas *eb)
+{
+	struct i915_vma *vma = eb_get_batch_vma(eb);
+
+	/*
+	 * SNA is doing fancy tricks with compressing batch buffers, which leads
+	 * to negative relocation deltas. Usually that works out ok since the
+	 * relocate address is still positive, except when the batch is placed
+	 * very low in the GTT. Ensure this doesn't happen.
+	 *
+	 * Note that actual hangs have only been observed on gen7, but for
+	 * paranoia do it everywhere.
+	 */
+	if ((vma->exec_entry->flags & EXEC_OBJECT_PINNED) == 0)
+		vma->exec_entry->flags |= __EXEC_OBJECT_NEEDS_BIAS;
+
+	return vma->obj;
+}
+
 static struct i915_vma *eb_get_vma(struct eb_vmas *eb, unsigned long handle)
 {
 	if (eb->and < 0) {
@@ -1341,26 +1370,6 @@ gen8_dispatch_bsd_ring(struct drm_i915_private *dev_priv, struct drm_file *file)
 	return file_priv->bsd_ring;
 }
 
-static struct drm_i915_gem_object *
-eb_get_batch(struct eb_vmas *eb)
-{
-	struct i915_vma *vma = list_entry(eb->vmas.prev, typeof(*vma), exec_list);
-
-	/*
-	 * SNA is doing fancy tricks with compressing batch buffers, which leads
-	 * to negative relocation deltas. Usually that works out ok since the
-	 * relocate address is still positive, except when the batch is placed
-	 * very low in the GTT. Ensure this doesn't happen.
-	 *
-	 * Note that actual hangs have only been observed on gen7, but for
-	 * paranoia do it everywhere.
-	 */
-	if ((vma->exec_entry->flags & EXEC_OBJECT_PINNED) == 0)
-		vma->exec_entry->flags |= __EXEC_OBJECT_NEEDS_BIAS;
-
-	return vma->obj;
-}
-
 #define I915_USER_RINGS (4)
 
 static const enum intel_engine_id user_ring_map[I915_USER_RINGS + 1] = {