drm/i915: don't use the first stolen page on Broadwell

The spec says we just can't use it.

v2:
  - Add WA name (Ville).
  - Add a big comment explaining that we still didn't fix the problem
    where we inherit a framebuffer on the first page (Chris, Ville).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_stolen.c

@@ -51,6 +51,11 @@ int i915_gem_stolen_insert_node_in_range(struct drm_i915_private *dev_priv,
 	if (!drm_mm_initialized(&dev_priv->mm.stolen))
 		return -ENODEV;
 
+	/* See the comment at the drm_mm_init() call for more about this check.
+	 * WaSkipStolenMemoryFirstPage:bdw,chv (incomplete) */
+	if (INTEL_INFO(dev_priv)->gen == 8 && start < 4096)
+		start = 4096;
+
 	mutex_lock(&dev_priv->mm.stolen_lock);
 	ret = drm_mm_insert_node_in_range(&dev_priv->mm.stolen, node, size,
 					  alignment, start, end,
@@ -393,7 +398,17 @@ int i915_gem_init_stolen(struct drm_device *dev)
 	dev_priv->gtt.stolen_usable_size = dev_priv->gtt.stolen_size -
 					   reserved_total;
 
-	/* Basic memrange allocator for stolen space */
+	/*
+	 * Basic memrange allocator for stolen space.
+	 *
+	 * TODO: Notice that some platforms require us to not use the first page
+	 * of the stolen memory but their BIOSes may still put the framebuffer
+	 * on the first page. So we don't reserve this page for now because of
+	 * that. Our current solution is to just prevent new nodes from being
+	 * inserted on the first page - see the check we have at
+	 * i915_gem_stolen_insert_node_in_range(). We may want to fix the fbcon
+	 * problem later.
+	 */
 	drm_mm_init(&dev_priv->mm.stolen, 0, dev_priv->gtt.stolen_usable_size);
 
 	return 0;