drm/i915: Pin the ifbdev for the info->system_base GGTT mmapping

A long time ago (before 3.14) we relied on a permanent pinning of the
ifbdev to lock the fb in place inside the GGTT. However, the
introduction of stealing the BIOS framebuffer and reusing its address in
the GGTT for the fbdev has muddied waters and we use an inherited fb.
However, the inherited fb is only pinned whilst it is active and we no
longer have an explicit pin for the info->system_base mmapping used by
the fbdev. The result is that after some aperture pressure the fbdev may
be evicted, but we continue to write the fbcon into the same GGTT
address - overwriting anything else that may be put into that offset.
The effect is most pronounced across suspend/resume as
intel_fbdev_set_suspend() does a full clear over the whole scanout.

v2: Only unpin the intel_fb is we allocate it. If we inherit the fb from
the BIOS, we do not own the pinned vma (except for the reference we add
in this patch for our access via info->screen_base).

v3: Finish balancing the vma pinning for the normal !preallocated case.

v4: Try to simplify the pinning even further.
v5: Leak the VMA (cleaned up by object-free) to avoid complicated error paths.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: "Goel, Akash" <akash.goel@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Lukas Wunner <lukas@wunner.de>
Cc: drm-intel-fixes@lists.freedesktop.org
Link: http://patchwork.freedesktop.org/patch/msgid/1449245126-26158-1-git-send-email-chris@chris-wilson.co.uk
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_fbdev.c

@@ -163,13 +163,6 @@ static int intelfb_alloc(struct drm_fb_helper *helper,
 		goto out;
 	}
 
-	/* Flush everything out, we'll be doing GTT only from now on */
-	ret = intel_pin_and_fence_fb_obj(NULL, fb, NULL);
-	if (ret) {
-		DRM_ERROR("failed to pin obj: %d\n", ret);
-		goto out;
-	}
-
 	mutex_unlock(&dev->struct_mutex);
 
 	ifbdev->fb = to_intel_framebuffer(fb);
@@ -225,6 +218,14 @@ static int intelfb_create(struct drm_fb_helper *helper,
 
 	mutex_lock(&dev->struct_mutex);
 
+	/* Pin the GGTT vma for our access via info->screen_base.
+	 * This also validates that any existing fb inherited from the
+	 * BIOS is suitable for own access.
+	 */
+	ret = intel_pin_and_fence_fb_obj(NULL, &ifbdev->fb->base, NULL);
+	if (ret)
+		goto out_unlock;
+
 	info = drm_fb_helper_alloc_fbi(helper);
 	if (IS_ERR(info)) {
 		DRM_ERROR("Failed to allocate fb_info\n");
@@ -287,6 +288,7 @@ out_destroy_fbi:
 	drm_fb_helper_release_fbi(helper);
 out_unpin:
 	i915_gem_object_ggtt_unpin(obj);
+out_unlock:
 	mutex_unlock(&dev->struct_mutex);
 	return ret;
 }
@@ -524,6 +526,10 @@ static const struct drm_fb_helper_funcs intel_fb_helper_funcs = {
 static void intel_fbdev_destroy(struct drm_device *dev,
 				struct intel_fbdev *ifbdev)
 {
+	/* We rely on the object-free to release the VMA pinning for
+	 * the info->screen_base mmaping. Leaking the VMA is simpler than
+	 * trying to rectify all the possible error paths leading here.
+	 */
 
 	drm_fb_helper_unregister_fbi(&ifbdev->helper);
 	drm_fb_helper_release_fbi(&ifbdev->helper);