drm: Catch stop possible NULL pointer reference

This was caught by Weiss. Also added some comments to the
fb_changed and mode_changed variables to explain what they do.

Signed-off-by: Jakob Bornecrantz <jakob@vmware.com>
Tested-by: Thomas White <taw@bitwiz.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/drivers/gpu/drm/drm_crtc_helper.c b/drivers/gpu/drm/drm_crtc_helper.c
index 3da9cfa31848..6aaa2cb23365 100644
--- a/drivers/gpu/drm/drm_crtc_helper.c
+++ b/drivers/gpu/drm/drm_crtc_helper.c
@@ -706,8 +706,8 @@ int drm_crtc_helper_set_config(struct drm_mode_set *set)
         struct drm_encoder **save_encoders, *new_encoder;
         struct drm_framebuffer *old_fb = NULL;
         bool save_enabled;
-        bool mode_changed = false;
-        bool fb_changed = false;
+        bool mode_changed = false; /* if true do a full mode set */
+        bool fb_changed = false; /* if true and !mode_changed just do a flip */
         struct drm_connector *connector;
         int count = 0, ro, fail = 0;
         struct drm_crtc_helper_funcs *crtc_funcs;
@@ -758,6 +758,8 @@ int drm_crtc_helper_set_config(struct drm_mode_set *set)
                 if (set->crtc->fb == NULL) {
                         DRM_DEBUG("crtc has no fb, full mode set\n");
                         mode_changed = true;
+                } else if (set->fb == NULL) {
+                        mode_changed = true;
                 } else if ((set->fb->bits_per_pixel !=
                          set->crtc->fb->bits_per_pixel) ||
                          set->fb->depth != set->crtc->fb->depth)