drm: Only decouple the old_fb from the crtc is we call mode_set*

Otherwise when disabling the output we switch to the new fb (which is
likely NULL) and skip the call to mode_set -- leaking driver private
state on the old_fb.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=29857
Reported-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Airlie <airlied@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/drivers/gpu/drm/drm_crtc_helper.c b/drivers/gpu/drm/drm_crtc_helper.c
index 4238a19c36db..de152a58967d 100644
--- a/drivers/gpu/drm/drm_crtc_helper.c
+++ b/drivers/gpu/drm/drm_crtc_helper.c
@@ -637,13 +637,13 @@ int drm_crtc_helper_set_config(struct drm_mode_set *set)
                 mode_changed = true;
 
         if (mode_changed) {
-                old_fb = set->crtc->fb;
-                set->crtc->fb = set->fb;
                 set->crtc->enabled = (set->mode != NULL);
                 if (set->mode != NULL) {
                         DRM_DEBUG_KMS("attempting to set mode from"
                                         " userspace\n");
                         drm_mode_debug_printmodeline(set->mode);
+                        old_fb = set->crtc->fb;
+                        set->crtc->fb = set->fb;
                         if (!drm_crtc_helper_set_mode(set->crtc, set->mode,
                                                       set->x, set->y,
                                                       old_fb)) {