i915: temporary fix for DP MST docking station NULL pointer dereference

torvalds · torvalds · commit 27667f4744fc · 2015-08-02T11:02:16.000-07:00
Ted Ts'o reports that his Lenovo T540p ThinkPad crashes at boot if attached to the docking station. This is a regression that he was able to bisect to commit 8c7b5cc: "drm/i915: Use atomic helpers for computing changed flags:" The reason seems to be the new call to drm_atomic_helper_check_modeset() added to intel_modeset_compute_config(), which in turn calls update_connector_routing(), and somehow ends up picking a NULL crtc for the connector state, causing the subsequent drm_crtc_index() to OOPS. Daniel Vetter says that the fundamental issue seems to be confusion in the encoder selection, and this isn't the right fix, but while he chases down the proper fix, this at least avoids the NULL pointer dereference and makes Ted's docking station work again. Reported-bisected-and-tested-by: Theodore Ts'o <tytso@mit.edu> Cc: Daniel Vetter <daniel.vetter@intel.com> Cc: Mani Nikula <jani.nikula@linux.intel.com> Cc: Dave Airlie <airlied@gmail.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
@@ -230,10 +230,12 @@ update_connector_routing(struct drm_atomic_state *state, int conn_idx)
 	}
 
 	connector_state->best_encoder = new_encoder;
-	idx = drm_crtc_index(connector_state->crtc);
+	if (connector_state->crtc) {
+		idx = drm_crtc_index(connector_state->crtc);
 
-	crtc_state = state->crtc_states[idx];
-	crtc_state->mode_changed = true;
+		crtc_state = state->crtc_states[idx];
+		crtc_state->mode_changed = true;
+	}
 
 	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] using [ENCODER:%d:%s] on [CRTC:%d]\n",
 			 connector->base.id,
