drm/i915: Don't oops if the initial modeset fails

If the first modeset operation fails, we will attempt to restore the
previous configuration that we read out from the hardware. But as we
don't yet reconstruct the framebuffer information, we end up calling
the modeset code with an enabled crtc but with fb==NULL. This will
lead to an oops within the modeset code.

Check for NULL fb when restoring the configuration, and instead of
oopsing simply disable the pipe.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -10024,6 +10024,29 @@ intel_modeset_stage_output_state(struct drm_device *dev,
 	return 0;
 }
 
+static void disable_crtc_nofb(struct intel_crtc *crtc)
+{
+	struct drm_device *dev = crtc->base.dev;
+	struct intel_encoder *encoder;
+	struct intel_connector *connector;
+
+	DRM_DEBUG_KMS("Trying to restore without FB -> disabling pipe %c\n",
+		      pipe_name(crtc->pipe));
+
+	list_for_each_entry(connector, &dev->mode_config.connector_list, base.head) {
+		if (connector->new_encoder &&
+		    connector->new_encoder->new_crtc == crtc)
+			connector->new_encoder = NULL;
+	}
+
+	list_for_each_entry(encoder, &dev->mode_config.encoder_list, base.head) {
+		if (encoder->new_crtc == crtc)
+			encoder->new_crtc = NULL;
+	}
+
+	crtc->new_enabled = false;
+}
+
 static int intel_crtc_set_config(struct drm_mode_set *set)
 {
 	struct drm_device *dev;
@@ -10100,6 +10123,15 @@ static int intel_crtc_set_config(struct drm_mode_set *set)
 fail:
 		intel_set_config_restore_state(dev, config);
 
+		/*
+		 * HACK: if the pipe was on, but we didn't have a framebuffer,
+		 * force the pipe off to avoid oopsing in the modeset code
+		 * due to fb==NULL. This should only happen during boot since
+		 * we don't yet reconstruct the FB from the hardware state.
+		 */
+		if (to_intel_crtc(save_set.crtc)->new_enabled && !save_set.fb)
+			disable_crtc_nofb(to_intel_crtc(save_set.crtc));
+
 		/* Try to restore the config */
 		if (config->mode_changed &&
 		    intel_set_mode(save_set.crtc, save_set.mode,