drm/i915: Restore sane defaults for KMS on GEM error load

If we fail during GEM initialisation, we scrub the HW state by
performing a device level GPU resuet. However, we want to leave the
system in a usable state (with functioning KMS but no GEM) so after
scrubbing the HW state, we need to restore some sane defaults and
re-enable the low-level common parts of the GPU (such as the GMCH).

v2: Restore GTT entries.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180726085033.4044-2-chris@chris-wilson.co.ukReviewed-by: Michał Winiarski <michal.winiarski@intel.com>

drivers/gpu/drm/i915/i915_gem.c

@@ -5598,6 +5598,8 @@ int i915_gem_init(struct drm_i915_private *dev_priv)
 		i915_gem_cleanup_userptr(dev_priv);
 
 	if (ret == -EIO) {
+		mutex_lock(&dev_priv->drm.struct_mutex);
+
 		/*
 		 * Allow engine initialisation to fail by marking the GPU as
 		 * wedged. But we only want to do this where the GPU is angry,
@@ -5608,7 +5610,14 @@ int i915_gem_init(struct drm_i915_private *dev_priv)
 					"Failed to initialize GPU, declaring it wedged!\n");
 			i915_gem_set_wedged(dev_priv);
 		}
-		ret = 0;
+
+		/* Minimal basic recovery for KMS */
+		ret = i915_ggtt_enable_hw(dev_priv);
+		i915_gem_restore_gtt_mappings(dev_priv);
+		i915_gem_restore_fences(dev_priv);
+		intel_init_clock_gating(dev_priv);
+
+		mutex_unlock(&dev_priv->drm.struct_mutex);
 	}
 
 	i915_gem_drain_freed_objects(dev_priv);