drm/i915: Prevent stalling for a GTT read back from a read-only GPU target

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_drv.h

@@ -728,6 +728,12 @@ struct drm_i915_gem_object {
 	 */
 	unsigned int dirty : 1;
 
+	/**
+	 * This is set if the object has been written to since the last
+	 * GPU flush.
+	 */
+	unsigned int pending_gpu_write : 1;
+
 	/**
 	 * Fence register bits (if any) for this object.  Will be set
 	 * as needed when mapped into the GTT.

drivers/gpu/drm/i915/i915_gem.c

@@ -1643,6 +1643,7 @@ i915_gem_object_move_to_inactive(struct drm_i915_gem_object *obj)
 	obj->last_fenced_ring = NULL;
 
 	obj->active = 0;
+	obj->pending_gpu_write = false;
 	drm_gem_object_unreference(&obj->base);
 
 	WARN_ON(i915_verify_lists(dev));
@@ -2810,9 +2811,11 @@ i915_gem_object_set_to_gtt_domain(struct drm_i915_gem_object *obj, bool write)
 		return -EINVAL;
 
 	i915_gem_object_flush_gpu_write_domain(obj);
+	if (obj->pending_gpu_write || write) {
 		ret = i915_gem_object_wait_rendering(obj, true);
 		if (ret)
 			return ret;
+	}
 
 	i915_gem_object_flush_cpu_write_domain(obj);
 

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -775,6 +775,7 @@ i915_gem_execbuffer_move_to_active(struct list_head *objects,
 		i915_gem_object_move_to_active(obj, ring);
 		if (obj->base.write_domain) {
 			obj->dirty = 1;
+			obj->pending_gpu_write = true;
 			list_move_tail(&obj->gpu_write_list,
 				       &ring->gpu_write_list);
 			intel_mark_busy(ring->dev, obj);