drm/cma-helper: Fix locking in drm_fb_cma_debugfs_show

This function takes two locks, both of them the wrong ones. This
wasn't an oversight from my fb locking rework since both patches
landed in parallel. We really only need fb_lock when walking that
list, since everything we can reach from that is refcounted properly
already.

v2: Drop unused dev spotted by 0day.

Cc: Rob Clark <robdclark@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_fb_cma_helper.c

@@ -209,23 +209,11 @@ int drm_fb_cma_debugfs_show(struct seq_file *m, void *arg)
 	struct drm_info_node *node = (struct drm_info_node *) m->private;
 	struct drm_device *dev = node->minor->dev;
 	struct drm_framebuffer *fb;
-	int ret;
-
-	ret = mutex_lock_interruptible(&dev->mode_config.mutex);
-	if (ret)
-		return ret;
-
-	ret = mutex_lock_interruptible(&dev->struct_mutex);
-	if (ret) {
-		mutex_unlock(&dev->mode_config.mutex);
-		return ret;
-	}
 
+	mutex_lock(&dev->mode_config.fb_lock);
 	list_for_each_entry(fb, &dev->mode_config.fb_list, head)
 		drm_fb_cma_describe(fb, m);
-
-	mutex_unlock(&dev->struct_mutex);
-	mutex_unlock(&dev->mode_config.mutex);
+	mutex_unlock(&dev->mode_config.fb_lock);
 
 	return 0;
 }

drivers/gpu/drm/drm_gem_cma_helper.c

@@ -381,11 +381,8 @@ void drm_gem_cma_describe(struct drm_gem_cma_object *cma_obj,
 			  struct seq_file *m)
 {
 	struct drm_gem_object *obj = &cma_obj->base;
-	struct drm_device *dev = obj->dev;
 	uint64_t off;
 
-	WARN_ON(!mutex_is_locked(&dev->struct_mutex));
-
 	off = drm_vma_node_start(&obj->vma_node);
 
 	seq_printf(m, "%2d (%2d) %08llx %pad %p %zu",