drm: Don't race connector registration

I was under the misconception that the sysfs dev stuff can be fully
set up, and then registered all in one step with device_add. That's
true for properties and property groups, but not for parents and child
devices. Those must be fully registered before you can register a
child.

Add a bit of tracking to make sure that asynchronous mst connector
hotplugging gets this right. For consistency we rely upon the implicit
barriers of the connector->mutex, which is taken anyway, to ensure
that at least either the connector or device registration call will
work out.

Mildly tested since I can't reliably reproduce this on my mst box
here.
Reported-by: Dave Hansen <dave.hansen@intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1484237756-2720-1-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_connector.c

@@ -379,6 +379,9 @@ int drm_connector_register(struct drm_connector *connector)
 {
 	int ret = 0;
 
+	if (!connector->dev->registered)
+		return 0;
+
 	mutex_lock(&connector->mutex);
 	if (connector->registered)
 		goto unlock;

drivers/gpu/drm/drm_drv.c

@@ -745,6 +745,8 @@ int drm_dev_register(struct drm_device *dev, unsigned long flags)
 	if (ret)
 		goto err_minors;
 
+	dev->registered = true;
+
 	if (dev->driver->load) {
 		ret = dev->driver->load(dev, flags);
 		if (ret)
@@ -785,6 +787,8 @@ void drm_dev_unregister(struct drm_device *dev)
 
 	drm_lastclose(dev);
 
+	dev->registered = false;
+
 	if (drm_core_check_feature(dev, DRIVER_MODESET))
 		drm_modeset_unregister_all(dev);
 

include/drm/drmP.h

@@ -517,6 +517,7 @@ struct drm_device {
 	struct drm_minor *control;		/**< Control node */
 	struct drm_minor *primary;		/**< Primary node */
 	struct drm_minor *render;		/**< Render node */
+	bool registered;
 
 	/* currently active master for this device. Protected by master_mutex */
 	struct drm_master *master;