drm/connector: Add a way to init/add a connector in separate steps

Atm when the connector is added to the drm_mode_config::connector_list,
the connector may not be fully initialized yet. This is not a problem
for static connectors initialized/added during driver loading, for which
the driver ensures that look-ups via the above list are not possible
until all the connector and other required state is fully initialized
already. It's also not a problem for user space looking up either a
static or dynamic (see what this is below) connector, since this will be
only possible once the connector is registered.

A dynamic - atm only a DP MST - connector can be initialized and added
after the load time initialization is done. Such a connector may be
looked up by in-kernel users once it's added to the connector list. In
particular a hotplug handler could perform a detection on all the
connectors on the list and hence find a connector there which isn't yet
initialized. For instance the connector's helper hooks may be unset,
leading to a NULL dereference while the detect helper calls the
connector's drm_connector_helper_funcs::detect() or detect_ctx()
handler.

To resolve the above issue, add a way for dynamic connectors to
separately initialize the DRM core specific parts of the connector
without adding it to the connector list - by calling the new
drm_connector_dynamic_init() - and to add the connector to the list
later once all the initialization is complete and the connector is
registered - by calling the new drm_connector_dynamic_register().

Adding the above 2 functions was also motivated to make the distinction
of the interface between static and dynamic connectors clearer: Drivers
should manually initialize and register only dynamic connectors (with
the above 2 functions). A driver should only initialize a static
connector (with one of the drm_connector_init*, drmm_connector_init*
functions) while the registration of the connector will be done
automatically by DRM core.

v2: (Jani)
- Let initing DDC as well via drm_connector_init_core().
- Rename __drm_connector_init to drm_connector_init_core_and_add().

v3:
- Rename drm_connector_init_core() to drm_connector_dynamic_init().
  (Sima)
- Instead of exporting drm_connector_add(), move adding the connector
  to the registration step via a new drm_connector_dynamic_register().
  (Sima)
- Update drm_connector_dynamic_init()'s function documentation and the
  commit log according to the above changes.
- Update the commit log describing the problematic scenario during
  connector detection. (Maxime)

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com> (v1)
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241211230328.4012496-2-imre.deak@intel.com

drivers/gpu/drm/drm_connector.c

@@ -218,11 +218,11 @@ void drm_connector_free_work_fn(struct work_struct *work)
 	}
 }
 
-static int __drm_connector_init(struct drm_device *dev,
-				struct drm_connector *connector,
-				const struct drm_connector_funcs *funcs,
-				int connector_type,
-				struct i2c_adapter *ddc)
+static int drm_connector_init_only(struct drm_device *dev,
+				   struct drm_connector *connector,
+				   const struct drm_connector_funcs *funcs,
+				   int connector_type,
+				   struct i2c_adapter *ddc)
 {
 	struct drm_mode_config *config = &dev->mode_config;
 	int ret;
@@ -273,6 +273,7 @@ static int __drm_connector_init(struct drm_device *dev,
 	/* provide ddc symlink in sysfs */
 	connector->ddc = ddc;
 
+	INIT_LIST_HEAD(&connector->head);
 	INIT_LIST_HEAD(&connector->global_connector_list_entry);
 	INIT_LIST_HEAD(&connector->probed_modes);
 	INIT_LIST_HEAD(&connector->modes);
@@ -289,14 +290,6 @@ static int __drm_connector_init(struct drm_device *dev,
 
 	drm_connector_get_cmdline_mode(connector);
 
-	/* We should add connectors at the end to avoid upsetting the connector
-	 * index too much.
-	 */
-	spin_lock_irq(&config->connector_list_lock);
-	list_add_tail(&connector->head, &config->connector_list);
-	config->num_connector++;
-	spin_unlock_irq(&config->connector_list_lock);
-
 	if (connector_type != DRM_MODE_CONNECTOR_VIRTUAL &&
 	    connector_type != DRM_MODE_CONNECTOR_WRITEBACK)
 		drm_connector_attach_edid_property(connector);
@@ -333,6 +326,58 @@ out_put:
 	return ret;
 }
 
+static void drm_connector_add(struct drm_connector *connector)
+{
+	struct drm_device *dev = connector->dev;
+	struct drm_mode_config *config = &dev->mode_config;
+
+	/*
+	 * TODO: Change this to a WARN, once all drivers are converted to
+	 * call drm_connector_dynamic_init() for MST connectors.
+	 */
+	if (!list_empty(&connector->head))
+		return;
+
+	spin_lock_irq(&config->connector_list_lock);
+	list_add_tail(&connector->head, &config->connector_list);
+	config->num_connector++;
+	spin_unlock_irq(&config->connector_list_lock);
+}
+
+static void drm_connector_remove(struct drm_connector *connector)
+{
+	struct drm_device *dev = connector->dev;
+
+	/*
+	 * For dynamic connectors drm_connector_cleanup() can call this function
+	 * before the connector is registered and added to the list.
+	 */
+	if (list_empty(&connector->head))
+		return;
+
+	spin_lock_irq(&dev->mode_config.connector_list_lock);
+	list_del_init(&connector->head);
+	dev->mode_config.num_connector--;
+	spin_unlock_irq(&dev->mode_config.connector_list_lock);
+}
+
+static int drm_connector_init_and_add(struct drm_device *dev,
+				      struct drm_connector *connector,
+				      const struct drm_connector_funcs *funcs,
+				      int connector_type,
+				      struct i2c_adapter *ddc)
+{
+	int ret;
+
+	ret = drm_connector_init_only(dev, connector, funcs, connector_type, ddc);
+	if (ret)
+		return ret;
+
+	drm_connector_add(connector);
+
+	return 0;
+}
+
 /**
  * drm_connector_init - Init a preallocated connector
  * @dev: DRM device
@@ -362,10 +407,51 @@ int drm_connector_init(struct drm_device *dev,
 	if (drm_WARN_ON(dev, !(funcs && funcs->destroy)))
 		return -EINVAL;
 
-	return __drm_connector_init(dev, connector, funcs, connector_type, NULL);
+	return drm_connector_init_and_add(dev, connector, funcs, connector_type, NULL);
 }
 EXPORT_SYMBOL(drm_connector_init);
 
+/**
+ * drm_connector_dynamic_init - Init a preallocated dynamic connector
+ * @dev: DRM device
+ * @connector: the connector to init
+ * @funcs: callbacks for this connector
+ * @connector_type: user visible type of the connector
+ * @ddc: pointer to the associated ddc adapter
+ *
+ * Initialises a preallocated dynamic connector. Connectors should be
+ * subclassed as part of driver connector objects. The connector
+ * structure should not be allocated with devm_kzalloc().
+ *
+ * Drivers should call this for dynamic connectors which can be hotplugged
+ * after drm_dev_register() has been called already, e.g. DP MST connectors.
+ * For all other - static - connectors, drivers should call one of the
+ * drm_connector_init*()/drmm_connector_init*() functions.
+ *
+ * After calling this function the drivers must call
+ * drm_connector_dynamic_register().
+ *
+ * To remove the connector the driver must call drm_connector_unregister()
+ * followed by drm_connector_put(). Putting the last reference will call the
+ * driver's &drm_connector_funcs.destroy hook, which in turn must call
+ * drm_connector_cleanup() and free the connector structure.
+ *
+ * Returns:
+ * Zero on success, error code on failure.
+ */
+int drm_connector_dynamic_init(struct drm_device *dev,
+			       struct drm_connector *connector,
+			       const struct drm_connector_funcs *funcs,
+			       int connector_type,
+			       struct i2c_adapter *ddc)
+{
+	if (drm_WARN_ON(dev, !(funcs && funcs->destroy)))
+		return -EINVAL;
+
+	return drm_connector_init_only(dev, connector, funcs, connector_type, ddc);
+}
+EXPORT_SYMBOL(drm_connector_dynamic_init);
+
 /**
  * drm_connector_init_with_ddc - Init a preallocated connector
  * @dev: DRM device
@@ -399,7 +485,7 @@ int drm_connector_init_with_ddc(struct drm_device *dev,
 	if (drm_WARN_ON(dev, !(funcs && funcs->destroy)))
 		return -EINVAL;
 
-	return __drm_connector_init(dev, connector, funcs, connector_type, ddc);
+	return drm_connector_init_and_add(dev, connector, funcs, connector_type, ddc);
 }
 EXPORT_SYMBOL(drm_connector_init_with_ddc);
 
@@ -443,7 +529,7 @@ int drmm_connector_init(struct drm_device *dev,
 	if (drm_WARN_ON(dev, funcs && funcs->destroy))
 		return -EINVAL;
 
-	ret = __drm_connector_init(dev, connector, funcs, connector_type, ddc);
+	ret = drm_connector_init_and_add(dev, connector, funcs, connector_type, ddc);
 	if (ret)
 		return ret;
 
@@ -660,10 +746,8 @@ void drm_connector_cleanup(struct drm_connector *connector)
 	connector->name = NULL;
 	fwnode_handle_put(connector->fwnode);
 	connector->fwnode = NULL;
-	spin_lock_irq(&dev->mode_config.connector_list_lock);
-	list_del(&connector->head);
-	dev->mode_config.num_connector--;
-	spin_unlock_irq(&dev->mode_config.connector_list_lock);
+
+	drm_connector_remove(connector);
 
 	WARN_ON(connector->state && !connector->funcs->atomic_destroy_state);
 	if (connector->state && connector->funcs->atomic_destroy_state)
@@ -750,6 +834,32 @@ unlock:
 }
 EXPORT_SYMBOL(drm_connector_register);
 
+/**
+ * drm_connector_dynamic_register - register a dynamic connector
+ * @connector: the connector to register
+ *
+ * Register userspace interfaces for a connector. Only call this for connectors
+ * initialized by calling drm_connector_dynamic_init(). All other connectors
+ * will be registered automatically when calling drm_dev_register().
+ *
+ * When the connector is no longer available the driver must call
+ * drm_connector_unregister().
+ *
+ * Returns:
+ * Zero on success, error code on failure.
+ */
+int drm_connector_dynamic_register(struct drm_connector *connector)
+{
+	/* Was the connector inited already? */
+	if (WARN_ON(!(connector->funcs && connector->funcs->destroy)))
+		return -EINVAL;
+
+	drm_connector_add(connector);
+
+	return drm_connector_register(connector);
+}
+EXPORT_SYMBOL(drm_connector_dynamic_register);
+
 /**
  * drm_connector_unregister - unregister a connector
  * @connector: the connector to unregister

include/drm/drm_connector.h

@@ -2129,6 +2129,11 @@ int drm_connector_init(struct drm_device *dev,
 		       struct drm_connector *connector,
 		       const struct drm_connector_funcs *funcs,
 		       int connector_type);
+int drm_connector_dynamic_init(struct drm_device *dev,
+			       struct drm_connector *connector,
+			       const struct drm_connector_funcs *funcs,
+			       int connector_type,
+			       struct i2c_adapter *ddc);
 int drm_connector_init_with_ddc(struct drm_device *dev,
 				struct drm_connector *connector,
 				const struct drm_connector_funcs *funcs,
@@ -2150,6 +2155,7 @@ int drmm_connector_hdmi_init(struct drm_device *dev,
 			     unsigned int max_bpc);
 void drm_connector_attach_edid_property(struct drm_connector *connector);
 int drm_connector_register(struct drm_connector *connector);
+int drm_connector_dynamic_register(struct drm_connector *connector);
 void drm_connector_unregister(struct drm_connector *connector);
 int drm_connector_attach_encoder(struct drm_connector *connector,
 				      struct drm_encoder *encoder);