drm/vkms: Allow to attach connectors and encoders via configfs

Create a default subgroup at
/config/vkms/connectors/connector/possible_encoders that will contain
symbolic links to the possible encoders for the connector.

Tested-by: Mark Yacoub <markyacoub@google.com>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
Co-developed-by: José Expósito <jose.exposito89@gmail.com>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Link: https://lore.kernel.org/r/20251016175618.10051-12-jose.exposito89@gmail.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

Author: Fomys

Documentation/gpu/vkms.rst

@@ -112,6 +112,7 @@ To finish the configuration, link the different pipeline items::
 
   sudo ln -s /config/vkms/my-vkms/crtcs/crtc0 /config/vkms/my-vkms/planes/plane0/possible_crtcs
   sudo ln -s /config/vkms/my-vkms/crtcs/crtc0 /config/vkms/my-vkms/encoders/encoder0/possible_crtcs
+  sudo ln -s /config/vkms/my-vkms/encoders/encoder0 /config/vkms/my-vkms/connectors/connector0/possible_encoders
 
 Since at least one primary plane is required, make sure to set the right type::
 
@@ -129,6 +130,7 @@ And removing the top level directory and its subdirectories::
 
   sudo rm /config/vkms/my-vkms/planes/*/possible_crtcs/*
   sudo rm /config/vkms/my-vkms/encoders/*/possible_crtcs/*
+  sudo rm /config/vkms/my-vkms/connectors/*/possible_encoders/*
   sudo rmdir /config/vkms/my-vkms/planes/*
   sudo rmdir /config/vkms/my-vkms/crtcs/*
   sudo rmdir /config/vkms/my-vkms/encoders/*

drivers/gpu/drm/vkms/vkms_configfs.c

@@ -88,11 +88,14 @@ struct vkms_configfs_encoder {
  *
  * @group: Top level configuration group that represents a connector.
  * Initialized when a new directory is created under "/config/vkms/connectors"
+ * @possible_encoders_group: Default subgroup of @group at
+ * "connector/possible_encoders"
  * @dev: The vkms_configfs_device this connector belongs to
  * @config: Configuration of the VKMS connector
  */
 struct vkms_configfs_connector {
 	struct config_group group;
+	struct config_group possible_encoders_group;
 	struct vkms_configfs_device *dev;
 	struct vkms_config_connector *config;
 };
@@ -126,6 +129,10 @@ struct vkms_configfs_connector {
 	container_of(to_config_group((item)), struct vkms_configfs_connector, \
 		     group)
 
+#define connector_possible_encoders_item_to_vkms_configfs_connector(item) \
+	container_of(to_config_group((item)), struct vkms_configfs_connector, \
+		     possible_encoders_group)
+
 static ssize_t crtc_writeback_show(struct config_item *item, char *page)
 {
 	struct vkms_configfs_crtc *crtc;
@@ -528,6 +535,55 @@ static const struct config_item_type connector_item_type = {
 	.ct_owner	= THIS_MODULE,
 };
 
+static int connector_possible_encoders_allow_link(struct config_item *src,
+						  struct config_item *target)
+{
+	struct vkms_configfs_connector *connector;
+	struct vkms_configfs_encoder *encoder;
+	int ret;
+
+	if (target->ci_type != &encoder_item_type)
+		return -EINVAL;
+
+	connector = connector_possible_encoders_item_to_vkms_configfs_connector(src);
+	encoder = encoder_item_to_vkms_configfs_encoder(target);
+
+	scoped_guard(mutex, &connector->dev->lock) {
+		if (connector->dev->enabled)
+			return -EBUSY;
+
+		ret = vkms_config_connector_attach_encoder(connector->config,
+							   encoder->config);
+	}
+
+	return ret;
+}
+
+static void connector_possible_encoders_drop_link(struct config_item *src,
+						  struct config_item *target)
+{
+	struct vkms_configfs_connector *connector;
+	struct vkms_configfs_encoder *encoder;
+
+	connector = connector_possible_encoders_item_to_vkms_configfs_connector(src);
+	encoder = encoder_item_to_vkms_configfs_encoder(target);
+
+	scoped_guard(mutex, &connector->dev->lock) {
+		vkms_config_connector_detach_encoder(connector->config,
+						     encoder->config);
+	}
+}
+
+static struct configfs_item_operations connector_possible_encoders_item_operations = {
+	.allow_link	= connector_possible_encoders_allow_link,
+	.drop_link	= connector_possible_encoders_drop_link,
+};
+
+static const struct config_item_type connector_possible_encoders_group_type = {
+	.ct_item_ops	= &connector_possible_encoders_item_operations,
+	.ct_owner	= THIS_MODULE,
+};
+
 static struct config_group *make_connector_group(struct config_group *group,
 						 const char *name)
 {
@@ -554,6 +610,12 @@ static struct config_group *make_connector_group(struct config_group *group,
 
 		config_group_init_type_name(&connector->group, name,
 					    &connector_item_type);
+
+		config_group_init_type_name(&connector->possible_encoders_group,
+					    "possible_encoders",
+					    &connector_possible_encoders_group_type);
+		configfs_add_default_group(&connector->possible_encoders_group,
+					   &connector->group);
 	}
 
 	return &connector->group;