driver/base: implement subsys_virtual_register()

Kay tells me the most appropriate place to expose workqueues to
userland would be /sys/devices/virtual/workqueues/WQ_NAME which is
symlinked to /sys/bus/workqueue/devices/WQ_NAME and that we're lacking
a way to do that outside of driver core as virtual_device_parent()
isn't exported and there's no inteface to conveniently create a
virtual subsystem.

This patch implements subsys_virtual_register() by factoring out
subsys_register() from subsys_system_register() and using it with
virtual_device_parent() as the origin directory.  It's identical to
subsys_system_register() other than the origin directory but we aren't
gonna restrict the device names which should be used under it.

This will be used to expose workqueue attributes to userland.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Kay Sievers <kay.sievers@vrfy.org>

drivers/base/base.h

@@ -101,6 +101,8 @@ static inline int hypervisor_init(void) { return 0; }
 extern int platform_bus_init(void);
 extern void cpu_dev_init(void);
 
+struct kobject *virtual_device_parent(struct device *dev);
+
 extern int bus_add_device(struct device *dev);
 extern void bus_probe_device(struct device *dev);
 extern void bus_remove_device(struct device *dev);

drivers/base/bus.c

@@ -1205,6 +1205,49 @@ static void system_root_device_release(struct device *dev)
 {
 	kfree(dev);
 }
+
+static int subsys_register(struct bus_type *subsys,
+			   const struct attribute_group **groups,
+			   struct kobject *parent_of_root)
+{
+	struct device *dev;
+	int err;
+
+	err = bus_register(subsys);
+	if (err < 0)
+		return err;
+
+	dev = kzalloc(sizeof(struct device), GFP_KERNEL);
+	if (!dev) {
+		err = -ENOMEM;
+		goto err_dev;
+	}
+
+	err = dev_set_name(dev, "%s", subsys->name);
+	if (err < 0)
+		goto err_name;
+
+	dev->kobj.parent = parent_of_root;
+	dev->groups = groups;
+	dev->release = system_root_device_release;
+
+	err = device_register(dev);
+	if (err < 0)
+		goto err_dev_reg;
+
+	subsys->dev_root = dev;
+	return 0;
+
+err_dev_reg:
+	put_device(dev);
+	dev = NULL;
+err_name:
+	kfree(dev);
+err_dev:
+	bus_unregister(subsys);
+	return err;
+}
+
 /**
  * subsys_system_register - register a subsystem at /sys/devices/system/
  * @subsys: system subsystem
@@ -1226,45 +1269,33 @@ static void system_root_device_release(struct device *dev)
 int subsys_system_register(struct bus_type *subsys,
 			   const struct attribute_group **groups)
 {
-	struct device *dev;
-	int err;
-
-	err = bus_register(subsys);
-	if (err < 0)
-		return err;
-
-	dev = kzalloc(sizeof(struct device), GFP_KERNEL);
-	if (!dev) {
-		err = -ENOMEM;
-		goto err_dev;
-	}
-
-	err = dev_set_name(dev, "%s", subsys->name);
-	if (err < 0)
-		goto err_name;
-
-	dev->kobj.parent = &system_kset->kobj;
-	dev->groups = groups;
-	dev->release = system_root_device_release;
-
-	err = device_register(dev);
-	if (err < 0)
-		goto err_dev_reg;
-
-	subsys->dev_root = dev;
-	return 0;
-
-err_dev_reg:
-	put_device(dev);
-	dev = NULL;
-err_name:
-	kfree(dev);
-err_dev:
-	bus_unregister(subsys);
-	return err;
+	return subsys_register(subsys, groups, &system_kset->kobj);
 }
 EXPORT_SYMBOL_GPL(subsys_system_register);
 
+/**
+ * subsys_virtual_register - register a subsystem at /sys/devices/virtual/
+ * @subsys: virtual subsystem
+ * @groups: default attributes for the root device
+ *
+ * All 'virtual' subsystems have a /sys/devices/system/<name> root device
+ * with the name of the subystem.  The root device can carry subsystem-wide
+ * attributes.  All registered devices are below this single root device.
+ * There's no restriction on device naming.  This is for kernel software
+ * constructs which need sysfs interface.
+ */
+int subsys_virtual_register(struct bus_type *subsys,
+			    const struct attribute_group **groups)
+{
+	struct kobject *virtual_dir;
+
+	virtual_dir = virtual_device_parent(NULL);
+	if (!virtual_dir)
+		return -ENOMEM;
+
+	return subsys_register(subsys, groups, virtual_dir);
+}
+
 int __init buses_init(void)
 {
 	bus_kset = kset_create_and_add("bus", &bus_uevent_ops, NULL);

drivers/base/core.c

@@ -690,7 +690,7 @@ void device_initialize(struct device *dev)
 	set_dev_node(dev, -1);
 }
 
-static struct kobject *virtual_device_parent(struct device *dev)
+struct kobject *virtual_device_parent(struct device *dev)
 {
 	static struct kobject *virtual_dir = NULL;
 

include/linux/device.h

@@ -302,6 +302,8 @@ void subsys_interface_unregister(struct subsys_interface *sif);
 
 int subsys_system_register(struct bus_type *subsys,
 			   const struct attribute_group **groups);
+int subsys_virtual_register(struct bus_type *subsys,
+			    const struct attribute_group **groups);
 
 /**
  * struct class - device classes