/*
   * Sample kset and ktype implementation
   *
   * Copyright (C) 2004-2007 Greg Kroah-Hartman <greg@kroah.com>
   * Copyright (C) 2007 Novell Inc.
   *
   * Released under the GPL version 2 only.
   *
   */
  #include <linux/kobject.h>
  #include <linux/string.h>
  #include <linux/sysfs.h>
  #include <linux/slab.h>
  #include <linux/module.h>
  #include <linux/init.h>
  
  /*
   * This module shows how to create a kset in sysfs called
   * /sys/kernel/kset-example
   * Then tree kobjects are created and assigned to this kset, "foo", "baz",
   * and "bar".  In those kobjects, attributes of the same name are also
   * created and if an integer is written to these files, it can be later
   * read out of it.
   */
  
  
  /*
   * This is our "object" that we will create a few of and register them with
   * sysfs.
   */
  struct foo_obj {
  	struct kobject kobj;
  	int foo;
  	int baz;
  	int bar;
  };
  #define to_foo_obj(x) container_of(x, struct foo_obj, kobj)
  
  /* a custom attribute that works just for a struct foo_obj. */
  struct foo_attribute {
  	struct attribute attr;
  	ssize_t (*show)(struct foo_obj *foo, struct foo_attribute *attr, char *buf);
  	ssize_t (*store)(struct foo_obj *foo, struct foo_attribute *attr, const char *buf, size_t count);
  };
  #define to_foo_attr(x) container_of(x, struct foo_attribute, attr)
  
  /*
   * The default show function that must be passed to sysfs.  This will be
   * called by sysfs for whenever a show function is called by the user on a
   * sysfs file associated with the kobjects we have registered.  We need to
   * transpose back from a "default" kobject to our custom struct foo_obj and
   * then call the show function for that specific object.
   */
  static ssize_t foo_attr_show(struct kobject *kobj,
  			     struct attribute *attr,
  			     char *buf)
  {
  	struct foo_attribute *attribute;
  	struct foo_obj *foo;
  
  	attribute = to_foo_attr(attr);
  	foo = to_foo_obj(kobj);
  
  	if (!attribute->show)
  		return -EIO;
  
  	return attribute->show(foo, attribute, buf);
  }
  
  /*
   * Just like the default show function above, but this one is for when the
   * sysfs "store" is requested (when a value is written to a file.)
   */
  static ssize_t foo_attr_store(struct kobject *kobj,
  			      struct attribute *attr,
  			      const char *buf, size_t len)
  {
  	struct foo_attribute *attribute;
  	struct foo_obj *foo;
  
  	attribute = to_foo_attr(attr);
  	foo = to_foo_obj(kobj);
  
  	if (!attribute->store)
  		return -EIO;
  
  	return attribute->store(foo, attribute, buf, len);
  }
  
  /* Our custom sysfs_ops that we will associate with our ktype later on */
  static const struct sysfs_ops foo_sysfs_ops = {
  	.show = foo_attr_show,
  	.store = foo_attr_store,
  };
  
  /*
   * The release function for our object.  This is REQUIRED by the kernel to
   * have.  We free the memory held in our object here.
   *
   * NEVER try to get away with just a "blank" release function to try to be
   * smarter than the kernel.  Turns out, no one ever is...
   */
  static void foo_release(struct kobject *kobj)
  {
  	struct foo_obj *foo;
  
  	foo = to_foo_obj(kobj);
  	kfree(foo);
  }
  
  /*
   * The "foo" file where the .foo variable is read from and written to.
   */
  static ssize_t foo_show(struct foo_obj *foo_obj, struct foo_attribute *attr,
  			char *buf)
  {
  	return sprintf(buf, "%d
  ", foo_obj->foo);
  }
  
  static ssize_t foo_store(struct foo_obj *foo_obj, struct foo_attribute *attr,
  			 const char *buf, size_t count)
  {
  	sscanf(buf, "%du", &foo_obj->foo);
  	return count;
  }
  
  static struct foo_attribute foo_attribute =
  	__ATTR(foo, 0666, foo_show, foo_store);
  
  /*
   * More complex function where we determine which variable is being accessed by
   * looking at the attribute for the "baz" and "bar" files.
   */
  static ssize_t b_show(struct foo_obj *foo_obj, struct foo_attribute *attr,
  		      char *buf)
  {
  	int var;
  
  	if (strcmp(attr->attr.name, "baz") == 0)
  		var = foo_obj->baz;
  	else
  		var = foo_obj->bar;
  	return sprintf(buf, "%d
  ", var);
  }
  
  static ssize_t b_store(struct foo_obj *foo_obj, struct foo_attribute *attr,
  		       const char *buf, size_t count)
  {
  	int var;
  
  	sscanf(buf, "%du", &var);
  	if (strcmp(attr->attr.name, "baz") == 0)
  		foo_obj->baz = var;
  	else
  		foo_obj->bar = var;
  	return count;
  }
  
  static struct foo_attribute baz_attribute =
  	__ATTR(baz, 0666, b_show, b_store);
  static struct foo_attribute bar_attribute =
  	__ATTR(bar, 0666, b_show, b_store);
  
  /*
   * Create a group of attributes so that we can create and destroy them all
   * at once.
   */
  static struct attribute *foo_default_attrs[] = {
  	&foo_attribute.attr,
  	&baz_attribute.attr,
  	&bar_attribute.attr,
  	NULL,	/* need to NULL terminate the list of attributes */
  };
  
  /*
   * Our own ktype for our kobjects.  Here we specify our sysfs ops, the
   * release function, and the set of default attributes we want created
   * whenever a kobject of this type is registered with the kernel.
   */
  static struct kobj_type foo_ktype = {
  	.sysfs_ops = &foo_sysfs_ops,
  	.release = foo_release,
  	.default_attrs = foo_default_attrs,
  };
  
  static struct kset *example_kset;
  static struct foo_obj *foo_obj;
  static struct foo_obj *bar_obj;
  static struct foo_obj *baz_obj;
  
  static struct foo_obj *create_foo_obj(const char *name)
  {
  	struct foo_obj *foo;
  	int retval;
  
  	/* allocate the memory for the whole object */
  	foo = kzalloc(sizeof(*foo), GFP_KERNEL);
  	if (!foo)
  		return NULL;
  
  	/*
  	 * As we have a kset for this kobject, we need to set it before calling
  	 * the kobject core.
  	 */
  	foo->kobj.kset = example_kset;
  
  	/*
  	 * Initialize and add the kobject to the kernel.  All the default files
  	 * will be created here.  As we have already specified a kset for this
  	 * kobject, we don't have to set a parent for the kobject, the kobject
  	 * will be placed beneath that kset automatically.
  	 */
  	retval = kobject_init_and_add(&foo->kobj, &foo_ktype, NULL, "%s", name);
  	if (retval) {
  		kobject_put(&foo->kobj);
  		return NULL;
  	}
  
  	/*
  	 * We are always responsible for sending the uevent that the kobject
  	 * was added to the system.
  	 */
  	kobject_uevent(&foo->kobj, KOBJ_ADD);
  
  	return foo;
  }
  
  static void destroy_foo_obj(struct foo_obj *foo)
  {
  	kobject_put(&foo->kobj);
  }
  
  static int __init example_init(void)
  {
  	/*
  	 * Create a kset with the name of "kset_example",
  	 * located under /sys/kernel/
  	 */
  	example_kset = kset_create_and_add("kset_example", NULL, kernel_kobj);
  	if (!example_kset)
  		return -ENOMEM;
  
  	/*
  	 * Create three objects and register them with our kset
  	 */
  	foo_obj = create_foo_obj("foo");
  	if (!foo_obj)
  		goto foo_error;
  
  	bar_obj = create_foo_obj("bar");
  	if (!bar_obj)
  		goto bar_error;
  
  	baz_obj = create_foo_obj("baz");
  	if (!baz_obj)
  		goto baz_error;
  
  	return 0;
  
  baz_error:
  	destroy_foo_obj(bar_obj);
  bar_error:
  	destroy_foo_obj(foo_obj);
  foo_error:
  	kset_unregister(example_kset);
  	return -EINVAL;
  }
  
  static void __exit example_exit(void)
  {
  	destroy_foo_obj(baz_obj);
  	destroy_foo_obj(bar_obj);
  	destroy_foo_obj(foo_obj);
  	kset_unregister(example_kset);
  }
  
  module_init(example_init);
  module_exit(example_exit);
  MODULE_LICENSE("GPL");
  MODULE_AUTHOR("Greg Kroah-Hartman <greg@kroah.com>");