The kobject Infrastructure
  
  Patrick Mochel <mochel@osdl.org>
  
  Updated: 3 June 2003
  
  
  Copyright (c)  2003 Patrick Mochel
  Copyright (c)  2003 Open Source Development Labs
  
  
  0. Introduction
  
  The kobject infrastructure performs basic object management that larger
  data structures and subsystems can leverage, rather than reimplement
  similar functionality. This functionality primarily concerns:
  
  - Object reference counting.
  - Maintaining lists (sets) of objects.
  - Object set locking.
  - Userspace representation. 
  
  The infrastructure consists of a number of object types to support
  this functionality. Their programming interfaces are described below
  in detail, and briefly here:
  
  - kobjects	a simple object.
  - kset		a set of objects of a certain type.
  - ktype		a set of helpers for objects of a common type. 
  - subsystem	a controlling object for a number of ksets.
  
  
  The kobject infrastructure maintains a close relationship with the
  sysfs filesystem. Each kobject that is registered with the kobject
  core receives a directory in sysfs. Attributes about the kobject can
  then be exported. Please see Documentation/filesystems/sysfs.txt for
  more information. 
  
  The kobject infrastructure provides a flexible programming interface,
  and allows kobjects and ksets to be used without being registered
  (i.e. with no sysfs representation). This is also described later. 
  
  
  1. kobjects
  
  1.1 Description
  
  
  struct kobject is a simple data type that provides a foundation for
  more complex object types. It provides a set of basic fields that
  almost all complex data types share. kobjects are intended to be
  embedded in larger data structures and replace fields they duplicate. 

  1.2 Definition

  
  struct kobject {
  	char			name[KOBJ_NAME_LEN];
  	atomic_t		refcount;
  	struct list_head	entry;
  	struct kobject		* parent;
  	struct kset		* kset;
  	struct kobj_type	* ktype;
  	struct dentry		* dentry;
  };
  
  void kobject_init(struct kobject *);
  int kobject_add(struct kobject *);
  int kobject_register(struct kobject *);
  
  void kobject_del(struct kobject *);
  void kobject_unregister(struct kobject *);
  
  struct kobject * kobject_get(struct kobject *);
  void kobject_put(struct kobject *);
  
  
  1.3 kobject Programming Interface
  
  kobjects may be dynamically added and removed from the kobject core
  using kobject_register() and kobject_unregister(). Registration
  includes inserting the kobject in the list of its dominant kset and
  creating a directory for it in sysfs.
  
  Alternatively, one may use a kobject without adding it to its kset's list
  or exporting it via sysfs, by simply calling kobject_init(). An
  initialized kobject may later be added to the object hierarchy by
  calling kobject_add(). An initialized kobject may be used for
  reference counting.
  
  Note: calling kobject_init() then kobject_add() is functionally
  equivalent to calling kobject_register().
  
  When a kobject is unregistered, it is removed from its kset's list,
  removed from the sysfs filesystem, and its reference count is decremented.
  List and sysfs removal happen in kobject_del(), and may be called
  manually. kobject_put() decrements the reference count, and may also
  be called manually. 
  
  A kobject's reference count may be incremented with kobject_get(),
  which returns a valid reference to a kobject; and decremented with 
  kobject_put(). An object's reference count may only be incremented if
  it is already positive. 
  
  When a kobject's reference count reaches 0, the method struct
  kobj_type::release() (which the kobject's kset points to) is called.
  This allows any memory allocated for the object to be freed.
  
  
  NOTE!!! 
  
  It is _imperative_ that you supply a destructor for dynamically
  allocated kobjects to free them if you are using kobject reference
  counts. The reference count controls the lifetime of the object.
  If it goes to 0, then it is assumed that the object will
  be freed and cannot be used. 
  
  More importantly, you must free the object there, and not immediately
  after an unregister call. If someone else is referencing the object
  (e.g. through a sysfs file), they will obtain a reference to the
  object, assume it's valid and operate on it. If the object is
  unregistered and freed in the meantime, the operation will then
  reference freed memory and go boom. 
  
  This can be prevented, in the simplest case, by defining a release
  method and freeing the object from there only. Note that this will not
  secure reference count/object management models that use a dual
  reference count or do other wacky things with the reference count
  (like the networking layer). 
  
  
  1.4 sysfs
  
  Each kobject receives a directory in sysfs. This directory is created
  under the kobject's parent directory. 
  
  If a kobject does not have a parent when it is registered, its parent
  becomes its dominant kset. 
  
  If a kobject does not have a parent nor a dominant kset, its directory
  is created at the top-level of the sysfs partition. This should only
  happen for kobjects that are embedded in a struct subsystem. 
  
  
  
  2. ksets
  
  2.1 Description
  
  A kset is a set of kobjects that are embedded in the same type. 
  
  
  struct kset {
  	struct subsystem	* subsys;
  	struct kobj_type	* ktype;
  	struct list_head	list;
  	struct kobject		kobj;
  };
  
  
  void kset_init(struct kset * k);
  int kset_add(struct kset * k);
  int kset_register(struct kset * k);
  void kset_unregister(struct kset * k);
  
  struct kset * kset_get(struct kset * k);
  void kset_put(struct kset * k);
  
  struct kobject * kset_find_obj(struct kset *, char *);
  
  
  The type that the kobjects are embedded in is described by the ktype
  pointer. The subsystem that the kobject belongs to is pointed to by the
  subsys pointer. 
  
  A kset contains a kobject itself, meaning that it may be registered in
  the kobject hierarchy and exported via sysfs. More importantly, the
  kset may be embedded in a larger data type, and may be part of another
  kset (of that object type). 
  
  For example, a block device is an object (struct gendisk) that is
  contained in a set of block devices. It may also contain a set of
  partitions (struct hd_struct) that have been found on the device. The
  following code snippet illustrates how to express this properly.
  
  	 struct gendisk * disk;
  	 ...
  	 disk->kset.kobj.kset = &block_kset;
  	 disk->kset.ktype = &partition_ktype;
  	 kset_register(&disk->kset);
  
  - The kset that the disk's embedded object belongs to is the
    block_kset, and is pointed to by disk->kset.kobj.kset. 
  
  - The type of objects on the disk's _subordinate_ list are partitions, 
    and is set in disk->kset.ktype. 
  
  - The kset is then registered, which handles initializing and adding
    the embedded kobject to the hierarchy. 
  
  
  2.2 kset Programming Interface 
  
  All kset functions, except kset_find_obj(), eventually forward the
  calls to their embedded kobjects after performing kset-specific
  operations. ksets offer a similar programming model to kobjects: they
  may be used after they are initialized, without registering them in
  the hierarchy. 
  
  kset_find_obj() may be used to locate a kobject with a particular
  name. The kobject, if found, is returned. 
  
  
  2.3 sysfs
  
  ksets are represented in sysfs when their embedded kobjects are
  registered. They follow the same rules of parenting, with one
  exception. If a kset does not have a parent, nor is its embedded
  kobject part of another kset, the kset's parent becomes its dominant
  subsystem. 
  
  If the kset does not have a parent, its directory is created at the
  sysfs root. This should only happen when the kset registered is
  embedded in a subsystem itself. 
  
  
  3. struct ktype
  
  3.1. Description
  
  struct kobj_type {
  	void (*release)(struct kobject *);
  	struct sysfs_ops	* sysfs_ops;
  	struct attribute	** default_attrs;
  };
  
  
  Object types require specific functions for converting between the
  generic object and the more complex type. struct kobj_type provides
  the object-specific fields, which include:
  
  - release: Called when the kobject's reference count reaches 0. This
    should convert the object to the more complex type and free it. 
  
  - sysfs_ops: Provides conversion functions for sysfs access. Please
    see the sysfs documentation for more information. 
  
  - default_attrs: Default attributes to be exported via sysfs when the
    object is registered.Note that the last attribute has to be
    initialized to NULL ! You can find a complete implementation

    in block/genhd.c

  
  
  Instances of struct kobj_type are not registered; only referenced by
  the kset. A kobj_type may be referenced by an arbitrary number of
  ksets, as there may be disparate sets of identical objects. 
  
  
  
  4. subsystems
  
  4.1 Description
  
  A subsystem represents a significant entity of code that maintains an
  arbitrary number of sets of objects of various types. Since the number
  of ksets and the type of objects they contain are variable, a
  generic representation of a subsystem is minimal. 
  
  
  struct subsystem {
  	struct kset		kset;
  	struct rw_semaphore	rwsem;
  };
  
  int subsystem_register(struct subsystem *);
  void subsystem_unregister(struct subsystem *);
  
  struct subsystem * subsys_get(struct subsystem * s);
  void subsys_put(struct subsystem * s);
  
  
  A subsystem contains an embedded kset so:
  
  - It can be represented in the object hierarchy via the kset's
    embedded kobject. 
  
  - It can maintain a default list of objects of one type. 
  
  Additional ksets may attach to the subsystem simply by referencing the
  subsystem before they are registered. (This one-way reference means
  that there is no way to determine the ksets that are attached to the
  subsystem.) 
  
  All ksets that are attached to a subsystem share the subsystem's R/W
  semaphore. 
  
  
  4.2 subsystem Programming Interface.
  
  The subsystem programming interface is simple and does not offer the
  flexibility that the kset and kobject programming interfaces do. They
  may be registered and unregistered, as well as reference counted. Each
  call forwards the calls to their embedded ksets (which forward the
  calls to their embedded kobjects).
  
  
  4.3 Helpers
  
  A number of macros are available to make dealing with subsystems and
  their embedded objects easier. 
  
  
  decl_subsys(name,type)
  
  Declares a subsystem named '<name>_subsys', with an embedded kset of
  type <type>. For example, 
  
  decl_subsys(devices,&ktype_devices);
  
  is equivalent to doing:
  
  struct subsystem device_subsys = {
         .kset = {
  	     .kobj = {
  		   .name = "devices",
  	     },
  	     .ktype = &ktype_devices,
  	}
  }; 
  
  
  The objects that are registered with a subsystem that use the
  subsystem's default list must have their kset ptr set properly. These
  objects may have embedded kobjects, ksets, or other subsystems. The
  following helpers make setting the kset easier: 
  
  
  kobj_set_kset_s(obj,subsys)
  
  - Assumes that obj->kobj exists, and is a struct kobject. 
  - Sets the kset of that kobject to the subsystem's embedded kset.
  
  
  kset_set_kset_s(obj,subsys)
  
  - Assumes that obj->kset exists, and is a struct kset.
  - Sets the kset of the embedded kobject to the subsystem's 
    embedded kset. 
  
  subsys_set_kset(obj,subsys)
  
  - Assumes obj->subsys exists, and is a struct subsystem.
  - Sets obj->subsys.kset.kobj.kset to the subsystem's embedded kset.
  
  
  4.4 sysfs
  
  subsystems are represented in sysfs via their embedded kobjects. They
  follow the same rules as previously mentioned with no exceptions. They
  typically receive a top-level directory in sysfs, except when their
  embedded kobject is part of another kset, or the parent of the
  embedded kobject is explicitly set. 
  
  Note that the subsystem's embedded kset must be 'attached' to the
  subsystem itself in order to use its rwsem. This is done after
  kset_add() has been called. (Not before, because kset_add() uses its
  subsystem for a default parent if it doesn't already have one).