Documentation/kobject.txt - arm/linux-arm64-legacy - Git at Google

 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 Defintion

 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 drivers/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).
	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 Defintion

	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 drivers/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).