kobject: update documentation

- 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

1.2 Definition

struct kobject {

	const char		* k_name;

	char			name[KOBJ_NAME_LEN];

	atomic_t		refcount;

	struct kref		kref;

	struct list_head	entry;

	struct kobject		* parent;

	struct kset		* kset;

	struct kobj_type	* ktype;

	struct dentry		* dentry;

	struct sysfs_dirent	* sd;

	wait_queue_head_t	poll;

};

void kobject_init(struct kobject *);

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. 

is created at the top-level of the sysfs partition.

struct kset {

	struct subsystem	* subsys;

	struct kobj_type	* ktype;

	struct list_head	list;

	struct kobject		kobj;

	struct kset_uevent_ops	* uevent_ops;

};

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. 

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_find_obj() may be used to locate a kobject with a particular

name. The kobject, if found, is returned. 

There are also some helper functions which names point to the formerly

existing "struct subsystem", whose functions have been taken over by

ksets.

decl_subsys(name,type,uevent_ops)

Declares a kset named '<name>_subsys' of type <type> with

uevent_ops <uevent_ops>. For example,

decl_subsys(devices, &ktype_device, &device_uevent_ops);

is equivalent to doing:

struct kset devices_subsys = {

     .kobj = {

	   .name = "devices",

     },

     .ktype = &ktype_devices,

     .uevent_ops = &device_uevent_ops,

};

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 or ksets. 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 kset <subsys>.

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 kset <subsys>.

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.

void subsystem_init(struct kset *s);

int subsystem_register(struct kset *s);

void subsystem_unregister(struct kset *s);

struct kset *subsys_get(struct kset *s);

void kset_put(struct kset *s);

These are just wrappers around the respective kset_* functions.

2.3 sysfs

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).