+++ /dev/null
-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.
-
-
-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 {
- const char * k_name;
- struct kref kref;
- struct list_head entry;
- struct kobject * parent;
- struct kset * kset;
- struct kobj_type * ktype;
- struct sysfs_dirent * sd;
- wait_queue_head_t poll;
-};
-
-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.
-
-
-
-2. ksets
-
-2.1 Description
-
-A kset is a set of kobjects that are embedded in the same type.
-
-
-struct kset {
- struct kobj_type * ktype;
- struct list_head list;
- struct kobject kobj;
- struct kset_uevent_ops * uevent_ops;
-};
-
-
-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.
-
-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.
-
-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 = {
- .ktype = &ktype_devices,
- .uevent_ops = &device_uevent_ops,
-};
-kobject_set_name(&devices_subsys, name);
-
-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 helper makes 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>.
-
-int subsystem_register(struct kset *s);
-void subsystem_unregister(struct kset *s);
-
-These are just wrappers around the respective kset_* functions.
-
-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.
-
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