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kobject: remove old, outdated documentation.
As we are replacing the documentation, it's easier to do this in a two stage pass, delete the old file and add the new one. Cc: Kay Sievers <kay.sievers@vrfy.org> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
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The kobject Infrastructure
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Patrick Mochel <mochel@osdl.org>
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Updated: 3 June 2003
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Copyright (c) 2003 Patrick Mochel
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Copyright (c) 2003 Open Source Development Labs
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0. Introduction
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The kobject infrastructure performs basic object management that larger
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data structures and subsystems can leverage, rather than reimplement
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similar functionality. This functionality primarily concerns:
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- Object reference counting.
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- Maintaining lists (sets) of objects.
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- Object set locking.
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- Userspace representation.
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The infrastructure consists of a number of object types to support
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this functionality. Their programming interfaces are described below
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in detail, and briefly here:
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- kobjects a simple object.
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- kset a set of objects of a certain type.
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- ktype a set of helpers for objects of a common type.
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The kobject infrastructure maintains a close relationship with the
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sysfs filesystem. Each kobject that is registered with the kobject
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core receives a directory in sysfs. Attributes about the kobject can
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then be exported. Please see Documentation/filesystems/sysfs.txt for
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more information.
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The kobject infrastructure provides a flexible programming interface,
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and allows kobjects and ksets to be used without being registered
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(i.e. with no sysfs representation). This is also described later.
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1. kobjects
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1.1 Description
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struct kobject is a simple data type that provides a foundation for
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more complex object types. It provides a set of basic fields that
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almost all complex data types share. kobjects are intended to be
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embedded in larger data structures and replace fields they duplicate.
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1.2 Definition
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struct kobject {
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const char * k_name;
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struct kref kref;
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struct list_head entry;
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struct kobject * parent;
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struct kset * kset;
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struct kobj_type * ktype;
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struct sysfs_dirent * sd;
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wait_queue_head_t poll;
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};
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void kobject_init(struct kobject *);
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int kobject_add(struct kobject *);
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int kobject_register(struct kobject *);
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void kobject_del(struct kobject *);
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void kobject_unregister(struct kobject *);
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struct kobject * kobject_get(struct kobject *);
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void kobject_put(struct kobject *);
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1.3 kobject Programming Interface
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kobjects may be dynamically added and removed from the kobject core
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using kobject_register() and kobject_unregister(). Registration
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includes inserting the kobject in the list of its dominant kset and
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creating a directory for it in sysfs.
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Alternatively, one may use a kobject without adding it to its kset's list
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or exporting it via sysfs, by simply calling kobject_init(). An
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initialized kobject may later be added to the object hierarchy by
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calling kobject_add(). An initialized kobject may be used for
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reference counting.
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Note: calling kobject_init() then kobject_add() is functionally
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equivalent to calling kobject_register().
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When a kobject is unregistered, it is removed from its kset's list,
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removed from the sysfs filesystem, and its reference count is decremented.
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List and sysfs removal happen in kobject_del(), and may be called
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manually. kobject_put() decrements the reference count, and may also
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be called manually.
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A kobject's reference count may be incremented with kobject_get(),
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which returns a valid reference to a kobject; and decremented with
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kobject_put(). An object's reference count may only be incremented if
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it is already positive.
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When a kobject's reference count reaches 0, the method struct
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kobj_type::release() (which the kobject's kset points to) is called.
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This allows any memory allocated for the object to be freed.
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NOTE!!!
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It is _imperative_ that you supply a destructor for dynamically
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allocated kobjects to free them if you are using kobject reference
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counts. The reference count controls the lifetime of the object.
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If it goes to 0, then it is assumed that the object will
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be freed and cannot be used.
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More importantly, you must free the object there, and not immediately
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after an unregister call. If someone else is referencing the object
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(e.g. through a sysfs file), they will obtain a reference to the
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object, assume it's valid and operate on it. If the object is
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unregistered and freed in the meantime, the operation will then
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reference freed memory and go boom.
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This can be prevented, in the simplest case, by defining a release
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method and freeing the object from there only. Note that this will not
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secure reference count/object management models that use a dual
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reference count or do other wacky things with the reference count
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(like the networking layer).
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1.4 sysfs
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Each kobject receives a directory in sysfs. This directory is created
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under the kobject's parent directory.
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If a kobject does not have a parent when it is registered, its parent
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becomes its dominant kset.
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If a kobject does not have a parent nor a dominant kset, its directory
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is created at the top-level of the sysfs partition.
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2. ksets
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2.1 Description
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A kset is a set of kobjects that are embedded in the same type.
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struct kset {
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struct kobj_type * ktype;
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struct list_head list;
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struct kobject kobj;
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struct kset_uevent_ops * uevent_ops;
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};
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void kset_init(struct kset * k);
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int kset_add(struct kset * k);
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int kset_register(struct kset * k);
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void kset_unregister(struct kset * k);
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struct kset * kset_get(struct kset * k);
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void kset_put(struct kset * k);
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struct kobject * kset_find_obj(struct kset *, char *);
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The type that the kobjects are embedded in is described by the ktype
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pointer.
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A kset contains a kobject itself, meaning that it may be registered in
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the kobject hierarchy and exported via sysfs. More importantly, the
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kset may be embedded in a larger data type, and may be part of another
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kset (of that object type).
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For example, a block device is an object (struct gendisk) that is
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contained in a set of block devices. It may also contain a set of
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partitions (struct hd_struct) that have been found on the device. The
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following code snippet illustrates how to express this properly.
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struct gendisk * disk;
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...
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disk->kset.kobj.kset = &block_kset;
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disk->kset.ktype = &partition_ktype;
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kset_register(&disk->kset);
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- The kset that the disk's embedded object belongs to is the
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block_kset, and is pointed to by disk->kset.kobj.kset.
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- The type of objects on the disk's _subordinate_ list are partitions,
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and is set in disk->kset.ktype.
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- The kset is then registered, which handles initializing and adding
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the embedded kobject to the hierarchy.
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2.2 kset Programming Interface
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All kset functions, except kset_find_obj(), eventually forward the
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calls to their embedded kobjects after performing kset-specific
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operations. ksets offer a similar programming model to kobjects: they
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may be used after they are initialized, without registering them in
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the hierarchy.
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kset_find_obj() may be used to locate a kobject with a particular
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name. The kobject, if found, is returned.
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There are also some helper functions which names point to the formerly
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existing "struct subsystem", whose functions have been taken over by
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ksets.
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decl_subsys(name,type,uevent_ops)
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Declares a kset named '<name>_subsys' of type <type> with
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uevent_ops <uevent_ops>. For example,
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decl_subsys(devices, &ktype_device, &device_uevent_ops);
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is equivalent to doing:
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struct kset devices_subsys = {
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.ktype = &ktype_devices,
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.uevent_ops = &device_uevent_ops,
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};
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kobject_set_name(&devices_subsys, name);
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The objects that are registered with a subsystem that use the
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subsystem's default list must have their kset ptr set properly. These
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objects may have embedded kobjects or ksets. The
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following helper makes setting the kset easier:
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kobj_set_kset_s(obj,subsys)
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- Assumes that obj->kobj exists, and is a struct kobject.
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- Sets the kset of that kobject to the kset <subsys>.
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int subsystem_register(struct kset *s);
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void subsystem_unregister(struct kset *s);
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These are just wrappers around the respective kset_* functions.
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2.3 sysfs
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ksets are represented in sysfs when their embedded kobjects are
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registered. They follow the same rules of parenting, with one
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exception. If a kset does not have a parent, nor is its embedded
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kobject part of another kset, the kset's parent becomes its dominant
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subsystem.
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If the kset does not have a parent, its directory is created at the
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sysfs root. This should only happen when the kset registered is
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embedded in a subsystem itself.
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3. struct ktype
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3.1. Description
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struct kobj_type {
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void (*release)(struct kobject *);
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struct sysfs_ops * sysfs_ops;
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struct attribute ** default_attrs;
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};
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Object types require specific functions for converting between the
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generic object and the more complex type. struct kobj_type provides
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the object-specific fields, which include:
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- release: Called when the kobject's reference count reaches 0. This
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should convert the object to the more complex type and free it.
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- sysfs_ops: Provides conversion functions for sysfs access. Please
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see the sysfs documentation for more information.
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- default_attrs: Default attributes to be exported via sysfs when the
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object is registered.Note that the last attribute has to be
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initialized to NULL ! You can find a complete implementation
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in block/genhd.c
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Instances of struct kobj_type are not registered; only referenced by
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the kset. A kobj_type may be referenced by an arbitrary number of
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ksets, as there may be disparate sets of identical objects.
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