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1 | The kobject Infrastructure | ||
2 | |||
3 | Patrick Mochel <mochel@osdl.org> | ||
4 | |||
5 | Updated: 3 June 2003 | ||
6 | |||
7 | |||
8 | Copyright (c) 2003 Patrick Mochel | ||
9 | Copyright (c) 2003 Open Source Development Labs | ||
10 | |||
11 | |||
12 | 0. Introduction | ||
13 | |||
14 | The kobject infrastructure performs basic object management that larger | ||
15 | data structures and subsystems can leverage, rather than reimplement | ||
16 | similar functionality. This functionality primarily concerns: | ||
17 | |||
18 | - Object reference counting. | ||
19 | - Maintaining lists (sets) of objects. | ||
20 | - Object set locking. | ||
21 | - Userspace representation. | ||
22 | |||
23 | The infrastructure consists of a number of object types to support | ||
24 | this functionality. Their programming interfaces are described below | ||
25 | in detail, and briefly here: | ||
26 | |||
27 | - kobjects a simple object. | ||
28 | - kset a set of objects of a certain type. | ||
29 | - ktype a set of helpers for objects of a common type. | ||
30 | |||
31 | |||
32 | The kobject infrastructure maintains a close relationship with the | ||
33 | sysfs filesystem. Each kobject that is registered with the kobject | ||
34 | core receives a directory in sysfs. Attributes about the kobject can | ||
35 | then be exported. Please see Documentation/filesystems/sysfs.txt for | ||
36 | more information. | ||
37 | |||
38 | The kobject infrastructure provides a flexible programming interface, | ||
39 | and allows kobjects and ksets to be used without being registered | ||
40 | (i.e. with no sysfs representation). This is also described later. | ||
41 | |||
42 | |||
43 | 1. kobjects | ||
44 | |||
45 | 1.1 Description | ||
46 | |||
47 | |||
48 | struct kobject is a simple data type that provides a foundation for | ||
49 | more complex object types. It provides a set of basic fields that | ||
50 | almost all complex data types share. kobjects are intended to be | ||
51 | embedded in larger data structures and replace fields they duplicate. | ||
52 | |||
53 | 1.2 Definition | ||
54 | |||
55 | struct kobject { | ||
56 | const char * k_name; | ||
57 | struct kref kref; | ||
58 | struct list_head entry; | ||
59 | struct kobject * parent; | ||
60 | struct kset * kset; | ||
61 | struct kobj_type * ktype; | ||
62 | struct sysfs_dirent * sd; | ||
63 | wait_queue_head_t poll; | ||
64 | }; | ||
65 | |||
66 | void kobject_init(struct kobject *); | ||
67 | int kobject_add(struct kobject *); | ||
68 | int kobject_register(struct kobject *); | ||
69 | |||
70 | void kobject_del(struct kobject *); | ||
71 | void kobject_unregister(struct kobject *); | ||
72 | |||
73 | struct kobject * kobject_get(struct kobject *); | ||
74 | void kobject_put(struct kobject *); | ||
75 | |||
76 | |||
77 | 1.3 kobject Programming Interface | ||
78 | |||
79 | kobjects may be dynamically added and removed from the kobject core | ||
80 | using kobject_register() and kobject_unregister(). Registration | ||
81 | includes inserting the kobject in the list of its dominant kset and | ||
82 | creating a directory for it in sysfs. | ||
83 | |||
84 | Alternatively, one may use a kobject without adding it to its kset's list | ||
85 | or exporting it via sysfs, by simply calling kobject_init(). An | ||
86 | initialized kobject may later be added to the object hierarchy by | ||
87 | calling kobject_add(). An initialized kobject may be used for | ||
88 | reference counting. | ||
89 | |||
90 | Note: calling kobject_init() then kobject_add() is functionally | ||
91 | equivalent to calling kobject_register(). | ||
92 | |||
93 | When a kobject is unregistered, it is removed from its kset's list, | ||
94 | removed from the sysfs filesystem, and its reference count is decremented. | ||
95 | List and sysfs removal happen in kobject_del(), and may be called | ||
96 | manually. kobject_put() decrements the reference count, and may also | ||
97 | be called manually. | ||
98 | |||
99 | A kobject's reference count may be incremented with kobject_get(), | ||
100 | which returns a valid reference to a kobject; and decremented with | ||
101 | kobject_put(). An object's reference count may only be incremented if | ||
102 | it is already positive. | ||
103 | |||
104 | When a kobject's reference count reaches 0, the method struct | ||
105 | kobj_type::release() (which the kobject's kset points to) is called. | ||
106 | This allows any memory allocated for the object to be freed. | ||
107 | |||
108 | |||
109 | NOTE!!! | ||
110 | |||
111 | It is _imperative_ that you supply a destructor for dynamically | ||
112 | allocated kobjects to free them if you are using kobject reference | ||
113 | counts. The reference count controls the lifetime of the object. | ||
114 | If it goes to 0, then it is assumed that the object will | ||
115 | be freed and cannot be used. | ||
116 | |||
117 | More importantly, you must free the object there, and not immediately | ||
118 | after an unregister call. If someone else is referencing the object | ||
119 | (e.g. through a sysfs file), they will obtain a reference to the | ||
120 | object, assume it's valid and operate on it. If the object is | ||
121 | unregistered and freed in the meantime, the operation will then | ||
122 | reference freed memory and go boom. | ||
123 | |||
124 | This can be prevented, in the simplest case, by defining a release | ||
125 | method and freeing the object from there only. Note that this will not | ||
126 | secure reference count/object management models that use a dual | ||
127 | reference count or do other wacky things with the reference count | ||
128 | (like the networking layer). | ||
129 | |||
130 | |||
131 | 1.4 sysfs | ||
132 | |||
133 | Each kobject receives a directory in sysfs. This directory is created | ||
134 | under the kobject's parent directory. | ||
135 | |||
136 | If a kobject does not have a parent when it is registered, its parent | ||
137 | becomes its dominant kset. | ||
138 | |||
139 | If a kobject does not have a parent nor a dominant kset, its directory | ||
140 | is created at the top-level of the sysfs partition. | ||
141 | |||
142 | |||
143 | |||
144 | 2. ksets | ||
145 | |||
146 | 2.1 Description | ||
147 | |||
148 | A kset is a set of kobjects that are embedded in the same type. | ||
149 | |||
150 | |||
151 | struct kset { | ||
152 | struct kobj_type * ktype; | ||
153 | struct list_head list; | ||
154 | struct kobject kobj; | ||
155 | struct kset_uevent_ops * uevent_ops; | ||
156 | }; | ||
157 | |||
158 | |||
159 | void kset_init(struct kset * k); | ||
160 | int kset_add(struct kset * k); | ||
161 | int kset_register(struct kset * k); | ||
162 | void kset_unregister(struct kset * k); | ||
163 | |||
164 | struct kset * kset_get(struct kset * k); | ||
165 | void kset_put(struct kset * k); | ||
166 | |||
167 | struct kobject * kset_find_obj(struct kset *, char *); | ||
168 | |||
169 | |||
170 | The type that the kobjects are embedded in is described by the ktype | ||
171 | pointer. | ||
172 | |||
173 | A kset contains a kobject itself, meaning that it may be registered in | ||
174 | the kobject hierarchy and exported via sysfs. More importantly, the | ||
175 | kset may be embedded in a larger data type, and may be part of another | ||
176 | kset (of that object type). | ||
177 | |||
178 | For example, a block device is an object (struct gendisk) that is | ||
179 | contained in a set of block devices. It may also contain a set of | ||
180 | partitions (struct hd_struct) that have been found on the device. The | ||
181 | following code snippet illustrates how to express this properly. | ||
182 | |||
183 | struct gendisk * disk; | ||
184 | ... | ||
185 | disk->kset.kobj.kset = &block_kset; | ||
186 | disk->kset.ktype = &partition_ktype; | ||
187 | kset_register(&disk->kset); | ||
188 | |||
189 | - The kset that the disk's embedded object belongs to is the | ||
190 | block_kset, and is pointed to by disk->kset.kobj.kset. | ||
191 | |||
192 | - The type of objects on the disk's _subordinate_ list are partitions, | ||
193 | and is set in disk->kset.ktype. | ||
194 | |||
195 | - The kset is then registered, which handles initializing and adding | ||
196 | the embedded kobject to the hierarchy. | ||
197 | |||
198 | |||
199 | 2.2 kset Programming Interface | ||
200 | |||
201 | All kset functions, except kset_find_obj(), eventually forward the | ||
202 | calls to their embedded kobjects after performing kset-specific | ||
203 | operations. ksets offer a similar programming model to kobjects: they | ||
204 | may be used after they are initialized, without registering them in | ||
205 | the hierarchy. | ||
206 | |||
207 | kset_find_obj() may be used to locate a kobject with a particular | ||
208 | name. The kobject, if found, is returned. | ||
209 | |||
210 | There are also some helper functions which names point to the formerly | ||
211 | existing "struct subsystem", whose functions have been taken over by | ||
212 | ksets. | ||
213 | |||
214 | |||
215 | decl_subsys(name,type,uevent_ops) | ||
216 | |||
217 | Declares a kset named '<name>_subsys' of type <type> with | ||
218 | uevent_ops <uevent_ops>. For example, | ||
219 | |||
220 | decl_subsys(devices, &ktype_device, &device_uevent_ops); | ||
221 | |||
222 | is equivalent to doing: | ||
223 | |||
224 | struct kset devices_subsys = { | ||
225 | .ktype = &ktype_devices, | ||
226 | .uevent_ops = &device_uevent_ops, | ||
227 | }; | ||
228 | kobject_set_name(&devices_subsys, name); | ||
229 | |||
230 | The objects that are registered with a subsystem that use the | ||
231 | subsystem's default list must have their kset ptr set properly. These | ||
232 | objects may have embedded kobjects or ksets. The | ||
233 | following helper makes setting the kset easier: | ||
234 | |||
235 | |||
236 | kobj_set_kset_s(obj,subsys) | ||
237 | |||
238 | - Assumes that obj->kobj exists, and is a struct kobject. | ||
239 | - Sets the kset of that kobject to the kset <subsys>. | ||
240 | |||
241 | int subsystem_register(struct kset *s); | ||
242 | void subsystem_unregister(struct kset *s); | ||
243 | |||
244 | These are just wrappers around the respective kset_* functions. | ||
245 | |||
246 | 2.3 sysfs | ||
247 | |||
248 | ksets are represented in sysfs when their embedded kobjects are | ||
249 | registered. They follow the same rules of parenting, with one | ||
250 | exception. If a kset does not have a parent, nor is its embedded | ||
251 | kobject part of another kset, the kset's parent becomes its dominant | ||
252 | subsystem. | ||
253 | |||
254 | If the kset does not have a parent, its directory is created at the | ||
255 | sysfs root. This should only happen when the kset registered is | ||
256 | embedded in a subsystem itself. | ||
257 | |||
258 | |||
259 | 3. struct ktype | ||
260 | |||
261 | 3.1. Description | ||
262 | |||
263 | struct kobj_type { | ||
264 | void (*release)(struct kobject *); | ||
265 | struct sysfs_ops * sysfs_ops; | ||
266 | struct attribute ** default_attrs; | ||
267 | }; | ||
268 | |||
269 | |||
270 | Object types require specific functions for converting between the | ||
271 | generic object and the more complex type. struct kobj_type provides | ||
272 | the object-specific fields, which include: | ||
273 | |||
274 | - release: Called when the kobject's reference count reaches 0. This | ||
275 | should convert the object to the more complex type and free it. | ||
276 | |||
277 | - sysfs_ops: Provides conversion functions for sysfs access. Please | ||
278 | see the sysfs documentation for more information. | ||
279 | |||
280 | - default_attrs: Default attributes to be exported via sysfs when the | ||
281 | object is registered.Note that the last attribute has to be | ||
282 | initialized to NULL ! You can find a complete implementation | ||
283 | in block/genhd.c | ||
284 | |||
285 | |||
286 | Instances of struct kobj_type are not registered; only referenced by | ||
287 | the kset. A kobj_type may be referenced by an arbitrary number of | ||
288 | ksets, as there may be disparate sets of identical objects. | ||
289 | |||