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Greg Kroah-Hartman36d78d62007-11-27 11:28:26 -08001Everything you never wanted to know about kobjects, ksets, and ktypes
2
3Greg Kroah-Hartman <gregkh@suse.de>
4
5Based on an original article by Jon Corbet for lwn.net written October 1,
62003 and located at http://lwn.net/Articles/51437/
7
8Last updated December 19, 2007
9
10
11Part of the difficulty in understanding the driver model - and the kobject
12abstraction upon which it is built - is that there is no obvious starting
13place. Dealing with kobjects requires understanding a few different types,
14all of which make reference to each other. In an attempt to make things
15easier, we'll take a multi-pass approach, starting with vague terms and
16adding detail as we go. To that end, here are some quick definitions of
17some terms we will be working with.
18
19 - A kobject is an object of type struct kobject. Kobjects have a name
20 and a reference count. A kobject also has a parent pointer (allowing
21 objects to be arranged into hierarchies), a specific type, and,
22 usually, a representation in the sysfs virtual filesystem.
23
24 Kobjects are generally not interesting on their own; instead, they are
25 usually embedded within some other structure which contains the stuff
26 the code is really interested in.
27
28 No structure should EVER have more than one kobject embedded within it.
29 If it does, the reference counting for the object is sure to be messed
30 up and incorrect, and your code will be buggy. So do not do this.
31
32 - A ktype is the type of object that embeds a kobject. Every structure
33 that embeds a kobject needs a corresponding ktype. The ktype controls
34 what happens to the kobject when it is created and destroyed.
35
36 - A kset is a group of kobjects. These kobjects can be of the same ktype
37 or belong to different ktypes. The kset is the basic container type for
38 collections of kobjects. Ksets contain their own kobjects, but you can
39 safely ignore that implementation detail as the kset core code handles
40 this kobject automatically.
41
42 When you see a sysfs directory full of other directories, generally each
43 of those directories corresponds to a kobject in the same kset.
44
45We'll look at how to create and manipulate all of these types. A bottom-up
46approach will be taken, so we'll go back to kobjects.
47
48
49Embedding kobjects
50
51It is rare for kernel code to create a standalone kobject, with one major
52exception explained below. Instead, kobjects are used to control access to
53a larger, domain-specific object. To this end, kobjects will be found
54embedded in other structures. If you are used to thinking of things in
55object-oriented terms, kobjects can be seen as a top-level, abstract class
56from which other classes are derived. A kobject implements a set of
57capabilities which are not particularly useful by themselves, but which are
58nice to have in other objects. The C language does not allow for the
59direct expression of inheritance, so other techniques - such as structure
60embedding - must be used.
61
62So, for example, the UIO code has a structure that defines the memory
63region associated with a uio device:
64
65struct uio_mem {
66 struct kobject kobj;
67 unsigned long addr;
68 unsigned long size;
69 int memtype;
70 void __iomem *internal_addr;
71};
72
73If you have a struct uio_mem structure, finding its embedded kobject is
74just a matter of using the kobj member. Code that works with kobjects will
75often have the opposite problem, however: given a struct kobject pointer,
76what is the pointer to the containing structure? You must avoid tricks
77(such as assuming that the kobject is at the beginning of the structure)
78and, instead, use the container_of() macro, found in <linux/kernel.h>:
79
80 container_of(pointer, type, member)
81
82where pointer is the pointer to the embedded kobject, type is the type of
83the containing structure, and member is the name of the structure field to
84which pointer points. The return value from container_of() is a pointer to
85the given type. So, for example, a pointer "kp" to a struct kobject
86embedded within a struct uio_mem could be converted to a pointer to the
87containing uio_mem structure with:
88
89 struct uio_mem *u_mem = container_of(kp, struct uio_mem, kobj);
90
91Programmers often define a simple macro for "back-casting" kobject pointers
92to the containing type.
93
94
95Initialization of kobjects
96
97Code which creates a kobject must, of course, initialize that object. Some
98of the internal fields are setup with a (mandatory) call to kobject_init():
99
100 void kobject_init(struct kobject *kobj, struct kobj_type *ktype);
101
102The ktype is required for a kobject to be created properly, as every kobject
103must have an associated kobj_type. After calling kobject_init(), to
104register the kobject with sysfs, the function kobject_add() must be called:
105
106 int kobject_add(struct kobject *kobj, struct kobject *parent, const char *fmt, ...);
107
108This sets up the parent of the kobject and the name for the kobject
109properly. If the kobject is to be associated with a specific kset,
110kobj->kset must be assigned before calling kobject_add(). If a kset is
111associated with a kobject, then the parent for the kobject can be set to
112NULL in the call to kobject_add() and then the kobject's parent will be the
113kset itself.
114
115As the name of the kobject is set when it is added to the kernel, the name
116of the kobject should never be manipulated directly. If you must change
117the name of the kobject, call kobject_rename():
118
119 int kobject_rename(struct kobject *kobj, const char *new_name);
120
Rusty Russell0732b492008-12-09 08:32:14 +1030121kobject_rename does not perform any locking or have a solid notion of
122what names are valid so the caller must provide their own sanity checking
Eric W. Biederman030c1d22008-05-08 14:41:00 -0700123and serialization.
124
Greg Kroah-Hartman36d78d62007-11-27 11:28:26 -0800125There is a function called kobject_set_name() but that is legacy cruft and
126is being removed. If your code needs to call this function, it is
127incorrect and needs to be fixed.
128
129To properly access the name of the kobject, use the function
130kobject_name():
131
132 const char *kobject_name(const struct kobject * kobj);
133
134There is a helper function to both initialize and add the kobject to the
Matt LaPlante19f59462009-04-27 15:06:31 +0200135kernel at the same time, called surprisingly enough kobject_init_and_add():
Greg Kroah-Hartman36d78d62007-11-27 11:28:26 -0800136
137 int kobject_init_and_add(struct kobject *kobj, struct kobj_type *ktype,
138 struct kobject *parent, const char *fmt, ...);
139
140The arguments are the same as the individual kobject_init() and
141kobject_add() functions described above.
142
143
144Uevents
145
146After a kobject has been registered with the kobject core, you need to
147announce to the world that it has been created. This can be done with a
148call to kobject_uevent():
149
150 int kobject_uevent(struct kobject *kobj, enum kobject_action action);
151
152Use the KOBJ_ADD action for when the kobject is first added to the kernel.
153This should be done only after any attributes or children of the kobject
154have been initialized properly, as userspace will instantly start to look
155for them when this call happens.
156
157When the kobject is removed from the kernel (details on how to do that is
158below), the uevent for KOBJ_REMOVE will be automatically created by the
159kobject core, so the caller does not have to worry about doing that by
160hand.
161
162
163Reference counts
164
165One of the key functions of a kobject is to serve as a reference counter
166for the object in which it is embedded. As long as references to the object
167exist, the object (and the code which supports it) must continue to exist.
168The low-level functions for manipulating a kobject's reference counts are:
169
170 struct kobject *kobject_get(struct kobject *kobj);
171 void kobject_put(struct kobject *kobj);
172
173A successful call to kobject_get() will increment the kobject's reference
174counter and return the pointer to the kobject.
175
176When a reference is released, the call to kobject_put() will decrement the
177reference count and, possibly, free the object. Note that kobject_init()
178sets the reference count to one, so the code which sets up the kobject will
179need to do a kobject_put() eventually to release that reference.
180
181Because kobjects are dynamic, they must not be declared statically or on
182the stack, but instead, always allocated dynamically. Future versions of
183the kernel will contain a run-time check for kobjects that are created
184statically and will warn the developer of this improper usage.
185
186If all that you want to use a kobject for is to provide a reference counter
187for your structure, please use the struct kref instead; a kobject would be
188overkill. For more information on how to use struct kref, please see the
189file Documentation/kref.txt in the Linux kernel source tree.
190
191
192Creating "simple" kobjects
193
194Sometimes all that a developer wants is a way to create a simple directory
195in the sysfs hierarchy, and not have to mess with the whole complication of
196ksets, show and store functions, and other details. This is the one
197exception where a single kobject should be created. To create such an
198entry, use the function:
199
200 struct kobject *kobject_create_and_add(char *name, struct kobject *parent);
201
202This function will create a kobject and place it in sysfs in the location
203underneath the specified parent kobject. To create simple attributes
204associated with this kobject, use:
205
206 int sysfs_create_file(struct kobject *kobj, struct attribute *attr);
207or
208 int sysfs_create_group(struct kobject *kobj, struct attribute_group *grp);
209
210Both types of attributes used here, with a kobject that has been created
211with the kobject_create_and_add(), can be of type kobj_attribute, so no
212special custom attribute is needed to be created.
213
214See the example module, samples/kobject/kobject-example.c for an
215implementation of a simple kobject and attributes.
216
217
218
219ktypes and release methods
220
221One important thing still missing from the discussion is what happens to a
222kobject when its reference count reaches zero. The code which created the
223kobject generally does not know when that will happen; if it did, there
224would be little point in using a kobject in the first place. Even
225predictable object lifecycles become more complicated when sysfs is brought
226in as other portions of the kernel can get a reference on any kobject that
227is registered in the system.
228
229The end result is that a structure protected by a kobject cannot be freed
230before its reference count goes to zero. The reference count is not under
231the direct control of the code which created the kobject. So that code must
232be notified asynchronously whenever the last reference to one of its
233kobjects goes away.
234
235Once you registered your kobject via kobject_add(), you must never use
236kfree() to free it directly. The only safe way is to use kobject_put(). It
237is good practice to always use kobject_put() after kobject_init() to avoid
238errors creeping in.
239
240This notification is done through a kobject's release() method. Usually
241such a method has a form like:
242
243 void my_object_release(struct kobject *kobj)
244 {
245 struct my_object *mine = container_of(kobj, struct my_object, kobj);
246
247 /* Perform any additional cleanup on this object, then... */
248 kfree(mine);
249 }
250
251One important point cannot be overstated: every kobject must have a
252release() method, and the kobject must persist (in a consistent state)
253until that method is called. If these constraints are not met, the code is
254flawed. Note that the kernel will warn you if you forget to provide a
255release() method. Do not try to get rid of this warning by providing an
256"empty" release function; you will be mocked mercilessly by the kobject
257maintainer if you attempt this.
258
259Note, the name of the kobject is available in the release function, but it
260must NOT be changed within this callback. Otherwise there will be a memory
261leak in the kobject core, which makes people unhappy.
262
263Interestingly, the release() method is not stored in the kobject itself;
264instead, it is associated with the ktype. So let us introduce struct
265kobj_type:
266
267 struct kobj_type {
268 void (*release)(struct kobject *);
269 struct sysfs_ops *sysfs_ops;
270 struct attribute **default_attrs;
271 };
272
273This structure is used to describe a particular type of kobject (or, more
274correctly, of containing object). Every kobject needs to have an associated
275kobj_type structure; a pointer to that structure must be specified when you
276call kobject_init() or kobject_init_and_add().
277
278The release field in struct kobj_type is, of course, a pointer to the
279release() method for this type of kobject. The other two fields (sysfs_ops
280and default_attrs) control how objects of this type are represented in
281sysfs; they are beyond the scope of this document.
282
283The default_attrs pointer is a list of default attributes that will be
284automatically created for any kobject that is registered with this ktype.
285
286
287ksets
288
289A kset is merely a collection of kobjects that want to be associated with
290each other. There is no restriction that they be of the same ktype, but be
291very careful if they are not.
292
293A kset serves these functions:
294
295 - It serves as a bag containing a group of objects. A kset can be used by
296 the kernel to track "all block devices" or "all PCI device drivers."
297
298 - A kset is also a subdirectory in sysfs, where the associated kobjects
299 with the kset can show up. Every kset contains a kobject which can be
300 set up to be the parent of other kobjects; the top-level directories of
301 the sysfs hierarchy are constructed in this way.
302
303 - Ksets can support the "hotplugging" of kobjects and influence how
304 uevent events are reported to user space.
305
306In object-oriented terms, "kset" is the top-level container class; ksets
307contain their own kobject, but that kobject is managed by the kset code and
308should not be manipulated by any other user.
309
310A kset keeps its children in a standard kernel linked list. Kobjects point
311back to their containing kset via their kset field. In almost all cases,
David Brigadaacccafe2008-06-11 13:27:32 -0400312the kobjects belonging to a kset have that kset (or, strictly, its embedded
Greg Kroah-Hartman36d78d62007-11-27 11:28:26 -0800313kobject) in their parent.
314
315As a kset contains a kobject within it, it should always be dynamically
316created and never declared statically or on the stack. To create a new
317kset use:
318 struct kset *kset_create_and_add(const char *name,
319 struct kset_uevent_ops *u,
320 struct kobject *parent);
321
322When you are finished with the kset, call:
323 void kset_unregister(struct kset *kset);
324to destroy it.
325
326An example of using a kset can be seen in the
327samples/kobject/kset-example.c file in the kernel tree.
328
329If a kset wishes to control the uevent operations of the kobjects
330associated with it, it can use the struct kset_uevent_ops to handle it:
331
332struct kset_uevent_ops {
333 int (*filter)(struct kset *kset, struct kobject *kobj);
334 const char *(*name)(struct kset *kset, struct kobject *kobj);
335 int (*uevent)(struct kset *kset, struct kobject *kobj,
336 struct kobj_uevent_env *env);
337};
338
339
340The filter function allows a kset to prevent a uevent from being emitted to
341userspace for a specific kobject. If the function returns 0, the uevent
342will not be emitted.
343
344The name function will be called to override the default name of the kset
345that the uevent sends to userspace. By default, the name will be the same
346as the kset itself, but this function, if present, can override that name.
347
348The uevent function will be called when the uevent is about to be sent to
349userspace to allow more environment variables to be added to the uevent.
350
351One might ask how, exactly, a kobject is added to a kset, given that no
352functions which perform that function have been presented. The answer is
353that this task is handled by kobject_add(). When a kobject is passed to
354kobject_add(), its kset member should point to the kset to which the
355kobject will belong. kobject_add() will handle the rest.
356
357If the kobject belonging to a kset has no parent kobject set, it will be
358added to the kset's directory. Not all members of a kset do necessarily
359live in the kset directory. If an explicit parent kobject is assigned
360before the kobject is added, the kobject is registered with the kset, but
361added below the parent kobject.
362
363
364Kobject removal
365
366After a kobject has been registered with the kobject core successfully, it
367must be cleaned up when the code is finished with it. To do that, call
368kobject_put(). By doing this, the kobject core will automatically clean up
369all of the memory allocated by this kobject. If a KOBJ_ADD uevent has been
370sent for the object, a corresponding KOBJ_REMOVE uevent will be sent, and
371any other sysfs housekeeping will be handled for the caller properly.
372
373If you need to do a two-stage delete of the kobject (say you are not
374allowed to sleep when you need to destroy the object), then call
375kobject_del() which will unregister the kobject from sysfs. This makes the
376kobject "invisible", but it is not cleaned up, and the reference count of
377the object is still the same. At a later time call kobject_put() to finish
378the cleanup of the memory associated with the kobject.
379
380kobject_del() can be used to drop the reference to the parent object, if
381circular references are constructed. It is valid in some cases, that a
382parent objects references a child. Circular references _must_ be broken
383with an explicit call to kobject_del(), so that a release functions will be
384called, and the objects in the former circle release each other.
385
386
387Example code to copy from
388
389For a more complete example of using ksets and kobjects properly, see the
390sample/kobject/kset-example.c code.