linux/Documentation/kobject.txt
<<
>>
Prefs
   1=====================================================================
   2Everything you never wanted to know about kobjects, ksets, and ktypes
   3=====================================================================
   4
   5:Author: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
   6:Last updated: December 19, 2007
   7
   8Based on an original article by Jon Corbet for lwn.net written October 1,
   92003 and located at http://lwn.net/Articles/51437/
  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==================
  51
  52It is rare for kernel code to create a standalone kobject, with one major
  53exception explained below.  Instead, kobjects are used to control access to
  54a larger, domain-specific object.  To this end, kobjects will be found
  55embedded in other structures.  If you are used to thinking of things in
  56object-oriented terms, kobjects can be seen as a top-level, abstract class
  57from which other classes are derived.  A kobject implements a set of
  58capabilities which are not particularly useful by themselves, but which are
  59nice to have in other objects.  The C language does not allow for the
  60direct expression of inheritance, so other techniques - such as structure
  61embedding - must be used.
  62
  63(As an aside, for those familiar with the kernel linked list implementation,
  64this is analogous as to how "list_head" structs are rarely useful on
  65their own, but are invariably found embedded in the larger objects of
  66interest.)
  67
  68So, for example, the UIO code in drivers/uio/uio.c has a structure that
  69defines the memory region associated with a uio device::
  70
  71    struct uio_map {
  72        struct kobject kobj;
  73        struct uio_mem *mem;
  74    };
  75
  76If you have a struct uio_map structure, finding its embedded kobject is
  77just a matter of using the kobj member.  Code that works with kobjects will
  78often have the opposite problem, however: given a struct kobject pointer,
  79what is the pointer to the containing structure?  You must avoid tricks
  80(such as assuming that the kobject is at the beginning of the structure)
  81and, instead, use the container_of() macro, found in <linux/kernel.h>::
  82
  83    container_of(pointer, type, member)
  84
  85where:
  86
  87  * "pointer" is the pointer to the embedded kobject,
  88  * "type" is the type of the containing structure, and
  89  * "member" is the name of the structure field to which "pointer" points.
  90
  91The return value from container_of() is a pointer to the corresponding
  92container type. So, for example, a pointer "kp" to a struct kobject
  93embedded *within* a struct uio_map could be converted to a pointer to the
  94*containing* uio_map structure with::
  95
  96    struct uio_map *u_map = container_of(kp, struct uio_map, kobj);
  97
  98For convenience, programmers often define a simple macro for "back-casting"
  99kobject pointers to the containing type.  Exactly this happens in the
 100earlier drivers/uio/uio.c, as you can see here::
 101
 102    struct uio_map {
 103        struct kobject kobj;
 104        struct uio_mem *mem;
 105    };
 106
 107    #define to_map(map) container_of(map, struct uio_map, kobj)
 108
 109where the macro argument "map" is a pointer to the struct kobject in
 110question.  That macro is subsequently invoked with::
 111
 112    struct uio_map *map = to_map(kobj);
 113
 114
 115Initialization of kobjects
 116==========================
 117
 118Code which creates a kobject must, of course, initialize that object. Some
 119of the internal fields are setup with a (mandatory) call to kobject_init()::
 120
 121    void kobject_init(struct kobject *kobj, struct kobj_type *ktype);
 122
 123The ktype is required for a kobject to be created properly, as every kobject
 124must have an associated kobj_type.  After calling kobject_init(), to
 125register the kobject with sysfs, the function kobject_add() must be called::
 126
 127    int kobject_add(struct kobject *kobj, struct kobject *parent,
 128                    const char *fmt, ...);
 129
 130This sets up the parent of the kobject and the name for the kobject
 131properly.  If the kobject is to be associated with a specific kset,
 132kobj->kset must be assigned before calling kobject_add().  If a kset is
 133associated with a kobject, then the parent for the kobject can be set to
 134NULL in the call to kobject_add() and then the kobject's parent will be the
 135kset itself.
 136
 137As the name of the kobject is set when it is added to the kernel, the name
 138of the kobject should never be manipulated directly.  If you must change
 139the name of the kobject, call kobject_rename()::
 140
 141    int kobject_rename(struct kobject *kobj, const char *new_name);
 142
 143kobject_rename does not perform any locking or have a solid notion of
 144what names are valid so the caller must provide their own sanity checking
 145and serialization.
 146
 147There is a function called kobject_set_name() but that is legacy cruft and
 148is being removed.  If your code needs to call this function, it is
 149incorrect and needs to be fixed.
 150
 151To properly access the name of the kobject, use the function
 152kobject_name()::
 153
 154    const char *kobject_name(const struct kobject * kobj);
 155
 156There is a helper function to both initialize and add the kobject to the
 157kernel at the same time, called surprisingly enough kobject_init_and_add()::
 158
 159    int kobject_init_and_add(struct kobject *kobj, struct kobj_type *ktype,
 160                             struct kobject *parent, const char *fmt, ...);
 161
 162The arguments are the same as the individual kobject_init() and
 163kobject_add() functions described above.
 164
 165
 166Uevents
 167=======
 168
 169After a kobject has been registered with the kobject core, you need to
 170announce to the world that it has been created.  This can be done with a
 171call to kobject_uevent()::
 172
 173    int kobject_uevent(struct kobject *kobj, enum kobject_action action);
 174
 175Use the KOBJ_ADD action for when the kobject is first added to the kernel.
 176This should be done only after any attributes or children of the kobject
 177have been initialized properly, as userspace will instantly start to look
 178for them when this call happens.
 179
 180When the kobject is removed from the kernel (details on how to do that are
 181below), the uevent for KOBJ_REMOVE will be automatically created by the
 182kobject core, so the caller does not have to worry about doing that by
 183hand.
 184
 185
 186Reference counts
 187================
 188
 189One of the key functions of a kobject is to serve as a reference counter
 190for the object in which it is embedded. As long as references to the object
 191exist, the object (and the code which supports it) must continue to exist.
 192The low-level functions for manipulating a kobject's reference counts are::
 193
 194    struct kobject *kobject_get(struct kobject *kobj);
 195    void kobject_put(struct kobject *kobj);
 196
 197A successful call to kobject_get() will increment the kobject's reference
 198counter and return the pointer to the kobject.
 199
 200When a reference is released, the call to kobject_put() will decrement the
 201reference count and, possibly, free the object. Note that kobject_init()
 202sets the reference count to one, so the code which sets up the kobject will
 203need to do a kobject_put() eventually to release that reference.
 204
 205Because kobjects are dynamic, they must not be declared statically or on
 206the stack, but instead, always allocated dynamically.  Future versions of
 207the kernel will contain a run-time check for kobjects that are created
 208statically and will warn the developer of this improper usage.
 209
 210If all that you want to use a kobject for is to provide a reference counter
 211for your structure, please use the struct kref instead; a kobject would be
 212overkill.  For more information on how to use struct kref, please see the
 213file Documentation/kref.txt in the Linux kernel source tree.
 214
 215
 216Creating "simple" kobjects
 217==========================
 218
 219Sometimes all that a developer wants is a way to create a simple directory
 220in the sysfs hierarchy, and not have to mess with the whole complication of
 221ksets, show and store functions, and other details.  This is the one
 222exception where a single kobject should be created.  To create such an
 223entry, use the function::
 224
 225    struct kobject *kobject_create_and_add(char *name, struct kobject *parent);
 226
 227This function will create a kobject and place it in sysfs in the location
 228underneath the specified parent kobject.  To create simple attributes
 229associated with this kobject, use::
 230
 231    int sysfs_create_file(struct kobject *kobj, struct attribute *attr);
 232
 233or::
 234
 235    int sysfs_create_group(struct kobject *kobj, struct attribute_group *grp);
 236
 237Both types of attributes used here, with a kobject that has been created
 238with the kobject_create_and_add(), can be of type kobj_attribute, so no
 239special custom attribute is needed to be created.
 240
 241See the example module, samples/kobject/kobject-example.c for an
 242implementation of a simple kobject and attributes.
 243
 244
 245
 246ktypes and release methods
 247==========================
 248
 249One important thing still missing from the discussion is what happens to a
 250kobject when its reference count reaches zero. The code which created the
 251kobject generally does not know when that will happen; if it did, there
 252would be little point in using a kobject in the first place. Even
 253predictable object lifecycles become more complicated when sysfs is brought
 254in as other portions of the kernel can get a reference on any kobject that
 255is registered in the system.
 256
 257The end result is that a structure protected by a kobject cannot be freed
 258before its reference count goes to zero. The reference count is not under
 259the direct control of the code which created the kobject. So that code must
 260be notified asynchronously whenever the last reference to one of its
 261kobjects goes away.
 262
 263Once you registered your kobject via kobject_add(), you must never use
 264kfree() to free it directly. The only safe way is to use kobject_put(). It
 265is good practice to always use kobject_put() after kobject_init() to avoid
 266errors creeping in.
 267
 268This notification is done through a kobject's release() method. Usually
 269such a method has a form like::
 270
 271    void my_object_release(struct kobject *kobj)
 272    {
 273            struct my_object *mine = container_of(kobj, struct my_object, kobj);
 274
 275            /* Perform any additional cleanup on this object, then... */
 276            kfree(mine);
 277    }
 278
 279One important point cannot be overstated: every kobject must have a
 280release() method, and the kobject must persist (in a consistent state)
 281until that method is called. If these constraints are not met, the code is
 282flawed.  Note that the kernel will warn you if you forget to provide a
 283release() method.  Do not try to get rid of this warning by providing an
 284"empty" release function; you will be mocked mercilessly by the kobject
 285maintainer if you attempt this.
 286
 287Note, the name of the kobject is available in the release function, but it
 288must NOT be changed within this callback.  Otherwise there will be a memory
 289leak in the kobject core, which makes people unhappy.
 290
 291Interestingly, the release() method is not stored in the kobject itself;
 292instead, it is associated with the ktype. So let us introduce struct
 293kobj_type::
 294
 295    struct kobj_type {
 296            void (*release)(struct kobject *kobj);
 297            const struct sysfs_ops *sysfs_ops;
 298            struct attribute **default_attrs;
 299            const struct kobj_ns_type_operations *(*child_ns_type)(struct kobject *kobj);
 300            const void *(*namespace)(struct kobject *kobj);
 301    };
 302
 303This structure is used to describe a particular type of kobject (or, more
 304correctly, of containing object). Every kobject needs to have an associated
 305kobj_type structure; a pointer to that structure must be specified when you
 306call kobject_init() or kobject_init_and_add().
 307
 308The release field in struct kobj_type is, of course, a pointer to the
 309release() method for this type of kobject. The other two fields (sysfs_ops
 310and default_attrs) control how objects of this type are represented in
 311sysfs; they are beyond the scope of this document.
 312
 313The default_attrs pointer is a list of default attributes that will be
 314automatically created for any kobject that is registered with this ktype.
 315
 316
 317ksets
 318=====
 319
 320A kset is merely a collection of kobjects that want to be associated with
 321each other.  There is no restriction that they be of the same ktype, but be
 322very careful if they are not.
 323
 324A kset serves these functions:
 325
 326 - It serves as a bag containing a group of objects. A kset can be used by
 327   the kernel to track "all block devices" or "all PCI device drivers."
 328
 329 - A kset is also a subdirectory in sysfs, where the associated kobjects
 330   with the kset can show up.  Every kset contains a kobject which can be
 331   set up to be the parent of other kobjects; the top-level directories of
 332   the sysfs hierarchy are constructed in this way.
 333
 334 - Ksets can support the "hotplugging" of kobjects and influence how
 335   uevent events are reported to user space.
 336
 337In object-oriented terms, "kset" is the top-level container class; ksets
 338contain their own kobject, but that kobject is managed by the kset code and
 339should not be manipulated by any other user.
 340
 341A kset keeps its children in a standard kernel linked list.  Kobjects point
 342back to their containing kset via their kset field. In almost all cases,
 343the kobjects belonging to a kset have that kset (or, strictly, its embedded
 344kobject) in their parent.
 345
 346As a kset contains a kobject within it, it should always be dynamically
 347created and never declared statically or on the stack.  To create a new
 348kset use::
 349
 350  struct kset *kset_create_and_add(const char *name,
 351                                   struct kset_uevent_ops *u,
 352                                   struct kobject *parent);
 353
 354When you are finished with the kset, call::
 355
 356  void kset_unregister(struct kset *kset);
 357
 358to destroy it.  This removes the kset from sysfs and decrements its reference
 359count.  When the reference count goes to zero, the kset will be released.
 360Because other references to the kset may still exist, the release may happen
 361after kset_unregister() returns.
 362
 363An example of using a kset can be seen in the
 364samples/kobject/kset-example.c file in the kernel tree.
 365
 366If a kset wishes to control the uevent operations of the kobjects
 367associated with it, it can use the struct kset_uevent_ops to handle it::
 368
 369  struct kset_uevent_ops {
 370        int (*filter)(struct kset *kset, struct kobject *kobj);
 371        const char *(*name)(struct kset *kset, struct kobject *kobj);
 372        int (*uevent)(struct kset *kset, struct kobject *kobj,
 373                      struct kobj_uevent_env *env);
 374  };
 375
 376
 377The filter function allows a kset to prevent a uevent from being emitted to
 378userspace for a specific kobject.  If the function returns 0, the uevent
 379will not be emitted.
 380
 381The name function will be called to override the default name of the kset
 382that the uevent sends to userspace.  By default, the name will be the same
 383as the kset itself, but this function, if present, can override that name.
 384
 385The uevent function will be called when the uevent is about to be sent to
 386userspace to allow more environment variables to be added to the uevent.
 387
 388One might ask how, exactly, a kobject is added to a kset, given that no
 389functions which perform that function have been presented.  The answer is
 390that this task is handled by kobject_add().  When a kobject is passed to
 391kobject_add(), its kset member should point to the kset to which the
 392kobject will belong.  kobject_add() will handle the rest.
 393
 394If the kobject belonging to a kset has no parent kobject set, it will be
 395added to the kset's directory.  Not all members of a kset do necessarily
 396live in the kset directory.  If an explicit parent kobject is assigned
 397before the kobject is added, the kobject is registered with the kset, but
 398added below the parent kobject.
 399
 400
 401Kobject removal
 402===============
 403
 404After a kobject has been registered with the kobject core successfully, it
 405must be cleaned up when the code is finished with it.  To do that, call
 406kobject_put().  By doing this, the kobject core will automatically clean up
 407all of the memory allocated by this kobject.  If a KOBJ_ADD uevent has been
 408sent for the object, a corresponding KOBJ_REMOVE uevent will be sent, and
 409any other sysfs housekeeping will be handled for the caller properly.
 410
 411If you need to do a two-stage delete of the kobject (say you are not
 412allowed to sleep when you need to destroy the object), then call
 413kobject_del() which will unregister the kobject from sysfs.  This makes the
 414kobject "invisible", but it is not cleaned up, and the reference count of
 415the object is still the same.  At a later time call kobject_put() to finish
 416the cleanup of the memory associated with the kobject.
 417
 418kobject_del() can be used to drop the reference to the parent object, if
 419circular references are constructed.  It is valid in some cases, that a
 420parent objects references a child.  Circular references _must_ be broken
 421with an explicit call to kobject_del(), so that a release functions will be
 422called, and the objects in the former circle release each other.
 423
 424
 425Example code to copy from
 426=========================
 427
 428For a more complete example of using ksets and kobjects properly, see the
 429example programs samples/kobject/{kobject-example.c,kset-example.c},
 430which will be built as loadable modules if you select CONFIG_SAMPLE_KOBJECT.
 431