kobject: update the kobject/kset documentation

This provides a much-needed kobject and kset documentation update.

Thanks to Kay Sievers, Alan Stern, Jonathan Corbet, Randy Dunlap, Jan
Engelhardt, and others for reviewing and providing help with this
document.

Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

Documentation/kobject.txt

+Everything you never wanted to know about kobjects, ksets, and ktypes
+
+Greg Kroah-Hartman <gregkh@suse.de>
+
+Based on an original article by Jon Corbet for lwn.net written October 1,
+2003 and located at http://lwn.net/Articles/51437/
+
+Last updated December 19, 2007
+
+
+Part of the difficulty in understanding the driver model - and the kobject
+abstraction upon which it is built - is that there is no obvious starting
+place. Dealing with kobjects requires understanding a few different types,
+all of which make reference to each other. In an attempt to make things
+easier, we'll take a multi-pass approach, starting with vague terms and
+adding detail as we go. To that end, here are some quick definitions of
+some terms we will be working with.
+
+ - A kobject is an object of type struct kobject.  Kobjects have a name
+   and a reference count.  A kobject also has a parent pointer (allowing
+   objects to be arranged into hierarchies), a specific type, and,
+   usually, a representation in the sysfs virtual filesystem.
+
+   Kobjects are generally not interesting on their own; instead, they are
+   usually embedded within some other structure which contains the stuff
+   the code is really interested in.
+
+   No structure should EVER have more than one kobject embedded within it.
+   If it does, the reference counting for the object is sure to be messed
+   up and incorrect, and your code will be buggy.  So do not do this.
+
+ - A ktype is the type of object that embeds a kobject.  Every structure
+   that embeds a kobject needs a corresponding ktype.  The ktype controls
+   what happens to the kobject when it is created and destroyed.
+
+ - A kset is a group of kobjects.  These kobjects can be of the same ktype
+   or belong to different ktypes.  The kset is the basic container type for
+   collections of kobjects. Ksets contain their own kobjects, but you can
+   safely ignore that implementation detail as the kset core code handles
+   this kobject automatically.
+
+   When you see a sysfs directory full of other directories, generally each
+   of those directories corresponds to a kobject in the same kset.
+
+We'll look at how to create and manipulate all of these types. A bottom-up
+approach will be taken, so we'll go back to kobjects.
+
+
+Embedding kobjects
+
+It is rare for kernel code to create a standalone kobject, with one major
+exception explained below.  Instead, kobjects are used to control access to
+a larger, domain-specific object.  To this end, kobjects will be found
+embedded in other structures.  If you are used to thinking of things in
+object-oriented terms, kobjects can be seen as a top-level, abstract class
+from which other classes are derived.  A kobject implements a set of
+capabilities which are not particularly useful by themselves, but which are
+nice to have in other objects.  The C language does not allow for the
+direct expression of inheritance, so other techniques - such as structure
+embedding - must be used.
+
+So, for example, the UIO code has a structure that defines the memory
+region associated with a uio device:
+
+struct uio_mem {
+	struct kobject kobj;
+	unsigned long addr;
+	unsigned long size;
+	int memtype;
+	void __iomem *internal_addr;
+};
+
+If you have a struct uio_mem structure, finding its embedded kobject is
+just a matter of using the kobj member.  Code that works with kobjects will
+often have the opposite problem, however: given a struct kobject pointer,
+what is the pointer to the containing structure?  You must avoid tricks
+(such as assuming that the kobject is at the beginning of the structure)
+and, instead, use the container_of() macro, found in <linux/kernel.h>:
+
+	container_of(pointer, type, member)
+
+where pointer is the pointer to the embedded kobject, type is the type of
+the containing structure, and member is the name of the structure field to
+which pointer points.  The return value from container_of() is a pointer to
+the given type. So, for example, a pointer "kp" to a struct kobject
+embedded within a struct uio_mem could be converted to a pointer to the
+containing uio_mem structure with:
+
+    struct uio_mem *u_mem = container_of(kp, struct uio_mem, kobj);
+
+Programmers often define a simple macro for "back-casting" kobject pointers
+to the containing type.
+
+
+Initialization of kobjects
+
+Code which creates a kobject must, of course, initialize that object. Some
+of the internal fields are setup with a (mandatory) call to kobject_init():
+
+    void kobject_init(struct kobject *kobj, struct kobj_type *ktype);
+
+The ktype is required for a kobject to be created properly, as every kobject
+must have an associated kobj_type.  After calling kobject_init(), to
+register the kobject with sysfs, the function kobject_add() must be called:
+
+    int kobject_add(struct kobject *kobj, struct kobject *parent, const char *fmt, ...);
+
+This sets up the parent of the kobject and the name for the kobject
+properly.  If the kobject is to be associated with a specific kset,
+kobj->kset must be assigned before calling kobject_add().  If a kset is
+associated with a kobject, then the parent for the kobject can be set to
+NULL in the call to kobject_add() and then the kobject's parent will be the
+kset itself.
+
+As the name of the kobject is set when it is added to the kernel, the name
+of the kobject should never be manipulated directly.  If you must change
+the name of the kobject, call kobject_rename():
+
+    int kobject_rename(struct kobject *kobj, const char *new_name);
+
+There is a function called kobject_set_name() but that is legacy cruft and
+is being removed.  If your code needs to call this function, it is
+incorrect and needs to be fixed.
+
+To properly access the name of the kobject, use the function
+kobject_name():
+
+    const char *kobject_name(const struct kobject * kobj);
+
+There is a helper function to both initialize and add the kobject to the
+kernel at the same time, called supprisingly enough kobject_init_and_add():
+
+    int kobject_init_and_add(struct kobject *kobj, struct kobj_type *ktype,
+                             struct kobject *parent, const char *fmt, ...);
+
+The arguments are the same as the individual kobject_init() and
+kobject_add() functions described above.
+
+
+Uevents
+
+After a kobject has been registered with the kobject core, you need to
+announce to the world that it has been created.  This can be done with a
+call to kobject_uevent():
+
+    int kobject_uevent(struct kobject *kobj, enum kobject_action action);
+
+Use the KOBJ_ADD action for when the kobject is first added to the kernel.
+This should be done only after any attributes or children of the kobject
+have been initialized properly, as userspace will instantly start to look
+for them when this call happens.
+
+When the kobject is removed from the kernel (details on how to do that is
+below), the uevent for KOBJ_REMOVE will be automatically created by the
+kobject core, so the caller does not have to worry about doing that by
+hand.
+
+
+Reference counts
+
+One of the key functions of a kobject is to serve as a reference counter
+for the object in which it is embedded. As long as references to the object
+exist, the object (and the code which supports it) must continue to exist.
+The low-level functions for manipulating a kobject's reference counts are:
+
+    struct kobject *kobject_get(struct kobject *kobj);
+    void kobject_put(struct kobject *kobj);
+
+A successful call to kobject_get() will increment the kobject's reference
+counter and return the pointer to the kobject.
+
+When a reference is released, the call to kobject_put() will decrement the
+reference count and, possibly, free the object. Note that kobject_init()
+sets the reference count to one, so the code which sets up the kobject will
+need to do a kobject_put() eventually to release that reference.
+
+Because kobjects are dynamic, they must not be declared statically or on
+the stack, but instead, always allocated dynamically.  Future versions of
+the kernel will contain a run-time check for kobjects that are created
+statically and will warn the developer of this improper usage.
+
+If all that you want to use a kobject for is to provide a reference counter
+for your structure, please use the struct kref instead; a kobject would be
+overkill.  For more information on how to use struct kref, please see the
+file Documentation/kref.txt in the Linux kernel source tree.
+
+
+Creating "simple" kobjects
+
+Sometimes all that a developer wants is a way to create a simple directory
+in the sysfs hierarchy, and not have to mess with the whole complication of
+ksets, show and store functions, and other details.  This is the one
+exception where a single kobject should be created.  To create such an
+entry, use the function:
+
+    struct kobject *kobject_create_and_add(char *name, struct kobject *parent);
+
+This function will create a kobject and place it in sysfs in the location
+underneath the specified parent kobject.  To create simple attributes
+associated with this kobject, use:
+
+    int sysfs_create_file(struct kobject *kobj, struct attribute *attr);
+or
+    int sysfs_create_group(struct kobject *kobj, struct attribute_group *grp);
+
+Both types of attributes used here, with a kobject that has been created
+with the kobject_create_and_add(), can be of type kobj_attribute, so no
+special custom attribute is needed to be created.
+
+See the example module, samples/kobject/kobject-example.c for an
+implementation of a simple kobject and attributes.
+
+
+
+ktypes and release methods
+
+One important thing still missing from the discussion is what happens to a
+kobject when its reference count reaches zero. The code which created the
+kobject generally does not know when that will happen; if it did, there
+would be little point in using a kobject in the first place. Even
+predictable object lifecycles become more complicated when sysfs is brought
+in as other portions of the kernel can get a reference on any kobject that
+is registered in the system.
+
+The end result is that a structure protected by a kobject cannot be freed
+before its reference count goes to zero. The reference count is not under
+the direct control of the code which created the kobject. So that code must
+be notified asynchronously whenever the last reference to one of its
+kobjects goes away.
+
+Once you registered your kobject via kobject_add(), you must never use
+kfree() to free it directly. The only safe way is to use kobject_put(). It
+is good practice to always use kobject_put() after kobject_init() to avoid
+errors creeping in.
+
+This notification is done through a kobject's release() method. Usually
+such a method has a form like:
+
+    void my_object_release(struct kobject *kobj)
+    {
+    	    struct my_object *mine = container_of(kobj, struct my_object, kobj);
+
+	    /* Perform any additional cleanup on this object, then... */
+	    kfree(mine);
+    }
+
+One important point cannot be overstated: every kobject must have a
+release() method, and the kobject must persist (in a consistent state)
+until that method is called. If these constraints are not met, the code is
+flawed.  Note that the kernel will warn you if you forget to provide a
+release() method.  Do not try to get rid of this warning by providing an
+"empty" release function; you will be mocked mercilessly by the kobject
+maintainer if you attempt this.
+
+Note, the name of the kobject is available in the release function, but it
+must NOT be changed within this callback.  Otherwise there will be a memory
+leak in the kobject core, which makes people unhappy.
+
+Interestingly, the release() method is not stored in the kobject itself;
+instead, it is associated with the ktype. So let us introduce struct
+kobj_type:
+
+    struct kobj_type {
+	    void (*release)(struct kobject *);
+	    struct sysfs_ops	*sysfs_ops;
+	    struct attribute	**default_attrs;
+    };
+
+This structure is used to describe a particular type of kobject (or, more
+correctly, of containing object). Every kobject needs to have an associated
+kobj_type structure; a pointer to that structure must be specified when you
+call kobject_init() or kobject_init_and_add().
+
+The release field in struct kobj_type is, of course, a pointer to the
+release() method for this type of kobject. The other two fields (sysfs_ops
+and default_attrs) control how objects of this type are represented in
+sysfs; they are beyond the scope of this document.
+
+The default_attrs pointer is a list of default attributes that will be
+automatically created for any kobject that is registered with this ktype.
+
+
+ksets
+
+A kset is merely a collection of kobjects that want to be associated with
+each other.  There is no restriction that they be of the same ktype, but be
+very careful if they are not.
+
+A kset serves these functions:
+
+ - It serves as a bag containing a group of objects. A kset can be used by
+   the kernel to track "all block devices" or "all PCI device drivers."
+
+ - A kset is also a subdirectory in sysfs, where the associated kobjects
+   with the kset can show up.  Every kset contains a kobject which can be
+   set up to be the parent of other kobjects; the top-level directories of
+   the sysfs hierarchy are constructed in this way.
+
+ - Ksets can support the "hotplugging" of kobjects and influence how
+   uevent events are reported to user space.
+
+In object-oriented terms, "kset" is the top-level container class; ksets
+contain their own kobject, but that kobject is managed by the kset code and
+should not be manipulated by any other user.
+
+A kset keeps its children in a standard kernel linked list.  Kobjects point
+back to their containing kset via their kset field. In almost all cases,
+the kobjects belonging to a ket have that kset (or, strictly, its embedded
+kobject) in their parent.
+
+As a kset contains a kobject within it, it should always be dynamically
+created and never declared statically or on the stack.  To create a new
+kset use:
+  struct kset *kset_create_and_add(const char *name,
+				   struct kset_uevent_ops *u,
+				   struct kobject *parent);
+
+When you are finished with the kset, call:
+  void kset_unregister(struct kset *kset);
+to destroy it.
+
+An example of using a kset can be seen in the
+samples/kobject/kset-example.c file in the kernel tree.
+
+If a kset wishes to control the uevent operations of the kobjects
+associated with it, it can use the struct kset_uevent_ops to handle it:
+
+struct kset_uevent_ops {
+        int (*filter)(struct kset *kset, struct kobject *kobj);
+        const char *(*name)(struct kset *kset, struct kobject *kobj);
+        int (*uevent)(struct kset *kset, struct kobject *kobj,
+                      struct kobj_uevent_env *env);
+};
+
+
+The filter function allows a kset to prevent a uevent from being emitted to
+userspace for a specific kobject.  If the function returns 0, the uevent
+will not be emitted.
+
+The name function will be called to override the default name of the kset
+that the uevent sends to userspace.  By default, the name will be the same
+as the kset itself, but this function, if present, can override that name.
+
+The uevent function will be called when the uevent is about to be sent to
+userspace to allow more environment variables to be added to the uevent.
+
+One might ask how, exactly, a kobject is added to a kset, given that no
+functions which perform that function have been presented.  The answer is
+that this task is handled by kobject_add().  When a kobject is passed to
+kobject_add(), its kset member should point to the kset to which the
+kobject will belong.  kobject_add() will handle the rest.
+
+If the kobject belonging to a kset has no parent kobject set, it will be
+added to the kset's directory.  Not all members of a kset do necessarily
+live in the kset directory.  If an explicit parent kobject is assigned
+before the kobject is added, the kobject is registered with the kset, but
+added below the parent kobject.
+
+
+Kobject removal
+
+After a kobject has been registered with the kobject core successfully, it
+must be cleaned up when the code is finished with it.  To do that, call
+kobject_put().  By doing this, the kobject core will automatically clean up
+all of the memory allocated by this kobject.  If a KOBJ_ADD uevent has been
+sent for the object, a corresponding KOBJ_REMOVE uevent will be sent, and
+any other sysfs housekeeping will be handled for the caller properly.
+
+If you need to do a two-stage delete of the kobject (say you are not
+allowed to sleep when you need to destroy the object), then call
+kobject_del() which will unregister the kobject from sysfs.  This makes the
+kobject "invisible", but it is not cleaned up, and the reference count of
+the object is still the same.  At a later time call kobject_put() to finish
+the cleanup of the memory associated with the kobject.
+
+kobject_del() can be used to drop the reference to the parent object, if
+circular references are constructed.  It is valid in some cases, that a
+parent objects references a child.  Circular references _must_ be broken
+with an explicit call to kobject_del(), so that a release functions will be
+called, and the objects in the former circle release each other.
+
+
+Example code to copy from
+
+For a more complete example of using ksets and kobjects properly, see the
+sample/kobject/kset-example.c code.