kobject.txt: standardize document format
Each text file under Documentation follows a different format. Some doesn't even have titles! Change its representation to follow the adopted standard, using ReST markups for it to be parseable by Sphinx: - Add markups for titles; - mark literal blocks as such; - add needed whitespace/blank lines; - use :Author: and :Last updated: for authorship. Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Mauro Carvalho Chehab
Jonathan Corbet
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@ -1,13 +1,13 @@
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=====================================================================
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Everything you never wanted to know about kobjects, ksets, and ktypes
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=====================================================================
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Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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:Author: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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:Last updated: December 19, 2007
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Based on an original article by Jon Corbet for lwn.net written October 1,
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2003 and located at http://lwn.net/Articles/51437/
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Last updated December 19, 2007
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Part of the difficulty in understanding the driver model - and the kobject
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abstraction upon which it is built - is that there is no obvious starting
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place. Dealing with kobjects requires understanding a few different types,
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@ -47,6 +47,7 @@ approach will be taken, so we'll go back to kobjects.
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Embedding kobjects
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==================
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It is rare for kernel code to create a standalone kobject, with one major
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exception explained below. Instead, kobjects are used to control access to
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@ -65,7 +66,7 @@ their own, but are invariably found embedded in the larger objects of
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interest.)
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So, for example, the UIO code in drivers/uio/uio.c has a structure that
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defines the memory region associated with a uio device:
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defines the memory region associated with a uio device::
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struct uio_map {
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struct kobject kobj;
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@ -77,7 +78,7 @@ just a matter of using the kobj member. Code that works with kobjects will
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often have the opposite problem, however: given a struct kobject pointer,
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what is the pointer to the containing structure? You must avoid tricks
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(such as assuming that the kobject is at the beginning of the structure)
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and, instead, use the container_of() macro, found in <linux/kernel.h>:
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and, instead, use the container_of() macro, found in <linux/kernel.h>::
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container_of(pointer, type, member)
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@ -90,13 +91,13 @@ where:
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The return value from container_of() is a pointer to the corresponding
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container type. So, for example, a pointer "kp" to a struct kobject
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embedded *within* a struct uio_map could be converted to a pointer to the
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*containing* uio_map structure with:
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*containing* uio_map structure with::
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struct uio_map *u_map = container_of(kp, struct uio_map, kobj);
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For convenience, programmers often define a simple macro for "back-casting"
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kobject pointers to the containing type. Exactly this happens in the
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earlier drivers/uio/uio.c, as you can see here:
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earlier drivers/uio/uio.c, as you can see here::
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struct uio_map {
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struct kobject kobj;
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@ -106,23 +107,25 @@ earlier drivers/uio/uio.c, as you can see here:
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#define to_map(map) container_of(map, struct uio_map, kobj)
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where the macro argument "map" is a pointer to the struct kobject in
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question. That macro is subsequently invoked with:
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question. That macro is subsequently invoked with::
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struct uio_map *map = to_map(kobj);
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Initialization of kobjects
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==========================
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Code which creates a kobject must, of course, initialize that object. Some
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of the internal fields are setup with a (mandatory) call to kobject_init():
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of the internal fields are setup with a (mandatory) call to kobject_init()::
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void kobject_init(struct kobject *kobj, struct kobj_type *ktype);
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The ktype is required for a kobject to be created properly, as every kobject
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must have an associated kobj_type. After calling kobject_init(), to
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register the kobject with sysfs, the function kobject_add() must be called:
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register the kobject with sysfs, the function kobject_add() must be called::
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int kobject_add(struct kobject *kobj, struct kobject *parent, const char *fmt, ...);
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int kobject_add(struct kobject *kobj, struct kobject *parent,
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const char *fmt, ...);
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This sets up the parent of the kobject and the name for the kobject
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properly. If the kobject is to be associated with a specific kset,
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@ -133,7 +136,7 @@ kset itself.
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As the name of the kobject is set when it is added to the kernel, the name
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of the kobject should never be manipulated directly. If you must change
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the name of the kobject, call kobject_rename():
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the name of the kobject, call kobject_rename()::
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int kobject_rename(struct kobject *kobj, const char *new_name);
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@ -146,12 +149,12 @@ is being removed. If your code needs to call this function, it is
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incorrect and needs to be fixed.
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To properly access the name of the kobject, use the function
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kobject_name():
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kobject_name()::
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const char *kobject_name(const struct kobject * kobj);
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There is a helper function to both initialize and add the kobject to the
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kernel at the same time, called surprisingly enough kobject_init_and_add():
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kernel at the same time, called surprisingly enough kobject_init_and_add()::
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int kobject_init_and_add(struct kobject *kobj, struct kobj_type *ktype,
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struct kobject *parent, const char *fmt, ...);
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@ -161,10 +164,11 @@ kobject_add() functions described above.
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Uevents
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=======
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After a kobject has been registered with the kobject core, you need to
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announce to the world that it has been created. This can be done with a
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call to kobject_uevent():
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call to kobject_uevent()::
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int kobject_uevent(struct kobject *kobj, enum kobject_action action);
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@ -180,11 +184,12 @@ hand.
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Reference counts
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================
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One of the key functions of a kobject is to serve as a reference counter
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for the object in which it is embedded. As long as references to the object
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exist, the object (and the code which supports it) must continue to exist.
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The low-level functions for manipulating a kobject's reference counts are:
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The low-level functions for manipulating a kobject's reference counts are::
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struct kobject *kobject_get(struct kobject *kobj);
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void kobject_put(struct kobject *kobj);
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@ -209,21 +214,24 @@ file Documentation/kref.txt in the Linux kernel source tree.
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Creating "simple" kobjects
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==========================
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Sometimes all that a developer wants is a way to create a simple directory
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in the sysfs hierarchy, and not have to mess with the whole complication of
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ksets, show and store functions, and other details. This is the one
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exception where a single kobject should be created. To create such an
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entry, use the function:
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entry, use the function::
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struct kobject *kobject_create_and_add(char *name, struct kobject *parent);
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This function will create a kobject and place it in sysfs in the location
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underneath the specified parent kobject. To create simple attributes
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associated with this kobject, use:
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associated with this kobject, use::
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int sysfs_create_file(struct kobject *kobj, struct attribute *attr);
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or
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or::
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int sysfs_create_group(struct kobject *kobj, struct attribute_group *grp);
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Both types of attributes used here, with a kobject that has been created
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@ -236,6 +244,7 @@ implementation of a simple kobject and attributes.
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ktypes and release methods
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==========================
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One important thing still missing from the discussion is what happens to a
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kobject when its reference count reaches zero. The code which created the
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@ -257,7 +266,7 @@ is good practice to always use kobject_put() after kobject_init() to avoid
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errors creeping in.
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This notification is done through a kobject's release() method. Usually
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such a method has a form like:
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such a method has a form like::
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void my_object_release(struct kobject *kobj)
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{
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@ -281,7 +290,7 @@ leak in the kobject core, which makes people unhappy.
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Interestingly, the release() method is not stored in the kobject itself;
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instead, it is associated with the ktype. So let us introduce struct
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kobj_type:
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kobj_type::
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struct kobj_type {
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void (*release)(struct kobject *kobj);
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@ -306,6 +315,7 @@ automatically created for any kobject that is registered with this ktype.
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ksets
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=====
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A kset is merely a collection of kobjects that want to be associated with
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each other. There is no restriction that they be of the same ktype, but be
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@ -335,13 +345,16 @@ kobject) in their parent.
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As a kset contains a kobject within it, it should always be dynamically
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created and never declared statically or on the stack. To create a new
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kset use:
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kset use::
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struct kset *kset_create_and_add(const char *name,
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struct kset_uevent_ops *u,
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struct kobject *parent);
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When you are finished with the kset, call:
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When you are finished with the kset, call::
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void kset_unregister(struct kset *kset);
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to destroy it. This removes the kset from sysfs and decrements its reference
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count. When the reference count goes to zero, the kset will be released.
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Because other references to the kset may still exist, the release may happen
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@ -351,7 +364,7 @@ An example of using a kset can be seen in the
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samples/kobject/kset-example.c file in the kernel tree.
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If a kset wishes to control the uevent operations of the kobjects
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associated with it, it can use the struct kset_uevent_ops to handle it:
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associated with it, it can use the struct kset_uevent_ops to handle it::
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struct kset_uevent_ops {
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int (*filter)(struct kset *kset, struct kobject *kobj);
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@ -386,6 +399,7 @@ added below the parent kobject.
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Kobject removal
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===============
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After a kobject has been registered with the kobject core successfully, it
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must be cleaned up when the code is finished with it. To do that, call
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@ -409,6 +423,7 @@ called, and the objects in the former circle release each other.
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Example code to copy from
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=========================
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For a more complete example of using ksets and kobjects properly, see the
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example programs samples/kobject/{kobject-example.c,kset-example.c},
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