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diff --git a/Documentation/kobject.txt b/Documentation/kobject.txt new file mode 100644 index 000000000..fc9485d79 --- /dev/null +++ b/Documentation/kobject.txt @@ -0,0 +1,430 @@ +===================================================================== +Everything you never wanted to know about kobjects, ksets, and ktypes +===================================================================== + +:Author: Greg Kroah-Hartman <gregkh@linuxfoundation.org> +:Last updated: December 19, 2007 + +Based on an original article by Jon Corbet for lwn.net written October 1, +2003 and located at http://lwn.net/Articles/51437/ + +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. + +(As an aside, for those familiar with the kernel linked list implementation, +this is analogous as to how "list_head" structs are rarely useful on +their own, but are invariably found embedded in the larger objects of +interest.) + +So, for example, the UIO code in drivers/uio/uio.c has a structure that +defines the memory region associated with a uio device:: + + struct uio_map { + struct kobject kobj; + struct uio_mem *mem; + }; + +If you have a struct uio_map 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 corresponding +container type. So, for example, a pointer "kp" to a struct kobject +embedded *within* a struct uio_map could be converted to a pointer to the +*containing* uio_map structure with:: + + struct uio_map *u_map = container_of(kp, struct uio_map, kobj); + +For convenience, programmers often define a simple macro for "back-casting" +kobject pointers to the containing type. Exactly this happens in the +earlier drivers/uio/uio.c, as you can see here:: + + struct uio_map { + struct kobject kobj; + struct uio_mem *mem; + }; + + #define to_map(map) container_of(map, struct uio_map, kobj) + +where the macro argument "map" is a pointer to the struct kobject in +question. That macro is subsequently invoked with:: + + struct uio_map *map = to_map(kobj); + + +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); + +kobject_rename does not perform any locking or have a solid notion of +what names are valid so the caller must provide their own sanity checking +and serialization. + +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 surprisingly 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 are +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 *kobj); + const struct sysfs_ops *sysfs_ops; + struct attribute **default_attrs; + const struct kobj_ns_type_operations *(*child_ns_type)(struct kobject *kobj); + const void *(*namespace)(struct kobject *kobj); + }; + +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 kset 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. This removes the kset from sysfs and decrements its reference +count. When the reference count goes to zero, the kset will be released. +Because other references to the kset may still exist, the release may happen +after kset_unregister() returns. + +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 +example programs samples/kobject/{kobject-example.c,kset-example.c}, +which will be built as loadable modules if you select CONFIG_SAMPLE_KOBJECT. |