1、前言
Linux内核中有大量的驱动，而这些驱动往往具有类似的结构，根据面向对象的思想，可以将共同的部分提取为父类，而这个父类就是kobject，kobject结构体中包含了大量设备的必须信息，而三大类设备驱动都需要包含这个kobject结构，运用面向对象的思想来看问题，也就是继承来自kobject，一个kobject对象往往就对应sysfs中的一个目录，kobject是组成设备模型的基本结构，kobject需要处理的基本任务有如下：
（1）对象的引用计数
当一个内核对象被创建时，不可能知道对象的存活时间，跟踪该对象的的生命周期的一个方法就是使用引用计数，当内核中没有代码持有该对象的引用时，说明该对象可以被销毁了；
（2）sysfs表述
一个kobject对象往往对应sysfs中的一个目录，kobject被用来与内核交互并创建它的sysfs可见表述；
（3）数据结构关联
从整体上看，设备模型是一个友好而复杂的数据结构，通过kobject对象实现了大量的连接而构成了一个多层次的体系结构；
（4）热拔插事件处理
当系统中的硬件被热插拔时，在kobject子系统的控制下，将产生事件以通知用户空间。
2、kobject以及相关结构体
Linux内核源码中对kobject结构体的定义在include/linux/kobject.h文件中，实现在lib/kobjet.c，struct kobject结构体的定义如下所示：
struct kobject { const char *name; struct list_head entry; struct kobject *parent; struct kset *kset; struct kobj_type *ktype; struct kernfs_node *sd; /* sysfs directory entry */ struct kref kref; #ifdef CONFIG_DEBUG_KOBJECT_RELEASE struct delayed_work release; #endif unsigned int state_initialized:1; unsigned int state_in_sysfs:1; unsigned int state_add_uevent_sent:1; unsigned int state_remove_uevent_sent:1; unsigned int uevent_suppress:1; };
结构体常用成员解释：
name：表示kobject对象的名字，对应sysfs下的一个目录；
entry：在kobject中嵌入双向链表list_head结构；
parent：指向当前kobject父对象的指针；
kset：表示当前kobject对象所属的集合；
ktype：表示当前kobject对象所属类型；
sd：用于VFS文件系统的目录项，是设备与文件之间的桥梁，sysfs中的符号链接就是通过kernfs_node内的联合体实现的；
kref：是对kobject的引用计数，当引用计数为0时，就回调之前注册的release函数释放该对象；
state_initialized:1：初始化的标志位，在对象被初始化时置位，用于表示对象已被初始化；
state_in_sysfs:1：表示kobject对象在sysfs中的状态，在对应目录中被创建则置1，否则为0；
state_add_uevent_sent:1：添加设备的uevent事件是否发送标志，添加设备时会向用户空间发送uevent事件，请求新增设备；
state_remove_uevent_sent:1：删除设备的uevent事件是否发送标志，删除设备时会向用户空间发送uevent事件，请求卸载设备。
struct kobj_type定义了kobject的类型，该结构体既有操作的函数，也有默认的公共属性，该结构体定义如下：
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); void (*get_ownership)(struct kobject *kobj, kuid_t *uid, kgid_t *gid); };
结构体常用成员解释：
release：函数指针，当引用计数为0时，在kobject释放时调用；
sysfs_ops：定义了读写属性文件时调用的函数；
default_attrs：定义了这类kobject默认属性，指向attribute数组的指针。
struct sysfs_ops结构体是用于实现属性的的函数操作集，其定义如下所示：
struct sysfs_ops { ssize_t (*show)(struct kobject *, struct attribute *, char *); ssize_t (*store)(struct kobject *, struct attribute *, const char *, size_t); };
show：函数用于读取一个属性到用户空间，函数的第一个参数是要读取的kobject指针，它对应要读的目录，第二个参数是要读的属性，第三个参数存放读到的属性的缓冲区，当函数调用成功后，返回实际读取的数据长度；
store：函数用于将属性写入到内核，函数第一个参数是与写相关的kobject指针，它对应要写的目录，第二个参数是要写的属性，第三个参数是要写入的数据，第四个参数是要写入的参数长度，这个长度不能超过PAGESIZE个字节大小，只有当拥有属性写权限时，才能调用store()函数。
struct attribute定义了kobject的属性，该结构体定义如下：
struct attribute { const char *name; umode_t mode; #ifdef CONFIG_DEBUG_LOCK_ALLOC bool ignore_lockdep:1; struct lock_class_key *key; struct lock_class_key skey; #endif };
常用结构体成员解释：
name：属性文件的名称，属性文件将在kobject的sysfs目录中显示；
mode：属性文件的读写权限。
struct kset可以看成是在kobject上的拓展，它包含了一个kobject的链表，可以方便地表示sysfs中目录与子目录的关系，该结构体定义如下：
/** * struct kset - a set of kobjects of a specific type, belonging to a specific subsystem. * * A kset defines a group of kobjects. They can be individually * different "types" but overall these kobjects all want to be grouped * together and operated on in the same manner. ksets are used to * define the attribute callbacks and other common events that happen to * a kobject. * * @list: the list of all kobjects for this kset * @list_lock: a lock for iterating over the kobjects * @kobj: the embedded kobject for this kset (recursion, isn't it fun...) * @uevent_ops: the set of uevent operations for this kset. These are * called whenever a kobject has something happen to it so that the kset * can add new environment variables, or filter out the uevents if so * desired. */ struct kset { struct list_head list; spinlock_t list_lock; struct kobject kobj; const struct kset_uevent_ops *uevent_ops; };
结构体常用成员解释：
list：用来挂在链表上的结构，包含在一个kset的所有kobject构成了一个双向循坏链表，list_head是这个链表的头部，这个链表用来连接第一个和最后一个kobject对象，第一个kobject使用entry连接kset集合以及第二个kobject对象，第二个kobject对象使用entry连接第一个kobject对象和第三个kobject对象，最终形成一个kobject对象的链表；
list_lock：用于迭代kobjects的锁；
kobj：归属于该kset的所有的kobject的共有parent，这个parent就是体现内核设备组织结构的关键，同时，kset的引用计数就是内嵌的kobject对象的引用计数；
uevent_ops：此kset的uevent操作函数集。
struct kobj_attribute是kobject在attribute上的拓展，添加了专门读写kobject_attribute属性的函数，结构体定义如下：
struct kobj_attribute { struct attribute attr; ssize_t (*show)(struct kobject *kobj, struct kobj_attribute *attr, char *buf); ssize_t (*store)(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t count); };
结构体成员解释：
attr：kobject的公共属性，包括名称和读写权限；
show：函数指针，用于读取属性到用户空间；
store：函数指针，用于用户空间写属性到内核。
注意：无论是kobjec还是kset（说到底是kset内部的kobject），都提供了使用kobj_attribute的快速创建方法。
3、kobject实现操作
在Linux内核中提供了一系列kobject的实现方法，接下来，进行简要分析：
/* * populate_dir - populate directory with attributes. * @kobj: object we're working on. * * Most subsystems have a set of default attributes that are associated * with an object that registers with them. This is a helper called during * object registration that loops through the default attributes of the * subsystem and creates attributes files for them in sysfs. */ static int populate_dir(struct kobject *kobj) { struct kobj_type *t = get_ktype(kobj); struct attribute *attr; int error = 0; int i; if (t && t->default_attrs) { for (i = 0; (attr = t->default_attrs[i]) != NULL; i++) { error = sysfs_create_file(kobj, attr);//在sysfs目录创建属性文件 if (error) break; } } return error; } static int create_dir(struct kobject *kobj) { const struct kobj_ns_type_operations *ops; int error; error = sysfs_create_dir_ns(kobj, kobject_namespace(kobj));//创建sysfs目录 if (error) return error; error = populate_dir(kobj);//在sysfs目录中填充属性文件 if (error) { sysfs_remove_dir(kobj); return error; } /* * @kobj->sd may be deleted by an ancestor going away. Hold an * extra reference so that it stays until @kobj is gone. */ sysfs_get(kobj->sd);//引用计数加1 /* * If @kobj has ns_ops, its children need to be filtered based on * their namespace tags. Enable namespace support on @kobj->sd. */ ops = kobj_child_ns_ops(kobj); if (ops) { BUG_ON(ops->type <= KOBJ_NS_TYPE_NONE); BUG_ON(ops->type >= KOBJ_NS_TYPES); BUG_ON(!kobj_ns_type_registered(ops->type)); sysfs_enable_ns(kobj->sd); } return 0; }
上面两个函数中，create_dir()用于在sysfs中创建kobj相应的目录，populate_dir()创建kobj中默认属性对应的文件，create_dir()函数就是调用populate_dir()实现的。
static int get_kobj_path_length(struct kobject *kobj) { int length = 1; struct kobject *parent = kobj; /* walk up the ancestors until we hit the one pointing to the * root. * Add 1 to strlen for leading '/' of each level. */ do { if (kobject_name(parent) == NULL) return 0; length += strlen(kobject_name(parent)) + 1; parent = parent->parent; } while (parent); return length; } static void fill_kobj_path(struct kobject *kobj, char *path, int length) { struct kobject *parent; --length; for (parent = kobj; parent; parent = parent->parent) { int cur = strlen(kobject_name(parent)); /* back up enough to print this name with '/' */ length -= cur; strncpy(path + length, kobject_name(parent), cur); *(path + --length) = '/'; } pr_debug("kobject: '%s' (%p): %s: path = '%s' ", kobject_name(kobj), kobj, __func__, path); } /** * kobject_get_path - generate and return the path associated with a given kobj and kset pair. * * @kobj: kobject in question, with which to build the path * @gfp_mask: the allocation type used to allocate the path * * The result must be freed by the caller with kfree(). */ char *kobject_get_path(struct kobject *kobj, gfp_t gfp_mask) { char *path; int len; len = get_kobj_path_length(kobj); if (len == 0) return NULL; path = kzalloc(len, gfp_mask); if (!path) return NULL; fill_kobj_path(kobj, path, len); return path; }
在上面的三个函数中，前面两个函数是内部函数，get_kobj_path_length()函数用于获得kobj路径名的长度，fill_kobj_path()函数用于将kobj路径名填充到path缓冲区，然后将指针返回，kobject_get_path()函数就是调用这两个函数进行实现的。
/* add the kobject to its kset's list */ static void kobj_kset_join(struct kobject *kobj) { if (!kobj->kset) return; kset_get(kobj->kset);//引用计数加1 spin_lock(&kobj->kset->list_lock); list_add_tail(&kobj->entry, &kobj->kset->list);//在链表尾部插入节点 spin_unlock(&kobj->kset->list_lock); } /* remove the kobject from its kset's list */ static void kobj_kset_leave(struct kobject *kobj) { if (!kobj->kset) return; spin_lock(&kobj->kset->list_lock); list_del_init(&kobj->entry);//链表删除节点 spin_unlock(&kobj->kset->list_lock); kset_put(kobj->kset);//引用计数减1 }
上面两个函数的功能是相对的，kobj_kset_join()用于将kobj加入到kobj->kset的链表中，kobj_kset_leave()用于将kobj从kobj->kset的链表中删除。
static void kobject_init_internal(struct kobject *kobj) { if (!kobj) return; kref_init(&kobj->kref);//引用计数初始化 INIT_LIST_HEAD(&kobj->entry);//链表初始化 kobj->state_in_sysfs = 0; kobj->state_add_uevent_sent = 0; kobj->state_remove_uevent_sent = 0; kobj->state_initialized = 1; } static int kobject_add_internal(struct kobject *kobj) { int error = 0; struct kobject *parent; if (!kobj) return -ENOENT; if (!kobj->name || !kobj->name[0]) { WARN(1, "kobject: (%p): attempted to be registered with empty " "name! ", kobj); return -EINVAL; } parent = kobject_get(kobj->parent);//父kobject引用计数加1 /* join kset if set, use it as parent if we do not already have one */ if (kobj->kset) { if (!parent) parent = kobject_get(&kobj->kset->kobj); kobj_kset_join(kobj); kobj->parent = parent;//设置kobject的parent指针 } pr_debug("kobject: '%s' (%p): %s: parent: '%s', set: '%s' ", kobject_name(kobj), kobj, __func__, parent ? kobject_name(parent) : "<NULL>", kobj->kset ? kobject_name(&kobj->kset->kobj) : "<NULL>"); error = create_dir(kobj);//在sysfs中创建目录 if (error) { kobj_kset_leave(kobj); kobject_put(parent); kobj->parent = NULL; /* be noisy on error issues */ if (error == -EEXIST) pr_err("%s failed for %s with -EEXIST, don't try to register things with the same name in the same directory. ", __func__, kobject_name(kobj)); else pr_err("%s failed for %s (error: %d parent: %s) ", __func__, kobject_name(kobj), error, parent ? kobject_name(parent) : "'none'"); } else kobj->state_in_sysfs = 1; return error; }
kobject_init_internal()函数用来初始化kobj，kobject_add_internal()函数用来将kobj加入已有的结构中去，这两个函数有密切的联系，在kobject结构体中有很多变量，但是重要的只有两个，一个是kset，一个是parent，这两个变量表示了kobject在整个体系中的位置，决不能自行决定，需要外部参与设置，kobject的创建过程分为init和add两个阶段。kobject_init_internal()把一些结构体变量进行初始化后，等外界设置了parent和kset后，再调用kobject_add_internal()把kobject添加到适当的位置，并创建相应的sysfs目录及文件。
/** * kobject_set_name_vargs - Set the name of an kobject * @kobj: struct kobject to set the name of * @fmt: format string used to build the name * @vargs: vargs to format the string. */ int kobject_set_name_vargs(struct kobject *kobj, const char *fmt, va_list vargs) { const char *s; if (kobj->name && !fmt) return 0; s = kvasprintf_const(GFP_KERNEL, fmt, vargs); if (!s) return -ENOMEM; /* * ewww... some of these buggers have '/' in the name ... If * that's the case, we need to make sure we have an actual * allocated copy to modify, since kvasprintf_const may have * returned something from .rodata. */ if (strchr(s, '/')) { char *t; t = kstrdup(s, GFP_KERNEL); kfree_const(s); if (!t) return -ENOMEM; strreplace(t, '/', '!'); s = t; } kfree_const(kobj->name); kobj->name = s; return 0; } /** * kobject_set_name - Set the name of a kobject * @kobj: struct kobject to set the name of * @fmt: format string used to build the name * * This sets the name of the kobject. If you have already added the * kobject to the system, you must call kobject_rename() in order to * change the name of the kobject. */ int kobject_set_name(struct kobject *kobj, const char *fmt, ...) { va_list vargs; int retval; va_start(vargs, fmt); retval = kobject_set_name_vargs(kobj, fmt, vargs); va_end(vargs); return retval; }
kobject_set_name()函数的作用是设置kobj的名称，它通过调用kobject_set_name_vargs()来实现，需要注意的时，kobject_set_name()仅限于kobject添加到体系之前使用，因为该函数只是修改了名字，并未通知到用户空间。
/** * kobject_init - initialize a kobject structure * @kobj: pointer to the kobject to initialize * @ktype: pointer to the ktype for this kobject. * * This function will properly initialize a kobject such that it can then * be passed to the kobject_add() call. * * After this function is called, the kobject MUST be cleaned up by a call * to kobject_put(), not by a call to kfree directly to ensure that all of * the memory is cleaned up properly. */ void kobject_init(struct kobject *kobj, struct kobj_type *ktype) { char *err_str; if (!kobj) { err_str = "invalid kobject pointer!"; goto error; } if (!ktype) { err_str = "must have a ktype to be initialized properly! "; goto error; } if (kobj->state_initialized) { /* do not error out as sometimes we can recover */ printk(KERN_ERR "kobject (%p): tried to init an initialized " "object, something is seriously wrong. ", kobj); dump_stack(); } kobject_init_internal(kobj); kobj->ktype = ktype; return; error: printk(KERN_ERR "kobject (%p): %s ", kobj, err_str); dump_stack(); }
kobject_init()函数就是调用了kobject_init_internal()函数进行一些结构体变量初始化，然后又设置了ktype结构体成员，ktype的作用是管理一些默认属性。
static int kobject_add_varg(struct kobject *kobj, struct kobject *parent, const char *fmt, va_list vargs) { int retval; retval = kobject_set_name_vargs(kobj, fmt, vargs); if (retval) { printk(KERN_ERR "kobject: can not set name properly! "); return retval; } kobj->parent = parent; return kobject_add_internal(kobj); } /** * kobject_add - the main kobject add function * @kobj: the kobject to add * @parent: pointer to the parent of the kobject. * @fmt: format to name the kobject with. * * The kobject name is set and added to the kobject hierarchy in this * function. * * If @parent is set, then the parent of the @kobj will be set to it. * If @parent is NULL, then the parent of the @kobj will be set to the * kobject associated with the kset assigned to this kobject. If no kset * is assigned to the kobject, then the kobject will be located in the * root of the sysfs tree. * * If this function returns an error, kobject_put() must be called to * properly clean up the memory associated with the object. * Under no instance should the kobject that is passed to this function * be directly freed with a call to kfree(), that can leak memory. * * Note, no "add" uevent will be created with this call, the caller should set * up all of the necessary sysfs files for the object and then call * kobject_uevent() with the UEVENT_ADD parameter to ensure that * userspace is properly notified of this kobject's creation. */ int kobject_add(struct kobject *kobj, struct kobject *parent, const char *fmt, ...) { va_list args; int retval; if (!kobj) return -EINVAL; if (!kobj->state_initialized) { printk(KERN_ERR "kobject '%s' (%p): tried to add an " "uninitialized object, something is seriously wrong. ", kobject_name(kobj), kobj); dump_stack(); return -EINVAL; } va_start(args, fmt); retval = kobject_add_varg(kobj, parent, fmt, args); va_end(args); return retval; }
kobject_add()函数用于将kobj添加到体系中去，但是该函数还有一个附加的功能，设置kobj的名字，parent作为参数被传进来。
/** * kobject_init_and_add - initialize a kobject structure and add it to the kobject hierarchy * @kobj: pointer to the kobject to initialize * @ktype: pointer to the ktype for this kobject. * @parent: pointer to the parent of this kobject. * @fmt: the name of the kobject. * * This function combines the call to kobject_init() and * kobject_add(). The same type of error handling after a call to * kobject_add() and kobject lifetime rules are the same here. */ int kobject_init_and_add(struct kobject *kobj, struct kobj_type *ktype, struct kobject *parent, const char *fmt, ...) { va_list args; int retval; kobject_init(kobj, ktype); va_start(args, fmt); retval = kobject_add_varg(kobj, parent, fmt, args); va_end(args); return retval; }
kobject_init_and_add()其实是kobjec_init()和kobject_add()的合并，将kobject初始化后并添加到kobject的层次结构中去。
/** * kobject_rename - change the name of an object * @kobj: object in question. * @new_name: object's new name * * It is the responsibility of the caller to provide mutual * exclusion between two different calls of kobject_rename * on the same kobject and to ensure that new_name is valid and * won't conflict with other kobjects. */ int kobject_rename(struct kobject *kobj, const char *new_name) { int error = 0; const char *devpath = NULL; const char *dup_name = NULL, *name; char *devpath_string = NULL; char *envp[2]; kobj = kobject_get(kobj); if (!kobj) return -EINVAL; if (!kobj->parent) return -EINVAL; devpath = kobject_get_path(kobj, GFP_KERNEL); if (!devpath) { error = -ENOMEM; goto out; } devpath_string = kmalloc(strlen(devpath) + 15, GFP_KERNEL); if (!devpath_string) { error = -ENOMEM; goto out; } sprintf(devpath_string, "DEVPATH_OLD=%s", devpath); envp[0] = devpath_string; envp[1] = NULL; name = dup_name = kstrdup_const(new_name, GFP_KERNEL); if (!name) { error = -ENOMEM; goto out; } error = sysfs_rename_dir_ns(kobj, new_name, kobject_namespace(kobj)); if (error) goto out; /* Install the new kobject name */ dup_name = kobj->name; kobj->name = name; /* This function is mostly/only used for network interface. * Some hotplug package track interfaces by their name and * therefore want to know when the name is changed by the user. */ kobject_uevent_env(kobj, KOBJ_MOVE, envp); out: kfree_const(dup_name); kfree(devpath_string); kfree(devpath); kobject_put(kobj); return error; }
kobject_rename()函数实现的功能为，当kobj已经添加到系统后，可以使用此函数调用实现更改kobject的名字，它除了能完成kobject_set_name()的功能以外，还向用户空间通知这一消息。
/** * kobject_move - move object to another parent * @kobj: object in question. * @new_parent: object's new parent (can be NULL) */ int kobject_move(struct kobject *kobj, struct kobject *new_parent) { int error; struct kobject *old_parent; const char *devpath = NULL; char *devpath_string = NULL; char *envp[2]; kobj = kobject_get(kobj); if (!kobj) return -EINVAL; new_parent = kobject_get(new_parent); if (!new_parent) { if (kobj->kset) new_parent = kobject_get(&kobj->kset->kobj); } /* old object path */ devpath = kobject_get_path(kobj, GFP_KERNEL); if (!devpath) { error = -ENOMEM; goto out; } devpath_string = kmalloc(strlen(devpath) + 15, GFP_KERNEL); if (!devpath_string) { error = -ENOMEM; goto out; } sprintf(devpath_string, "DEVPATH_OLD=%s", devpath); envp[0] = devpath_string; envp[1] = NULL; error = sysfs_move_dir_ns(kobj, new_parent, kobject_namespace(kobj)); if (error) goto out; old_parent = kobj->parent; kobj->parent = new_parent; new_parent = NULL; kobject_put(old_parent); kobject_uevent_env(kobj, KOBJ_MOVE, envp); out: kobject_put(new_parent); kobject_put(kobj); kfree(devpath_string); kfree(devpath); return error; }
kobjec_move()函数实现的功能是在kobj添加到系统后，调用此函数能将kobj移动到新的parent下，同时会通知用户空间。
/** * kobject_del - unlink kobject from hierarchy. * @kobj: object. */ void kobject_del(struct kobject *kobj) { struct kernfs_node *sd; if (!kobj) return; sd = kobj->sd; sysfs_remove_dir(kobj); sysfs_put(sd); kobj->state_in_sysfs = 0; kobj_kset_leave(kobj); kobject_put(kobj->parent); kobj->parent = NULL; }
kobject_del()函数用于将kobj从系统中删除，和kobject_add()是相对的操作。
/** * kobject_get - increment refcount for object. * @kobj: object. */ struct kobject *kobject_get(struct kobject *kobj) { if (kobj) { if (!kobj->state_initialized) WARN(1, KERN_WARNING "kobject: '%s' (%p): is not " "initialized, yet kobject_get() is being " "called. ", kobject_name(kobj), kobj); kref_get(&kobj->kref); } return kobj; } /* * kobject_cleanup - free kobject resources. * @kobj: object to cleanup */ static void kobject_cleanup(struct kobject *kobj) { struct kobj_type *t = get_ktype(kobj); const char *name = kobj->name; pr_debug("kobject: '%s' (%p): %s, parent %p ", kobject_name(kobj), kobj, __func__, kobj->parent); if (t && !t->release) pr_debug("kobject: '%s' (%p): does not have a release() " "function, it is broken and must be fixed. ", kobject_name(kobj), kobj); /* send "remove" if the caller did not do it but sent "add" */ if (kobj->state_add_uevent_sent && !kobj->state_remove_uevent_sent) { pr_debug("kobject: '%s' (%p): auto cleanup 'remove' event ", kobject_name(kobj), kobj); kobject_uevent(kobj, KOBJ_REMOVE); } /* remove from sysfs if the caller did not do it */ if (kobj->state_in_sysfs) { pr_debug("kobject: '%s' (%p): auto cleanup kobject_del ", kobject_name(kobj), kobj); kobject_del(kobj); } if (t && t->release) { pr_debug("kobject: '%s' (%p): calling ktype release ", kobject_name(kobj), kobj); t->release(kobj); } /* free name if we allocated it */ if (name) { pr_debug("kobject: '%s': free name ", name); kfree_const(name); } } #ifdef CONFIG_DEBUG_KOBJECT_RELEASE static void kobject_delayed_cleanup(struct work_struct *work) { kobject_cleanup(container_of(to_delayed_work(work), struct kobject, release)); } #endif static void kobject_release(struct kref *kref) { struct kobject *kobj = container_of(kref, struct kobject, kref); #ifdef CONFIG_DEBUG_KOBJECT_RELEASE unsigned long delay = HZ + HZ * (get_random_int() & 0x3); pr_info("kobject: '%s' (%p): %s, parent %p (delayed %ld) ", kobject_name(kobj), kobj, __func__, kobj->parent, delay); INIT_DELAYED_WORK(&kobj->release, kobject_delayed_cleanup); schedule_delayed_work(&kobj->release, delay); #else kobject_cleanup(kobj); #endif } /** * kobject_put - decrement refcount for object. * @kobj: object. * * Decrement the refcount, and if 0, call kobject_cleanup(). */ void kobject_put(struct kobject *kobj) { if (kobj) { if (!kobj->state_initialized) WARN(1, KERN_WARNING "kobject: '%s' (%p): is not " "initialized, yet kobject_put() is being " "called. ", kobject_name(kobj), kobj); kref_put(&kobj->kref, kobject_release); } }
kobject_get()和kobject_put()函数完成的是kobject的引用计数功能，kobject_put()函数在当引用计数为0时，撤销整个kobject的存在：向用户空间发送REMOVE信息，从sysfs中删除相应的目录，并调用ktype中定义的release函数，并释放name所占用的空间。kobject_cleanup()函数用来释放kobject创建时所分配的资源。
上述提到到API接口，基本上概括了kobject从创建到删除，包括kobject的名字修改、位置修改以及引用计数的变动等功能。但是，kobject的创建仍然比较麻烦，因为ktype需要自己去实现，下面是Linux内核中为kobject提供的一种快速创建的方法：
首先是kobject的属性文件的读写函数定义：
/* default kobject attribute operations */ static ssize_t kobj_attr_show(struct kobject *kobj, struct attribute *attr, char *buf) { struct kobj_attribute *kattr; ssize_t ret = -EIO; kattr = container_of(attr, struct kobj_attribute, attr); if (kattr->show) ret = kattr->show(kobj, kattr, buf); return ret; } static ssize_t kobj_attr_store(struct kobject *kobj, struct attribute *attr, const char *buf, size_t count) { struct kobj_attribute *kattr; ssize_t ret = -EIO; kattr = container_of(attr, struct kobj_attribute, attr); if (kattr->store) ret = kattr->store(kobj, kattr, buf, count); return ret; } const struct sysfs_ops kobj_sysfs_ops = { .show = kobj_attr_show, .store = kobj_attr_store, };
然后是，kobject的释放函数，以及kobj_type结构体变量的定义，如下：
static void dynamic_kobj_release(struct kobject *kobj) { pr_debug("kobject: (%p): %s ", kobj, __func__); kfree(kobj); } static struct kobj_type dynamic_kobj_ktype = { .release = dynamic_kobj_release, .sysfs_ops = &kobj_sysfs_ops, };
这是Linux内核里面为kobject自身提供的一种kobj_type，名称叫做dynamic_kobj_type，它并没有提供默认的属性，但是提供了release函数和访问属性的方法。
/** * kobject_create - create a struct kobject dynamically * * This function creates a kobject structure dynamically and sets it up * to be a "dynamic" kobject with a default release function set up. * * If the kobject was not able to be created, NULL will be returned. * The kobject structure returned from here must be cleaned up with a * call to kobject_put() and not kfree(), as kobject_init() has * already been called on this structure. */ struct kobject *kobject_create(void) { struct kobject *kobj; kobj = kzalloc(sizeof(*kobj), GFP_KERNEL); if (!kobj) return NULL; kobject_init(kobj, &dynamic_kobj_ktype); return kobj; } /** * kobject_create_and_add - create a struct kobject dynamically and register it with sysfs * * @name: the name for the kobject * @parent: the parent kobject of this kobject, if any. * * This function creates a kobject structure dynamically and registers it * with sysfs. When you are finished with this structure, call * kobject_put() and the structure will be dynamically freed when * it is no longer being used. * * If the kobject was not able to be created, NULL will be returned. */ struct kobject *kobject_create_and_add(const char *name, struct kobject *parent) { struct kobject *kobj; int retval; kobj = kobject_create(); if (!kobj) return NULL; retval = kobject_add(kobj, parent, "%s", name); if (retval) { printk(KERN_WARNING "%s: kobject_add error: %d ", __func__, retval); kobject_put(kobj); kobj = NULL; } return kobj; }
在kobject_create()和kobject_create_add()函数中，使用了dynamic_kobj_ktype这个结构体变量，这是一种很好的偷懒方式，因为release()函数会释放掉kobj，所以这里的kobj必须是动态创建的，这里的kobject_create()和kobject_init()相对，kobject_create_and_add()和kobject_init_and_add()相对。需要注意的是，这里创建的kobject无法嵌入到其它结构，是独立存在的，所以用到的地方很少。
/** * kset_init - initialize a kset for use * @k: kset */ void kset_init(struct kset *k) { kobject_init_internal(&k->kobj); INIT_LIST_HEAD(&k->list); spin_lock_init(&k->list_lock); }
kset_init()函数用于将kset结构体初始化，和kobject初始化类似。
/** * kset_register - initialize and add a kset. * @k: kset. */ int kset_register(struct kset *k) { int err; if (!k) return -EINVAL; kset_init(k); err = kobject_add_internal(&k->kobj); if (err) return err; kobject_uevent(&k->kobj, KOBJ_ADD); return 0; }
kset_register()函数用来初始化并添加到kset，函数比较简单，它负责将kset中的kobject添加到系统，并向用户空间发布KOBJ_ADD消息，所以在调用之前，需要先设置好k->kobj.name、k->kobj.parent和k->kobj.kset这些成员。
/** * kset_unregister - remove a kset. * @k: kset. */ void kset_unregister(struct kset *k) { if (!k) return; kobject_del(&k->kobj); kobject_put(&k->kobj); }
kset_unregister()和kset_register()函数功能是相对的，用于移除kset，并减少引用计数。
/** * kset_find_obj - search for object in kset. * @kset: kset we're looking in. * @name: object's name. * * Lock kset via @kset->subsys, and iterate over @kset->list, * looking for a matching kobject. If matching object is found * take a reference and return the object. */ struct kobject *kset_find_obj(struct kset *kset, const char *name) { struct kobject *k; struct kobject *ret = NULL; spin_lock(&kset->list_lock); list_for_each_entry(k, &kset->list, entry) { if (kobject_name(k) && !strcmp(kobject_name(k), name)) { ret = kobject_get_unless_zero(k); break; } } spin_unlock(&kset->list_lock); return ret; }
kset_find_obj()函数功能为通过从kset的链表中寻找名为name的kobject，实现了链表的遍历。
与kobject类似，kset也提供看一种kobj_type，名称为kset_ktype，实现如下：
static void kset_release(struct kobject *kobj) { struct kset *kset = container_of(kobj, struct kset, kobj); pr_debug("kobject: '%s' (%p): %s ", kobject_name(kobj), kobj, __func__); kfree(kset); } void kset_get_ownership(struct kobject *kobj, kuid_t *uid, kgid_t *gid) { if (kobj->parent) kobject_get_ownership(kobj->parent, uid, gid); } static struct kobj_type kset_ktype = { .sysfs_ops = &kobj_sysfs_ops, .release = kset_release, .get_ownership = kset_get_ownership, }; /** * kset_create - create a struct kset dynamically * * @name: the name for the kset * @uevent_ops: a struct kset_uevent_ops for the kset * @parent_kobj: the parent kobject of this kset, if any. * * This function creates a kset structure dynamically. This structure can * then be registered with the system and show up in sysfs with a call to * kset_register(). When you are finished with this structure, if * kset_register() has been called, call kset_unregister() and the * structure will be dynamically freed when it is no longer being used. * * If the kset was not able to be created, NULL will be returned. */ static struct kset *kset_create(const char *name, const struct kset_uevent_ops *uevent_ops, struct kobject *parent_kobj) { struct kset *kset; int retval; kset = kzalloc(sizeof(*kset), GFP_KERNEL); if (!kset) return NULL; retval = kobject_set_name(&kset->kobj, "%s", name); if (retval) { kfree(kset); return NULL; } kset->uevent_ops = uevent_ops; kset->kobj.parent = parent_kobj; /* * The kobject of this kset will have a type of kset_ktype and belong to * no kset itself. That way we can properly free it when it is * finished being used. */ kset->kobj.ktype = &kset_ktype; kset->kobj.kset = NULL; return kset; } /** * kset_create_and_add - create a struct kset dynamically and add it to sysfs * * @name: the name for the kset * @uevent_ops: a struct kset_uevent_ops for the kset * @parent_kobj: the parent kobject of this kset, if any. * * This function creates a kset structure dynamically and registers it * with sysfs. When you are finished with this structure, call * kset_unregister() and the structure will be dynamically freed when it * is no longer being used. * * If the kset was not able to be created, NULL will be returned. */ struct kset *kset_create_and_add(const char *name, const struct kset_uevent_ops *uevent_ops, struct kobject *parent_kobj) { struct kset *kset; int error; kset = kset_create(name, uevent_ops, parent_kobj); if (!kset) return NULL; error = kset_register(kset); if (error) { kfree(kset); return NULL; } return kset; }
kset_create()和kset_create_and_add()就是使用kset_type来快速创建kset的函数，当需要在sysfs中创建单纯的目录时，可以使用这两个函数进行创建。
static inline struct kset *to_kset(struct kobject *kobj) { return kobj ? container_of(kobj, struct kset, kobj) : NULL; } static inline struct kset *kset_get(struct kset *k) { return k ? to_kset(kobject_get(&k->kobj)) : NULL; } static inline void kset_put(struct kset *k) { kobject_put(&k->kobj); } static inline struct kobj_type *get_ktype(struct kobject *kobj) { return kobj->ktype; }
上面这些函数是在kobject.h文件里面的内联函数，to_kset()函数通过kobj的指针获取kset结构体的首地址，kset_get()和kset_put()为引用计数操作，get_ktype()函数用于获取kobject的ktype类型。
4、 kobject的层次结构
内核用kobject结构将各个对象连接起来组成一个分层的结构体系，从而与模型化的子系统相匹配。kobject结构体中的parent指针成员、kset指针成员和链表list_head实现了kobject的层次结构，parent指针最重要的用途是在sysfs分层结构中定位对象，下图是一个简单的kobject、kset的分层结构图：
5、小节
本文主要对kobject结构体以及相关的结构体以及Linux内核提供的相关API做了简要介绍，kobject结构体和设备驱动模型和sysfs的实现密切相关，必须深刻理解。
参考：
https://blog.csdn.net/yuanmengliang/article/details/52700529
https://blog.csdn.net/qb_2008/article/details/6846779
https://www.cnblogs.com/xiaojiang1025/p/6193959.html
《LINUX设备驱动程序（第三版）》