使用的linux版本是2.6.18
1、概念与重要的数据结构
SYSFS是一个虚拟的文件系统，这个文件系统的目录结构实质上就是内核对象模型树。
Kobject:内核对象，是模型树中的基本组成单位
53 struct kobject {
54 const char * k_name;
55 char name[KOBJ_NAME_LEN];
56 struct kref kref;
57 struct list_head entry;
58 struct kobject * parent;
59 struct kset * kset;
60 struct kobj_type * ktype;
61 struct dentry * dentry;
62 wait_queue_head_t poll;
63 };
kset:对象集合
127 struct kset {
128 struct subsystem * subsys;
129 struct kobj_type * ktype;
130 struct list_head list;
131 spinlock_t list_lock;
132 struct kobject kobj;
133 struct kset_uevent_ops * uevent_ops;
134 };
ktype:描述kobject的属性，可通过对属性的操作来作一些信息的获取或更改
96 struct kobj_type {
97 void (*release)(struct kobject *);
98 struct sysfs_ops * sysfs_ops;
99 struct attribute ** default_attrs;
100 };
subsystem:子系统，实则上是一个kset，在高层次的概念
176 struct subsystem {
177 struct kset kset;
178 struct rw_semaphore rwsem;
179 };
2、结构图
<参见LDD3的sysfs部分> 关于kset与kobject之间的关系图。
从该图可以看出，结构的组成，主要就是kset和kobject来组成各种高层次的结构。
3、bus子系统基本结构
38 struct bus_type {
39 const char * name;
40
41 struct subsystem subsys;
42 struct kset drivers;
43 struct kset devices;
44 struct klist klist_devices;
45 struct klist klist_drivers;
46
47 struct blocking_notifier_head bus_notifier;
48
49 struct bus_attribute * bus_attrs;
50 struct device_attribute * dev_attrs;
51 struct driver_attribute * drv_attrs;
52
53 int (*match)(struct device * dev, struct device_driver * drv);
54 int (*uevent)(struct device *dev, char **envp,
55 int num_envp, char *buffer, int buffer_size);
56 int (*probe)(struct device * dev);
57 int (*remove)(struct device * dev);
58 void (*shutdown)(struct device * dev);
59
60 int (*suspend)(struct device * dev, pm_message_t state);
61 int (*suspend_late)(struct device * dev, pm_message_t state);
62 int (*resume_early)(struct device * dev);
63 int (*resume)(struct device * dev);
64 };
bus_type内嵌了一个 subsystem subsys,所以bus_type可以看作是一个类子系统，实则上就是一个子系统。
通过分析i2c总线的注册过程和对应在sysfs中的目录结构来看。
bus_type – i2c_bus_type
在/sys/目录下，看到目录实际上对应一个子系统。而我们要在这些子系统下新建一些子系统，对于i2c总线，我们要在bus子系统下挂接我们的i2c子系统。通过bus_register(&i2c_bus_type)来完成这一操作。具体的操作实际上就是插入一个kobject。
（１）对i2c的sub_sys的kobject设置名称，唯一的名称，这个名称会在/sys/bus/下看到。
kobject_set_name(i2c_bus_type.sub_sys.kset.kobject);
（２）然后就是设置这个子系统的父结点，也就是设定这个子系统的父子系统。所有总线的父子系统都是bus_subsys。
i2c_bus_type.sub_sys.kset.kobject.kset = bus_subsys.kset
（３）最后将i2c_bus_type子系统加入到bus_subsys集合内的子系统链表中。
i2c_bus_type.sub_sys.kset.kobject.parent = bus_subsys.kset.kobject
add_list_tail(i2c_bus_type.sub_sys.kset.kobject.entry,
bus_subsys.kset.list)
以上的操作，基本上把i2c_bus_type对应的子系统加入到bus_subsys下面了，在/sys/bus/下面可以看到i2c目录。
在bus_type中可以看到有两个kset,drivers和devices。这两个kset属于i2c子系统下的两个集合。所以将这两个kset挂接到i2c子系统下面。
（4）挂接devices到i2c子系统下面，完成后可在/sys/bus/i2c下看到devices目录
kobject_set_name(devices.kobject, “devices”);
devices.subsys = &i2c_bus_type.sub_sys;
devices.kobject.parent = i2c_bus_type.sub_sys.kset.kobject
add_list_tail(devices.kobject, i2c_bus_type.sub_sys.kset.list);
(5)同理，挂接drivers到i2c子系统下面，完成后可在/sys/bus/i2c下看到drivers目录
kobject_set_name(devices.kobject, “devices”);
devices.subsys = &i2c_bus_type.sub_sys;
devices.kobject.parent = i2c_bus_type.sub_sys.kset.kobject
add_list_tail(devices.kobject, i2c_bus_type.sub_sys.kset.list);
4、bus子系统的设备与驱动
394 struct device {
395 struct klist klist_children;
396 struct klist_node knode_parent; /* node in sibling list */
397 struct klist_node knode_driver;
398 struct klist_node knode_bus;
399 struct device * parent;
400
401 struct kobject kobj;
402 char bus_id[BUS_ID_SIZE]; /* position on parent bus */
403 struct device_type *type;
404 unsigned is_registered:1;
405 struct device_attribute uevent_attr;
406 struct device_attribute *devt_attr;
407
408 struct semaphore sem; /* semaphore to synchronize calls to
409 * its driver.
410 */
411
412 struct bus_type * bus; /* type of bus device is on */
413 struct device_driver *driver; /* which driver has allocated this
414 device */
415 void *driver_data; /* data private to the driver */
416 void *platform_data; /* Platform specific data, device
417 core doesn't touch it */
418 struct dev_pm_info power;
420 #ifdef CONFIG_NUMA
421 int numa_node; /* NUMA node this device is close to */
422 #endif
423 u64 *dma_mask; /* dma mask (if dma'able device) */
424 u64 coherent_dma_mask;/* Like dma_mask, but for
425 alloc_coherent mappings as
426 not all hardware supports
427 64 bit addresses for consistent
428 allocations such descriptors. */
429
430 struct list_head dma_pools; /* dma pools (if dma'ble) */
431
432 struct dma_coherent_mem *dma_mem; /* internal for coherent mem
433 override */
434 /* arch specific additions */
435 struct dev_archdata archdata;
436
437 spinlock_t devres_lock;
438 struct list_head devres_head;
439
440 /* class_device migration path */
441 struct list_head node;
442 struct class *class;
443 dev_t devt; /* dev_t, creates the sysfs "dev" */
444 struct attribute_group **groups; /* optional groups */
445 int uevent_suppress;
446
447 void (*release)(struct device * dev);
448 };
120 struct device_driver {
121 const char * name;
122 struct bus_type * bus;
123
124 struct completion unloaded;
125 struct kobject kobj;
126 struct klist klist_devices;
127 struct klist_node knode_bus;
128
129 struct module * owner;
130 const char * mod_name; /* used for built-in modules */
131 struct module_kobject * mkobj;
132
133 int (*probe) (struct device * dev);
134 int (*remove) (struct device * dev);
135 void (*shutdown) (struct device * dev);
136 int (*suspend) (struct device * dev, pm_message_t state);
137 int (*resume) (struct device * dev);
138
139 unsigned int multithread_probe:1;
140 };
注册驱动程序：
driver_register(&i2c_adapter_driver);
i2c_adapter_driver内嵌了一个kobject，也就是说它将以一个kobject的模型挂接在系统的模型树里，在sysfs中，作为一个目录节点。
kobject_set_name(i2c_adapter_driver.kobject, i2c_adatpter_driver.name)
i2c_adapter_driver.kobject.kset = i2c_adapter_driver.bus->sub_sys.kset