Linux设备模型――设备驱动模型和sysfs文件系统解读

转载自 http://blog.csdn.net/yj4231/article/details/7799245 作者：yj4231本文将对Linux系统中的sysfs进行简单的分析，要分析sysfs就必须分析内核的driver-model（驱动模型），两者是紧密联系的。在分析过程中，本文将以platform总线和spi主控制器的platform驱动为例来进行讲解。其实，platform机制是基于driver-model的，通过本文，也会对platform机制有个简单的了解。内核版本：2.6.30

1. What is sysfs？

个人理解：sysfs向用户空间展示了驱动设备的层次结构。我们都知道设备和对应的驱动都是由内核管理的，这些对于用户空间是不可见的。现在通过sysfs，可以在用户空间直观的了解设备驱动的层次结构。我们来看看sysfs的文件结构：[root@yj423 /sys]#lsblock class devices fs modulebus dev firmware kernel powerblock：块设备bus：系统中的总线class： 设备类型，比如输入设备dev：系统中已注册的设备节点的视图,有两个子目录char和block。devices：系统中所有设备拓扑结构视图fireware：固件fs：文件系统kernel：内核配置选项和状态信息module：模块power：系统的电源管理数据

2. kobject ,kset和ktype

要分析sysfs，首先就要分析kobject和kset，因为驱动设备的层次结构的构成就是由这两个东东来完成的。

2.1 kobject

kobject是一个对象的抽象，它用于管理对象。每个kobject对应着sysfs中的一个目录。kobject用struct kobject来描述。

struct kobject {
const char        *name;            /*在sysfs建立目录的名字*/
struct list_head    entry;        /*用于连接到所属kset的链表中*/
struct kobject        *parent;    /*父对象*/
struct kset        *kset;            /*属于哪个kset*/
struct kobj_type    *ktype;        /*类型*/
struct sysfs_dirent    *sd;        /*sysfs中与该对象对应的文件节点*/
struct kref        kref;            /*对象的应用计数*/
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;
};

2.2 kset

kset是一些kobject的集合，这些kobject可以有相同的ktype，也可以不同。同时，kset自己也包含一个kobject。在sysfs中，kset也是对应这一个目录，但是目录下面包含着其他的kojbect。kset使用struct kset来描述。

/**
* 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;		/*属于该kset的kobject链表*/
spinlock_t list_lock;
struct kobject kobj;	/*该kset内嵌的kobj*/

struct kset_uevent_ops *uevent_ops;
};

2.3 ktype

每个kobject对象都内嵌有一个ktype，该结构定义了kobject在创建和删除时所采取的行为。

struct kobj_type {
void (*release)(struct kobject *kobj);
struct sysfs_ops *sysfs_ops;
struct attribute **default_attrs;
};

struct sysfs_ops {
ssize_t    (*show)(struct kobject *, struct attribute *,char *);
ssize_t    (*store)(struct kobject *,struct attribute *,const char *, size_t);
};

/* FIXME
* The *owner field is no longer used.
* x86 tree has been cleaned up. The owner
* attribute is still left for other arches.
*/
struct attribute {
const char        *name;
struct module        *owner;
mode_t            mode;
};

当kobject的引用计数为0时，通过release方法来释放相关的资源。attribute为属性，每个属性在sysfs中都有对应的属性文件。sysfs_op的两个方法用于实现读取和写入属性文件时应该采取的行为。

2.4 kobject与kset的关系

下面这张图非常经典。最下面的kobj都属于一个kset，同时这些kobj的父对象就是kset内嵌的kobj。通过链表，kset可以获取所有属于它的kobj。从sysfs角度而言，kset代表一个文件夹，而下面的kobj就是这个文件夹里面的内容，而内容有可能是文件也有可能是文件夹。

3.举例

在上一节中，我们知道sys下有一个bus目录，这一将分析如何通过kobject创建bus目录。下面代码位于drivers/base/bus.c

int __init buses_init(void)
{
bus_kset = kset_create_and_add("bus", &bus_uevent_ops, NULL);
if (!bus_kset)
return -ENOMEM;
return 0;
}

static struct kset_uevent_ops bus_uevent_ops = {
.filter = bus_uevent_filter,
};

static int bus_uevent_filter(struct kset *kset, struct kobject *kobj)
{
struct kobj_type *ktype = get_ktype(kobj);

if (ktype == &bus_ktype)
return 1;
return 0;
}

这里直接调用kset_create_and_add，第一个参数为要创建的目录的名字，而第三个参数表示没有父对象。下面代码位于drivers/base/kobject.c

/**
* 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,
struct kset_uevent_ops *uevent_ops,
struct kobject *parent_kobj)
{
struct kset *kset;
int error;

kset = kset_create(name, uevent_ops, parent_kobj);	/*建立kset，设置某些字段*/
if (!kset)
return NULL;
error = kset_register(kset);	/*添加kset到sysfs*/
if (error) {
kfree(kset);
return NULL;
}
return kset;
}

这里主要调用了两个函数，接下分别来看下。

3.1 kset_create函数

下面代码位于drivers/base/kobject.c

/**
* 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,
struct kset_uevent_ops *uevent_ops,
struct kobject *parent_kobj)
{
struct kset *kset;

kset = kzalloc(sizeof(*kset), GFP_KERNEL);/*分配kset*/
if (!kset)
return NULL;
kobject_set_name(&kset->kobj, name);/*设置kobj->name*/
kset->uevent_ops = uevent_ops;
kset->kobj.parent = parent_kobj;	/*设置父对象*/

/*
* 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;			/*本keset不属于任何kset*/

return kset;
}

这个函数中，动态分配了kset结构，调用kobject_set_name设置kset->kobj->name为bus，也就是我们要创建的目录bus。同时这里kset->kobj.parent为NULL，也就是没有父对象。因为要创建的bus目录是在sysfs所在的根目录创建的，自然没有父对象。随后简要看下由kobject_set_name函数调用引发的一系列调用。

/**
* 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_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 *old_name = kobj->name;
char *s;

if (kobj->name && !fmt)
return 0;

kobj->name = kvasprintf(GFP_KERNEL, fmt, vargs);
if (!kobj->name)
return -ENOMEM;

/* ewww... some of these buggers have '/' in the name ... */
while ((s = strchr(kobj->name, '/')))
s[0] = '!';

kfree(old_name);
return 0;
}

/* Simplified asprintf. */
char *kvasprintf(gfp_t gfp, const char *fmt, va_list ap)
{
unsigned int len;
char *p;
va_list aq;

va_copy(aq, ap);
len = vsnprintf(NULL, 0, fmt, aq);
va_end(aq);

p = kmalloc(len+1, gfp);
if (!p)
return NULL;

vsnprintf(p, len+1, fmt, ap);

return p;
}

3.2 kset_register

下面代码位于drivers/base/kobject.c。

/**
* kset_register - initialize and add a kset.
* @k: kset.
*/
int kset_register(struct kset *k)
{
int err;

if (!k)
return -EINVAL;

kset_init(k);           /*初始化kset*/
err = kobject_add_internal(&k->kobj);  /*在sysfs中建立目录*/
if (err)
return err;
kobject_uevent(&k->kobj, KOBJ_ADD);
return 0;
}

这里面调用了3个函数。这里先介绍前两个函数。

3.2.1 kset_init

该函数用于初始化kset。下面代码位于drivers/base/kobject.c。

/**
* kset_init - initialize a kset for use
* @k: kset
*/
void kset_init(struct kset *k)
{
kobject_init_internal(&k->kobj);/*初始化kobject的某些字段*/
INIT_LIST_HEAD(&k->list);	/*初始化链表头*/
spin_lock_init(&k->list_lock);	/*初始化自旋锁*/
}

static void kobject_init_internal(struct kobject *kobj)
{
if (!kobj)
return;
kref_init(&kobj->kref);           /*初始化引用基计数*/
INIT_LIST_HEAD(&kobj->entry);    /*初始化链表头*/
kobj->state_in_sysfs = 0;
kobj->state_add_uevent_sent = 0;
kobj->state_remove_uevent_sent = 0;
kobj->state_initialized = 1;
}

3.2.2 kobject_add_internal

该函数将在sysfs中建立目录。下面代码位于drivers/base/kobject.c。

static int kobject_add_internal(struct kobject *kobj)
{
int error = 0;
struct kobject *parent;

if (!kobj)
return -ENOENT;
/*检查name字段是否存在*/
if (!kobj->name || !kobj->name[0]) {
WARN(1, "kobject: (%p): attempted to be registered with empty "
"name!\n", kobj);
return -EINVAL;
}

parent = kobject_get(kobj->parent);	/*有父对象则增加父对象引用计数*/

/* join kset if set, use it as parent if we do not already have one */
if (kobj->kset) {
if (!parent)
/*kobj属于某个kset，但是该kobj没有父对象，则以kset的kobj作为父对象*/
parent = kobject_get(&kobj->kset->kobj);
kobj_kset_join(kobj);		/*将kojbect添加到kset结构中的链表当中*/
kobj->parent = parent;
}

pr_debug("kobject: '%s' (%p): %s: parent: '%s', set: '%s'\n",
kobject_name(kobj), kobj, __func__,
parent ? kobject_name(parent) : "<NULL>",
kobj->kset ? kobject_name(&kobj->kset->kobj) : "<NULL>");

error = create_dir(kobj);	/*根据kobj->name在sys中建立目录*/
if (error) {
kobj_kset_leave(kobj);	/*删除链表项*/
kobject_put(parent);	/*减少引用计数*/
kobj->parent = NULL;

/* be noisy on error issues */
if (error == -EEXIST)
printk(KERN_ERR "%s failed for %s with "
"-EEXIST, don't try to register things with "
"the same name in the same directory.\n",
__func__, kobject_name(kobj));
else
printk(KERN_ERR "%s failed for %s (%d)\n",
__func__, kobject_name(kobj), error);
dump_stack();
} else
kobj->state_in_sysfs = 1;

return error;
}

在上面的kset_create中有kset->kobj.kset = NULL，因此if (kobj->kset)条件不满足。因此在这个函数中，对name进行了必要的检查之后，调用了create_dir在sysfs中创建目录。在create_dir执行完成以后会在sysfs的根目录（/sys/）建立文件夹bus。该函数的详细分析将在后面给出。至此，对bus目录的建立有了简单而直观的了解。我们可以看出kset其实就是表示一个文件夹，而kset本身也含有一个kobject，而该kobject的name字段即为该目录的名字，本例中为bus。

4. driver model

第2节所介绍的是最底层，最核心的内容。下面开始将描述较为高层的内容。Linux设备模型使用了三个数据结构分别来描述总线、设备和驱动。所有的设备和对应的驱动都必须挂载在某一个总线上，通过总线，可以绑定设备和驱动。这个属于分离的思想，将设备和驱动分开管理。同时驱动程序可以了解到所有它所支持的设备，同样的，设备也能知道它对应驱动程序。

4.1 bus

总线是处理器与一个设备或者多个设备之间的通道。在设备模型中，所有的设备都挂载在某一个总线上。总线使用struct bus_type来表述。下列代码位于include/linux/device.h。

struct bus_type {
const char        *name;
struct bus_attribute    *bus_attrs;
struct device_attribute    *dev_attrs;
struct driver_attribute    *drv_attrs;

int (*match)(struct device *dev, struct device_driver *drv);
int (*uevent)(struct device *dev, struct kobj_uevent_env *env);
int (*probe)(struct device *dev);
int (*remove)(struct device *dev);
void (*shutdown)(struct device *dev);

int (*suspend)(struct device *dev, pm_message_t state);
int (*suspend_late)(struct device *dev, pm_message_t state);
int (*resume_early)(struct device *dev);
int (*resume)(struct device *dev);

struct dev_pm_ops *pm;

struct bus_type_private *p;
};

/**
* struct bus_type_private - structure to hold the private to the driver core portions of the bus_type structure.
*
* @subsys - the struct kset that defines this bus.  This is the main kobject
* @drivers_kset - the list of drivers associated with this bus
* @devices_kset - the list of devices associated with this bus
* @klist_devices - the klist to iterate over the @devices_kset
* @klist_drivers - the klist to iterate over the @drivers_kset
* @bus_notifier - the bus notifier list for anything that cares about things
* on this bus.
* @bus - pointer back to the struct bus_type that this structure is associated
* with.
*
* This structure is the one that is the actual kobject allowing struct
* bus_type to be statically allocated safely.  Nothing outside of the driver
* core should ever touch these fields.
*/
struct bus_type_private {
struct kset subsys;
struct kset *drivers_kset;
struct kset *devices_kset;
struct klist klist_devices;
struct klist klist_drivers;
struct blocking_notifier_head bus_notifier;
unsigned int drivers_autoprobe:1;
struct bus_type *bus;
}；

我们看到每个bus_type都包含一个kset对象subsys，该kset在/sys/bus/目录下有着对应的一个目录，目录名即为字段name。后面我们将看到platform总线的建立。drivers_kset和devices_kset对应着两个目录，该两个目录下将包含该总线上的设备和相应的驱动程序。同时总线上的设备和驱动将分别保存在两个链表中：klist_devices和klist_drivers。

4.2 device

设备对象在driver-model中使用struct device来表示。下列代码位于include/linux/device.h。

struct device {
struct device		*parent;

struct device_private	*p;

struct kobject kobj;
const char		*init_name; /* initial name of the device */
struct device_type	*type;

struct semaphore	sem;	/* semaphore to synchronize calls to
* its driver.
*/

struct bus_type	*bus;		/* type of bus device is on */
struct device_driver *driver;	/* which driver has allocated this
device */
void		*driver_data;	/* data private to the driver */
void		*platform_data;	/* Platform specific data, device
core doesn't touch it */
struct dev_pm_info	power;

#ifdef CONFIG_NUMA
int		numa_node;	/* NUMA node this device is close to */
#endif
u64		*dma_mask;	/* dma mask (if dma'able device) */
u64		coherent_dma_mask;/* Like dma_mask, but for
alloc_coherent mappings as
not all hardware supports
64 bit addresses for consistent
allocations such descriptors. */

struct device_dma_parameters *dma_parms;

struct list_head	dma_pools;	/* dma pools (if dma'ble) */

struct dma_coherent_mem	*dma_mem; /* internal for coherent mem
override */
/* arch specific additions */
struct dev_archdata	archdata;

dev_t			devt;	/* dev_t, creates the sysfs "dev" */

spinlock_t		devres_lock;
struct list_head	devres_head;

struct klist_node	knode_class;
struct class		*class;
struct attribute_group	**groups;	/* optional groups */

void	(*release)(struct device *dev);
};

/**
* struct device_private - structure to hold the private to the driver core portions of the device structure.
*
* @klist_children - klist containing all children of this device
* @knode_parent - node in sibling list
* @knode_driver - node in driver list
* @knode_bus - node in bus list
* @device - pointer back to the struct class that this structure is
* associated with.
*
* Nothing outside of the driver core should ever touch these fields.
*/
struct device_private {
struct klist klist_children;
struct klist_node knode_parent;
struct klist_node knode_driver;
struct klist_node knode_bus;
struct device *device;
};

device本身包含一个kobject，也就是说这个device在sysfs的某个地方有着一个对应的目录。该device所挂载的bus由knode_bus指定。该device所对应的设备驱动由knode_driver指定。

4.3 driver

设备设备对象在driver-model中使用struct device_driver来表示。下列代码位于include/linux/device.h。

struct device_driver {
const char		*name;
struct bus_type		*bus;

struct module		*owner;
const char 		*mod_name;	/* used for built-in modules */

int (*probe) (struct device *dev);
int (*remove) (struct device *dev);
void (*shutdown) (struct device *dev);
int (*suspend) (struct device *dev, pm_message_t state);
int (*resume) (struct device *dev);
struct attribute_group **groups;

struct dev_pm_ops *pm;

struct driver_private *p;
};

struct driver_private {
struct kobject kobj;
struct klist klist_devices;
struct klist_node knode_bus;
struct module_kobject *mkobj;
struct device_driver *driver;
};

device_driver本身包含一个kobject，也就是说这个device_driver在sysfs的某个地方有着一个对应的目录。该设备驱动所支持的设备由klist_devices指定。该设备驱动所挂载的总线由knode_bus制定。

5. Bus举例

本节我们将以platform总线为例，来看看，/sys/bus/platform是如何建立的。platform总线的注册是由platform_bus_init函数完成的。该函数在内核启动阶段被调用，我们来简单看下调用过程：start_kernel() -> rest_init() ->kernel_init() -> do_basic_setup() -> driver_init() -> platform_bus_init()。注：kernel_init()是在rest_init函数中创建内核线程来执行的。

int __init platform_bus_init(void)
{
int error;

early_platform_cleanup();

error = device_register(&platform_bus);
if (error)
return error;
error =  bus_register(&platform_bus_type);
if (error)
device_unregister(&platform_bus);
return error;
}
struct bus_type platform_bus_type = {
.name		= "platform",
.dev_attrs	= platform_dev_attrs,
.match		= platform_match,
.uevent		= platform_uevent,
.pm		= PLATFORM_PM_OPS_PTR,
};
EXPORT_SYMBOL_GPL(platform_bus_type);

从bus_type，我们看到该总线的名字为platform。调用了两个函数，我们只关注bus_register函数。

/**
* bus_register - register a bus with the system.
* @bus: bus.
*
* Once we have that, we registered the bus with the kobject
* infrastructure, then register the children subsystems it has:
* the devices and drivers that belong to the bus.
*/
int bus_register(struct bus_type *bus)
{
int retval;
struct bus_type_private *priv;

priv = kzalloc(sizeof(struct bus_type_private), GFP_KERNEL);
if (!priv)
return -ENOMEM;
/*互相保存*/
priv->bus = bus;
bus->p = priv;

BLOCKING_INIT_NOTIFIER_HEAD(&priv->bus_notifier);
/*设定kobject->name*/
retval = kobject_set_name(&priv->subsys.kobj, "%s", bus->name);
if (retval)
goto out;

priv->subsys.kobj.kset = bus_kset;
priv->subsys.kobj.ktype = &bus_ktype;
priv->drivers_autoprobe = 1;

/*注册kset,在bus/建立目录XXX，XXX为bus->name*/
retval = kset_register(&priv->subsys);
if (retval)
goto out;

/*创建属性，在bus/XXX/建立文件uevent*/
retval = bus_create_file(bus, &bus_attr_uevent);
if (retval)
goto bus_uevent_fail;

/*创建kset，在bus/XXX/建立目录devices*/
priv->devices_kset = kset_create_and_add("devices", NULL,
&priv->subsys.kobj);
if (!priv->devices_kset) {
retval = -ENOMEM;
goto bus_devices_fail;
}

/*创建kset,在bus/XXX/建立目录drivers*/
priv->drivers_kset = kset_create_and_add("drivers", NULL,
&priv->subsys.kobj);
if (!priv->drivers_kset) {
retval = -ENOMEM;
goto bus_drivers_fail;
}
/*初始化2个内核链表,*/
klist_init(&priv->klist_devices, klist_devices_get, klist_devices_put);
klist_init(&priv->klist_drivers, NULL, NULL);

/*创建属性,在bus/XXX/建立文件drivers_autoprobe和drivers_probe*/
retval = add_probe_files(bus);
if (retval)
goto bus_probe_files_fail;
/*根据bus->bus_attribute创建属性，在bus/XXX/下建立相应的文件d*/
retval = bus_add_attrs(bus);
if (retval)
goto bus_attrs_fail;

pr_debug("bus: '%s': registered\n", bus->name);
return 0;

bus_attrs_fail:
remove_probe_files(bus);
bus_probe_files_fail:
kset_unregister(bus->p->drivers_kset);
bus_drivers_fail:
kset_unregister(bus->p->devices_kset);
bus_devices_fail:
bus_remove_file(bus, &bus_attr_uevent);
bus_uevent_fail:
kset_unregister(&bus->p->subsys);
kfree(bus->p);
out:
bus->p = NULL;
return retval;
}
EXPORT_SYMBOL_GPL(bus_register);

函数中，首先调用kobject_set_name设置了bus对象的subsys.kobject->name 为 platform，也就是说会建立一个名为platform的目录。kobject_set_name函数在3.1小节中已经给出。在这里还用到了bus_kset这个变量，这个变量就是在第3节buses_init函数中建立bus目录所对应的kset对象。接着，priv->subsys.kobj.kset = bus_kset，设置subsys的kobj在bus_kset对象包含的集合中，也就是说bus目录下将包含subsys对象所对应的目录，即platform。紧接着调用了kset_register，参数为&priv->subsys。该函数在3.2节中以给出。在该函数的调用过程中，将调用kobj_kset_join函数，该函数将kobject添加到kobject->kset的链表中。

/* add the kobject to its kset's list */
static void kobj_kset_join(struct kobject *kobj)
{
if (!kobj->kset)
return;

kset_get(kobj->kset);	/*增加kset引用计数*/
spin_lock(&kobj->kset->list_lock);
list_add_tail(&kobj->entry, &kobj->kset->list);	/*将kojbect添加到kset结构中的链表当中*/
spin_unlock(&kobj->kset->list_lock);
}

kset_register函数执行完成后，将在/sys/bus/下建立目录platform。此刻，我们先来看下kset和kobject之间的关系。然后，调用了bus_create_file函数在/sys/bus/platform/下建立文件uevent。

int bus_create_file(struct bus_type *bus, struct bus_attribute *attr)
{
int error;
if (bus_get(bus)) {
error = sysfs_create_file(&bus->p->subsys.kobj, &attr->attr);
bus_put(bus);
} else
error = -EINVAL;
return error;
}
EXPORT_SYMBOL_GPL(bus_create_file);

有关底层的sysfs将在后面叙述，这里只要关注参数&bus->p->subsys.kobj，表示在该kset下建立文件，也就是platform下建立。接着调用了2次kset_create_and_add，分别在/sys/bus/platform/下建立了文件夹devices和drivers。该函数位于第3节开始处。这里和第3节调用kset_create_and_add时的最主要一个区别就是：此时的parent参数不为NULL，而是&priv->subsys.kobj。也就是说，将要创建的kset的kobject->parent = &priv->subsys.kobj，也即新建的kset被包含在platform文件夹对应的kset中。我们来看下关系图：随后，调用了add_probe_files创建了属性文件drivers_autoprobe和drivers_probe。

static int add_probe_files(struct bus_type *bus)
{
int retval;

retval = bus_create_file(bus, &bus_attr_drivers_probe);
if (retval)
goto out;

retval = bus_create_file(bus, &bus_attr_drivers_autoprobe);
if (retval)
bus_remove_file(bus, &bus_attr_drivers_probe);
out:
return retval;
}

该函数只是简单的调用了两次bus_create_file，该函数已在前面叙述过。最后调用bus_add_attrs创建总线相关的属性文件。

/**
* bus_add_attrs - Add default attributes for this bus.
* @bus: Bus that has just been registered.
*/

static int bus_add_attrs(struct bus_type *bus)
{
int error = 0;
int i;

if (bus->bus_attrs) {
for (i = 0; attr_name(bus->bus_attrs[i]); i++) {
error = bus_create_file(bus, &bus->bus_attrs[i]);
if (error)
goto err;
}
}
done:
return error;
err:
while (--i >= 0)
bus_remove_file(bus, &bus->bus_attrs[i]);
goto done;
}

我们可以看到这个函数将根据bus_type->bus_arrts来创建属性文件。不过，在本例中，bus_arrts从未给出定义，因此次函数不做任何工作。好了，整个bus_register调用完成了，我们来看下sysfs中实际的情况。[root@yj423 platform]#pwd/sys/bus/platform[root@yj423 platform]#lsdevices drivers drivers_autoprobe drivers_probe uevent最后，我们对整个bus_register的过程进行一个小结。

6. device举例

本节将首先讲述如何在/sys/devices下建立虚拟的platform设备，然后再讲述如何在/sys/devices/platform/下建立子设备。

6.1 虚拟的platform设备

之所以叫虚拟是因为这个platform并不代表任何实际存在的设备，但是platform将是所有具体设备的父设备。在第5节，platform_bus_init函数中还调用了device_register，现在对其做出分析。

int __init platform_bus_init(void)
{
int error;

early_platform_cleanup();

error = device_register(&platform_bus);
if (error)
return error;
error =  bus_register(&platform_bus_type);
if (error)
device_unregister(&platform_bus);
return error;
}

struct device platform_bus = {
.init_name    = "platform",
};
EXPORT_SYMBOL_GPL(platform_bus)

下列函数位于drivers/base/core.c。

/**
* device_register - register a device with the system.
* @dev: pointer to the device structure
*
* This happens in two clean steps - initialize the device
* and add it to the system. The two steps can be called
* separately, but this is the easiest and most common.
* I.e. you should only call the two helpers separately if
* have a clearly defined need to use and refcount the device
* before it is added to the hierarchy.
*
* NOTE: _Never_ directly free @dev after calling this function, even
* if it returned an error! Always use put_device() to give up the
* reference initialized in this function instead.
*/
int device_register(struct device *dev)
{
device_initialize(dev);	/*初始化dev的某些字段*/
return device_add(dev); /*将设备添加到系统中*/
}

一个设备的注册分成两部，每步通过调用一个函数函数。首先先看第一步：下列函数位于drivers/base/core.c。

/**
* device_initialize - init device structure.
* @dev: device.
*
* This prepares the device for use by other layers by initializing
* its fields.
* It is the first half of device_register(), if called by
* that function, though it can also be called separately, so one
* may use @dev's fields. In particular, get_device()/put_device()
* may be used for reference counting of @dev after calling this
* function.
*
* NOTE: Use put_device() to give up your reference instead of freeing
* @dev directly once you have called this function.
*/
void device_initialize(struct device *dev)
{
dev->kobj.kset = devices_kset;        /*设置kobj属于哪个kset，/sys/devices/*/
kobject_init(&dev->kobj, &device_ktype);/*初始化dev->kobj*/
INIT_LIST_HEAD(&dev->dma_pools);    /*初始化链表头*/
init_MUTEX(&dev->sem);                /*初始化互斥体*/
spin_lock_init(&dev->devres_lock);    /*初始化自旋锁*/
INIT_LIST_HEAD(&dev->devres_head);    /*初始化链表头*/
device_init_wakeup(dev, 0);            /*设置该device不能唤醒*/
device_pm_init(dev);                /*设置该device可操作*/
set_dev_node(dev, -1);                /*设置NUMA节点*/
}

6.1.1 有关devices_kset

首先其中用到了devices_kset对象，这个对象和第3节当中的bus_kset是同样的性质，也就是说该对象表示一个目录。该对象的建立是在devices_init函数中完成的。

int __init devices_init(void)
{
devices_kset = kset_create_and_add("devices", &device_uevent_ops, NULL);
if (!devices_kset)
return -ENOMEM;
dev_kobj = kobject_create_and_add("dev", NULL);
if (!dev_kobj)
goto dev_kobj_err;
sysfs_dev_block_kobj = kobject_create_and_add("block", dev_kobj);
if (!sysfs_dev_block_kobj)
goto block_kobj_err;
sysfs_dev_char_kobj = kobject_create_and_add("char", dev_kobj);
if (!sysfs_dev_char_kobj)
goto char_kobj_err;

return 0;

char_kobj_err:
kobject_put(sysfs_dev_block_kobj);
block_kobj_err:
kobject_put(dev_kobj);
dev_kobj_err:
kset_unregister(devices_kset);
return -ENOMEM;
}

由此可见，devices_kset对象表示的目录为/sys下的devices目录。

6.1.2 kobject_init

下列函数位于lib/kojbect.c。

/**
* 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!\n";
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.\n", kobj);
dump_stack();
}

kobject_init_internal(kobj);
kobj->ktype = ktype;
return;

error:
printk(KERN_ERR "kobject (%p): %s\n", kobj, err_str);
dump_stack();
}
EXPORT_SYMBOL(kobject_init);

static void kobject_init_internal(struct kobject *kobj)
{
if (!kobj)
return;
kref_init(&kobj->kref);            /*初始化引用基计数*/
INIT_LIST_HEAD(&kobj->entry);    /*初始化链表头*/
kobj->state_in_sysfs = 0;
kobj->state_add_uevent_sent = 0;
kobj->state_remove_uevent_sent = 0;
kobj->state_initialized = 1;
}

该函数在做了一系列的必要检查后，调用kobject_init_internal初始化了kobject的某些字段。

6.1.3 device_init_wakeup

参数val为0，设置该device不能够唤醒。

#ifdef CONFIG_PM

/* changes to device_may_wakeup take effect on the next pm state change.
* by default, devices should wakeup if they can.
*/
static inline void device_init_wakeup(struct device *dev, int val)
{
dev->power.can_wakeup = dev->power.should_wakeup = !!val;
}
。。。。。。
#else /* !CONFIG_PM */

/* For some reason the next two routines work even without CONFIG_PM */
static inline void device_init_wakeup(struct device *dev, int val)
{
dev->power.can_wakeup = !!val;
}
。。。。。。
#endif

6.1.4 device_pm_init

设置电源的状态。

static inline void device_pm_init(struct device *dev)
{
dev->power.status = DPM_ON;    /*该device被认为可操作*/
}

6.1.5 set_dev_node

如果使用NUMA，则设置NUMA节点。

#ifdef CONFIG_NUMA
。。。。。。
static inline void set_dev_node(struct device *dev, int node)
{
dev->numa_node = node;
}
#else
。。。。。。
static inline void set_dev_node(struct device *dev, int node)
{
}
#endif

6.2 device_add

接下来是注册的第二步：调用device_add。

/**
* device_add - add device to device hierarchy.
* @dev: device.
*
* This is part 2 of device_register(), though may be called
* separately _iff_ device_initialize() has been called separately.
*
* This adds @dev to the kobject hierarchy via kobject_add(), adds it
* to the global and sibling lists for the device, then
* adds it to the other relevant subsystems of the driver model.
*
* NOTE: _Never_ directly free @dev after calling this function, even
* if it returned an error! Always use put_device() to give up your
* reference instead.
*/
int device_add(struct device *dev)
{
struct device *parent = NULL;
struct class_interface *class_intf;
int error = -EINVAL;

dev = get_device(dev);	/*增加引用计数*/
if (!dev)
goto done;

dev->p = kzalloc(sizeof(*dev->p), GFP_KERNEL);	/*分配device_private结构*/
if (!dev->p) {
error = -ENOMEM;
goto done;
}
dev->p->device = dev;	/*保存dev*/
klist_init(&dev->p->klist_children, klist_children_get,	/*初始化内核链表*/
klist_children_put);

/*
* for statically allocated devices, which should all be converted
* some day, we need to initialize the name. We prevent reading back
* the name, and force the use of dev_name()
*/
if (dev->init_name) {
dev_set_name(dev, dev->init_name); 	/*dev->kobject->name = dev->init_name*/
dev->init_name = NULL;
}

if (!dev_name(dev))	/*检查dev->kobject->name*/
goto name_error;

pr_debug("device: '%s': %s\n", dev_name(dev), __func__);

parent = get_device(dev->parent);	/*增加父设备引用计数*/
setup_parent(dev, parent);			/*设置dev->kobject->parent*/

/* use parent numa_node */
if (parent)
set_dev_node(dev, dev_to_node(parent));

/* first, register with generic layer. */
/* we require the name to be set before, and pass NULL */
/* 执行完以后，将在/sys/devices/下建立目录XXX，目录名XXX为dev->kobj->name*/
error = kobject_add(&dev->kobj, dev->kobj.parent, NULL);
if (error)
goto Error;

/* notify platform of device entry */
if (platform_notify)
platform_notify(dev);

/*在XXX下建立文件uevent*/
error = device_create_file(dev, &uevent_attr);
if (error)
goto attrError;

if (MAJOR(dev->devt)) {/*主设备号不为0*/
error = device_create_file(dev, &devt_attr);/*创建属性文件dev*/
if (error)
goto ueventattrError;

/* 在sys/dev/char/下建立symlink，名字为主设备号:次设备号，该链接指向XXX */
error = device_create_sys_dev_entry(dev);
if (error)
goto devtattrError;
}

error = device_add_class_symlinks(dev);
if (error)
goto SymlinkError;
error = device_add_attrs(dev);	/*添加类设备属型文件和属性组*/
if (error)
goto AttrsError;
error = bus_add_device(dev);	/*添加3个symlink*/
if (error)
goto BusError;
error = dpm_sysfs_add(dev);		/*创建power子目录，并在其下添加电源管理的属性组文件*/
if (error)
goto DPMError;
device_pm_add(dev);				/*将该device添加到电源管理链表中*/

/* Notify clients of device addition.  This call must come
* after dpm_sysf_add() and before kobject_uevent().
*/
if (dev->bus)
blocking_notifier_call_chain(&dev->bus->p->bus_notifier,
BUS_NOTIFY_ADD_DEVICE, dev);

kobject_uevent(&dev->kobj, KOBJ_ADD);	/*通知用户层*/
bus_attach_device(dev);					/*将设备添加到总线的设备链表中，并尝试获取驱动*/
if (parent)
klist_add_tail(&dev->p->knode_parent,	/*有父设备，则将该设备添加到父设备的儿子链表中*/
&parent->p->klist_children);

if (dev->class) {						/*该设备属于某个设备类*/
mutex_lock(&dev->class->p->class_mutex);
/* tie the class to the device */
klist_add_tail(&dev->knode_class,	/*将device添加到class的类设备链表中*/
&dev->class->p->class_devices);

/* notify any interfaces that the device is here */
list_for_each_entry(class_intf,
&dev->class->p->class_interfaces, node)
if (class_intf->add_dev)
class_intf->add_dev(dev, class_intf);
mutex_unlock(&dev->class->p->class_mutex);
}
done:
put_device(dev);
return error;
DPMError:
bus_remove_device(dev);
BusError:
device_remove_attrs(dev);
AttrsError:
device_remove_class_symlinks(dev);
SymlinkError:
if (MAJOR(dev->devt))
device_remove_sys_dev_entry(dev);
devtattrError:
if (MAJOR(dev->devt))
device_remove_file(dev, &devt_attr);
ueventattrError:
device_remove_file(dev, &uevent_attr);
attrError:
kobject_uevent(&dev->kobj, KOBJ_REMOVE);
kobject_del(&dev->kobj);
Error:
cleanup_device_parent(dev);
if (parent)
put_device(parent);
name_error:
kfree(dev->p);
dev->p = NULL;
goto done;
}

该函数调用了非常多的其他函数，接下来对主要的函数做出分析。

6.2.1 setup_parent函数

下列代码位于drivers/base/core.c。

static void setup_parent(struct device *dev, struct device *parent)
{
struct kobject *kobj;
kobj = get_device_parent(dev, parent);
if (kobj)
dev->kobj.parent = kobj;
}

static struct kobject *get_device_parent(struct device *dev,
struct device *parent)
{
/* class devices without a parent live in /sys/class/<classname>/ */
if (dev->class && (!parent || parent->class != dev->class))
return &dev->class->p->class_subsys.kobj;
/* all other devices keep their parent */
else if (parent)
return &parent->kobj;

return NULL;
}

该函数将设置dev对象的parent。在这里实际传入的parent为NULL，同时dev->class也没有定义过。因此这个函数什么都没有做。

6.2.2 kobject_add函数

下列代码位于lib/kobject.c。

/**
* 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 associted 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.\n",
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;
}
EXPORT_SYMBOL(kobject_add);

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!\n");
return retval;
}
kobj->parent = parent;
return kobject_add_internal(kobj);
}

static int kobject_add_internal(struct kobject *kobj)
{
int error = 0;
struct kobject *parent;

if (!kobj)
return -ENOENT;
/*检查name字段是否存在*/
if (!kobj->name || !kobj->name[0]) {
WARN(1, "kobject: (%p): attempted to be registered with empty "
"name!\n", kobj);
return -EINVAL;
}

parent = kobject_get(kobj->parent);    /*有父对象则增加父对象引用计数*/

/* join kset if set, use it as parent if we do not already have one */
if (kobj->kset) {
if (!parent)
/*kobj属于某个kset，但是该kobj没有父对象，则以kset的kobj作为父对象*/
parent = kobject_get(&kobj->kset->kobj);
kobj_kset_join(kobj);        /*将kojbect添加到kset结构中的链表当中*/
kobj->parent = parent;
}

pr_debug("kobject: '%s' (%p): %s: parent: '%s', set: '%s'\n",
kobject_name(kobj), kobj, __func__,
parent ? kobject_name(parent) : "<NULL>",
kobj->kset ? kobject_name(&kobj->kset->kobj) : "<NULL>");

error = create_dir(kobj);    /*根据kobj->name在sys中建立目录*/
if (error) {
kobj_kset_leave(kobj);    /*删除链表项*/
kobject_put(parent);    /*减少引用计数*/
kobj->parent = NULL;

/* be noisy on error issues */
if (error == -EEXIST)
printk(KERN_ERR "%s failed for %s with "
"-EEXIST, don't try to register things with "
"the same name in the same directory.\n",
__func__, kobject_name(kobj));
else
printk(KERN_ERR "%s failed for %s (%d)\n",
__func__, kobject_name(kobj), error);
dump_stack();
} else
kobj->state_in_sysfs = 1;

return error;
}

在调用时，参数parent为NULL，且dev->kobj.kset在6.1节device_initialize函数中设置为devices_kset。而devices_kset对应着/sys/devices目录，因此该函数调用完成后将在/sys/devices目录下生成目录platform。但是这里比较奇怪的是，为什么platform目录没有对应的kset对象？？？

6.2.3 device_create_sys_dev_entry函数

在调用该函数之前，会在/sys/devices/platform/下生成属性文件。接着如果该device的设备号不为0，则创建属性文件dev，并调用本函数。但是，在本例中设备号devt从未设置过，显然为0，那么本函数实际并未执行。下列代码位于drivers/base/core.c。

static int device_create_sys_dev_entry(struct device *dev)
{
struct kobject *kobj = device_to_dev_kobj(dev);
int error = 0;
char devt_str[15];

if (kobj) {
format_dev_t(devt_str, dev->devt);
error = sysfs_create_link(kobj, &dev->kobj, devt_str);
}

return error;
}
/**
* device_to_dev_kobj - select a /sys/dev/ directory for the device
* @dev: device
*
* By default we select char/ for new entries.  Setting class->dev_obj
* to NULL prevents an entry from being created.  class->dev_kobj must
* be set (or cleared) before any devices are registered to the class
* otherwise device_create_sys_dev_entry() and
* device_remove_sys_dev_entry() will disagree about the the presence
* of the link.
*/
static struct kobject *device_to_dev_kobj(struct device *dev)
{
struct kobject *kobj;

if (dev->class)
kobj = dev->class->dev_kobj;
else
kobj = sysfs_dev_char_kobj;

return kobj;
}

6.2.4 device_add_class_symlinks函数

由于dev->class为NULL，本函数其实没做任何工作。下列代码位于drivers/base/core.c。

static int device_add_class_symlinks(struct device *dev)
{
int error;

if (!dev->class)
return 0;

error = sysfs_create_link(&dev->kobj,
&dev->class->p->class_subsys.kobj,
"subsystem");
if (error)
goto out;

#ifdef CONFIG_SYSFS_DEPRECATED
/* stacked class devices need a symlink in the class directory */
if (dev->kobj.parent != &dev->class->p->class_subsys.kobj &&
device_is_not_partition(dev)) {
error = sysfs_create_link(&dev->class->p->class_subsys.kobj,
&dev->kobj, dev_name(dev));
if (error)
goto out_subsys;
}

if (dev->parent && device_is_not_partition(dev)) {
struct device *parent = dev->parent;
char *class_name;

/*
* stacked class devices have the 'device' link
* pointing to the bus device instead of the parent
*/
while (parent->class && !parent->bus && parent->parent)
parent = parent->parent;

error = sysfs_create_link(&dev->kobj,
&parent->kobj,
"device");
if (error)
goto out_busid;

class_name = make_class_name(dev->class->name,
&dev->kobj);
if (class_name)
error = sysfs_create_link(&dev->parent->kobj,
&dev->kobj, class_name);
kfree(class_name);
if (error)
goto out_device;
}
return 0;

out_device:
if (dev->parent && device_is_not_partition(dev))
sysfs_remove_link(&dev->kobj, "device");
out_busid:
if (dev->kobj.parent != &dev->class->p->class_subsys.kobj &&
device_is_not_partition(dev))
sysfs_remove_link(&dev->class->p->class_subsys.kobj,
dev_name(dev));
#else
/* link in the class directory pointing to the device */
error = sysfs_create_link(&dev->class->p->class_subsys.kobj,
&dev->kobj, dev_name(dev));
if (error)
goto out_subsys;

if (dev->parent && device_is_not_partition(dev)) {
error = sysfs_create_link(&dev->kobj, &dev->parent->kobj,
"device");
if (error)
goto out_busid;
}
return 0;

out_busid:
sysfs_remove_link(&dev->class->p->class_subsys.kobj, dev_name(dev));
#endif

out_subsys:
sysfs_remove_link(&dev->kobj, "subsystem");
out:
return error;
}

6.2.5 device_add_attrs函数

同样dev->class为空，什么都没干。下列代码位于drivers/base/core.c。

static int device_add_attrs(struct device *dev)
{
struct class *class = dev->class;
struct device_type *type = dev->type;
int error;

if (class) {
error = device_add_attributes(dev, class->dev_attrs);
if (error)
return error;
}

if (type) {
error = device_add_groups(dev, type->groups);
if (error)
goto err_remove_class_attrs;
}

error = device_add_groups(dev, dev->groups);
if (error)
goto err_remove_type_groups;

return 0;

err_remove_type_groups:
if (type)
device_remove_groups(dev, type->groups);
err_remove_class_attrs:
if (class)
device_remove_attributes(dev, class->dev_attrs);

return error;
}

6.2.6 bus_add_device函数

由于dev->bus未指定，因此这个函数什么都没干。该函数将创建三个symlink，在sysfs中建立总线和设备间的关系。下列代码位于drivers/base/bus.c。

/**
* bus_add_device - add device to bus
* @dev: device being added
*
* - Add the device to its bus's list of devices.
* - Create link to device's bus.
*/
int bus_add_device(struct device *dev)
{
struct bus_type *bus = bus_get(dev->bus);
int error = 0;

if (bus) {
pr_debug("bus: '%s': add device %s\n", bus->name, dev_name(dev));
error = device_add_attrs(bus, dev);
if (error)
goto out_put;

/*在sys/bus/XXX/devices下建立symlink,名字为设备名，该链接指向/sys/devices/下的某个目录*/
error = sysfs_create_link(&bus->p->devices_kset->kobj,
&dev->kobj, dev_name(dev));
if (error)
goto out_id;

/*在sys/devices/的某个目录下建立symlink,名字为subsystem，该链接指向/sys/bus/下的某个目录*/
error = sysfs_create_link(&dev->kobj,
&dev->bus->p->subsys.kobj, "subsystem");
if (error)
goto out_subsys;

/*在sys/devices/的某个目录下建立symlink,名字为bus，该链接指向/sys/bus/下的某个目录*/
error = make_deprecated_bus_links(dev);
if (error)
goto out_deprecated;
}
return 0;

out_deprecated:
sysfs_remove_link(&dev->kobj, "subsystem");
out_subsys:
sysfs_remove_link(&bus->p->devices_kset->kobj, dev_name(dev));
out_id:
device_remove_attrs(bus, dev);
out_put:
bus_put(dev->bus);
return error;
}

6.2.7 dpm_sysfs_add函数

下列代码位于drivers/base/power/sysfs.c。

int dpm_sysfs_add(struct device * dev)
{
return sysfs_create_group(&dev->kobj, &pm_attr_group);
}

static DEVICE_ATTR(wakeup, 0644, wake_show, wake_store);

static struct attribute * power_attrs[] = {
&dev_attr_wakeup.attr,
NULL,
};
static struct attribute_group pm_attr_group = {
.name    = "power",
.attrs    = power_attrs,
};

该函数将在XXX目录下建立power子目录，并在该子目录下建立属性文件wakeup。在本例中，将在/sys/bus/platform下建立子目录power并在子目录下建立wakeup文件。

6.2.8 device_pm_add函数

下列代码位于drivers/base/power/main.c。

/**
*	device_pm_add - add a device to the list of active devices
*	@dev:	Device to be added to the list
*/
void device_pm_add(struct device *dev)
{
pr_debug("PM: Adding info for %s:%s\n",
dev->bus ? dev->bus->name : "No Bus",
kobject_name(&dev->kobj));
mutex_lock(&dpm_list_mtx);
if (dev->parent) {
if (dev->parent->power.status >= DPM_SUSPENDING)
dev_warn(dev, "parent %s should not be sleeping\n",
dev_name(dev->parent));
} else if (transition_started) {
/*
* We refuse to register parentless devices while a PM
* transition is in progress in order to avoid leaving them
* unhandled down the road
*/
dev_WARN(dev, "Parentless device registered during a PM transaction\n");
}

list_add_tail(&dev->power.entry, &dpm_list); /*将该设备添加到链表中*/
mutex_unlock(&dpm_list_mtx);
}

该函数只是将设备添加到电源管理链表中。

6.2.9 bus_attach_device函数

在本例中，由于bus未指定，该函数实际不做任何工作。下列代码位于drivers/base/bus.c。

/**
* bus_attach_device - add device to bus
* @dev: device tried to attach to a driver
*
* - Add device to bus's list of devices.
* - Try to attach to driver.
*/
void bus_attach_device(struct device *dev)
{
struct bus_type *bus = dev->bus;
int ret = 0;

if (bus) {
if (bus->p->drivers_autoprobe)
ret = device_attach(dev);	/*尝试获取驱动*/
WARN_ON(ret < 0);
if (ret >= 0)		/*将设备挂在到总线中*/
klist_add_tail(&dev->p->knode_bus,
&bus->p->klist_devices);
}
}

/**
* device_attach - try to attach device to a driver.
* @dev: device.
*
* Walk the list of drivers that the bus has and call
* driver_probe_device() for each pair. If a compatible
* pair is found, break out and return.
*
* Returns 1 if the device was bound to a driver;
* 0 if no matching device was found;
* -ENODEV if the device is not registered.
*
* When called for a USB interface, @dev->parent->sem must be held.
*/
int device_attach(struct device *dev)
{
int ret = 0;

down(&dev->sem);
if (dev->driver) {    /*如果已指定驱动，即已绑定*/
ret = device_bind_driver(dev);    /*在sysfs中建立链接关系*/
if (ret == 0)
ret = 1;
else {
dev->driver = NULL;
ret = 0;
}
} else {        /*尚未绑定，尝试绑定，遍历该总线上的所有驱动*/
ret = bus_for_each_drv(dev->bus, NULL, dev, __device_attach);
}
up(&dev->sem);
return ret;
}
EXPORT_SYMBOL_GPL(device_attach);

如果bus存在的话，将会调用device_attach函数进行绑定工作。该函数首先判断dev->driver，如果非0，表示该设备已经绑定了驱动，只要在sysfs中建立链接关系即可。为0表示没有绑定，接着调用bus_for_each_drv，注意作为参数传入的__device_attach，这是个函数，后面会调用它。我们来看下bus_for_each_drv：

/**
* bus_for_each_drv - driver iterator
* @bus: bus we're dealing with.
* @start: driver to start iterating on.
* @data: data to pass to the callback.
* @fn: function to call for each driver.
*
* This is nearly identical to the device iterator above.
* We iterate over each driver that belongs to @bus, and call
* @fn for each. If @fn returns anything but 0, we break out
* and return it. If @start is not NULL, we use it as the head
* of the list.
*
* NOTE: we don't return the driver that returns a non-zero
* value, nor do we leave the reference count incremented for that
* driver. If the caller needs to know that info, it must set it
* in the callback. It must also be sure to increment the refcount
* so it doesn't disappear before returning to the caller.
*/
int bus_for_each_drv(struct bus_type *bus, struct device_driver *start,
void *data, int (*fn)(struct device_driver *, void *))
{
struct klist_iter i;
struct device_driver *drv;
int error = 0;

if (!bus)
return -EINVAL;

klist_iter_init_node(&bus->p->klist_drivers, &i,
start ? &start->p->knode_bus : NULL);
while ((drv = next_driver(&i)) && !error)
error = fn(drv, data);
klist_iter_exit(&i);
return error;
}
EXPORT_SYMBOL_GPL(bus_for_each_drv);

该函数将遍历总线的drivers目录下的所有驱动，也就是/sys/bus/XXX/drivers/下的目录，为该driver调用fn函数，也就是__device_attach。我们来看下：

static int __device_attach(struct device_driver *drv, void *data)
{
struct device *dev = data;

if (!driver_match_device(drv, dev))   /*进行匹配工作*/
return 0;

return driver_probe_device(drv, dev);
}

static inline int driver_match_device(struct device_driver *drv,
struct device *dev)
{
return drv->bus->match ? drv->bus->match(dev, drv) : 1;
}

/**
* driver_probe_device - attempt to bind device & driver together
* @drv: driver to bind a device to
* @dev: device to try to bind to the driver
*
* This function returns -ENODEV if the device is not registered,
* 1 if the device is bound sucessfully and 0 otherwise.
*
* This function must be called with @dev->sem held.  When called for a
* USB interface, @dev->parent->sem must be held as well.
*/
int driver_probe_device(struct device_driver *drv, struct device *dev)
{
int ret = 0;

if (!device_is_registered(dev))    /*该device是否已在sysfs中*/
return -ENODEV;

pr_debug("bus: '%s': %s: matched device %s with driver %s\n",
drv->bus->name, __func__, dev_name(dev), drv->name);

ret = really_probe(dev, drv);/*device已在sysfs，调用really_probe*/

return ret;
}

该函数首先调用driver_match_device函数，后者将会调用总线的match方法，如果有的话，来进行匹配工作。如果没有该方法，则返回1，表示匹配成功。我们这里是针对platform总线，该总线的方法将在7.6.2节中看到。随后，又调用了driver_probe_device函数。该函数将首先判断该device是否已在sysfs中，如果在则调用really_probe，否则返回出错。really_probe将会调用驱动的probe并完成绑定的工作。该函数将在7.6.2节中分析。

6.2.10 小结

在本例中，当device_register调用完成以后，将在/sys/devices/下建立目录platform，并在platfrom下建立属性文件uevent和子目录power，最后在power子目录下建立wakeup属性文件。最后以函数调用过程的总结来结束第6.2小结。

6.3 spi主控制器的平台设备

本节对一个特定的platform设备进行讲解，那就是spi主控制器的平台设备。在内核的启动阶段，platform设备将被注册进内核。我们来看下。下列代码位于arch/arm/mach-s3c2440/mach-smdk2440.c

static struct resource s3c_spi0_resource[] = {
[0] = {
.start = S3C24XX_PA_SPI,
.end   = S3C24XX_PA_SPI + 0x1f,
.flags = IORESOURCE_MEM,
},
[1] = {
.start = IRQ_SPI0,
.end   = IRQ_SPI0,
.flags = IORESOURCE_IRQ,
}

};

static u64 s3c_device_spi0_dmamask = 0xffffffffUL;

struct platform_device s3c_device_spi0 = {
.name          = "s3c2410-spi",
.id          = 0,
.num_resources      = ARRAY_SIZE(s3c_spi0_resource),
.resource      = s3c_spi0_resource,
.dev              = {
.dma_mask = &s3c_device_spi0_dmamask,
.coherent_dma_mask = 0xffffffffUL
}
};

static struct platform_device *smdk2440_devices[] __initdata = {
&s3c_device_usb,
&s3c_device_lcd,
&s3c_device_wdt,
&s3c_device_i2c0,
&s3c_device_iis,
&s3c_device_spi0,
};

static void __init smdk2440_machine_init(void)
{
s3c24xx_fb_set_platdata(&smdk2440_fb_info);
s3c_i2c0_set_platdata(NULL);

platform_add_devices(smdk2440_devices, ARRAY_SIZE(smdk2440_devices));
smdk_machine_init();
}

在smdk2440_machine_init函数中，通过调用platform_add_devices将设备注册到内核中。接着来看下该函数。

6.3.1 platform_add_devices

/*** platform_add_devices - add a numbers of platform devices* @devs: array of platform devices to add* @num: number of platform devices in array*/int platform_add_devices(struct platform_device **devs, int num){int i, ret = 0;for (i = 0; i < num; i++) {ret = platform_device_register(devs[i]);if (ret) {while (--i >= 0)platform_device_unregister(devs[i]);break;}}return ret;}EXPORT_SYMBOL_GPL(platform_add_devices);

该函数将根据devs指针数组，调用platform_device_register将platform设备逐一注册进内核。

6.3.2 platform_device_register

/*** platform_device_register - add a platform-level device* @pdev: platform device we're adding*/int platform_device_register(struct platform_device *pdev){device_initialize(&pdev->dev);return platform_device_add(pdev);}EXPORT_SYMBOL_GPL(platform_device_register);

调用了两个函数，第一个函数在6.1节已经分析过。我们来看下第二个函数。

6.3.2 platform_device_register

/*** platform_device_add - add a platform device to device hierarchy* @pdev: platform device we're adding** This is part 2 of platform_device_register(), though may be called* separately _iff_ pdev was allocated by platform_device_alloc().*/int platform_device_add(struct platform_device *pdev){int i, ret = 0;if (!pdev)return -EINVAL;if (!pdev->dev.parent)pdev->dev.parent = &platform_bus;	/*该设备的父设备是platform设备，/sys/devices/platform*/pdev->dev.bus = &platform_bus_type;		/*设备挂载到platform总线上*/if (pdev->id != -1)dev_set_name(&pdev->dev, "%s.%d", pdev->name,  pdev->id);elsedev_set_name(&pdev->dev, pdev->name);/*pdev->dev->kobj->name = pdev->name*//*遍历平台设备的资源，并将资源添加到资源树中*/for (i = 0; i < pdev->num_resources; i++) {struct resource *p, *r = &pdev->resource[i];if (r->name == NULL)r->name = dev_name(&pdev->dev);	/*获取dev->kobject->name*/p = r->parent;if (!p) {	/*p空*/if (resource_type(r) == IORESOURCE_MEM)p = &iomem_resource;else if (resource_type(r) == IORESOURCE_IO)p = &ioport_resource;}if (p && insert_resource(p, r)) {	/*将资源添加到资源树中*/printk(KERN_ERR"%s: failed to claim resource %d\n",dev_name(&pdev->dev), i);ret = -EBUSY;goto failed;}}pr_debug("Registering platform device '%s'. Parent at %s\n",dev_name(&pdev->dev), dev_name(pdev->dev.parent));ret = device_add(&pdev->dev);	/*添加设备*/if (ret == 0)return ret;failed:while (--i >= 0) {struct resource *r = &pdev->resource[i];unsigned long type = resource_type(r);if (type == IORESOURCE_MEM || type == IORESOURCE_IO)release_resource(r);}return ret;}EXPORT_SYMBOL_GPL(platform_device_add);

在这个函数的最后赫然出现了device_add函数。我们回忆下在6.1节中device_register的注册过程，该函数只调用了两个函数，一个是device_initialize函数，另一个就是device_add。本节的platform_device_register函数，首先也是调用了device_initialize，但是随后他做了一些其他的工作，最后调用了device_add。那么这个"其他的工作"干了些什么呢？首先，它将该SPI主控制对应的平台设备的父设备设为虚拟的platform设备（platform_bus），然后将该平台设备挂在至platform总线（platform_bus_type）上，这两步尤为重要，后面我们将看到。然后，调用了dev_set_name设置了pdev->dev-kobj.name，也就是该设备对象的名字，这里的名字为s3c2410-spi.0，这个名字将被用来建立一个目录。最后，将平台的相关资源添加到资源树中。这不是本篇文章讨论的重点所在，所以不做过多说明。在"其他的工作""干完之后，调用了device_add函数。那么后面的函数调用过程将和6.2小结的一致。由于“其他的工作”的原因，实际执行的过程和结果将有所区别。我们来分析下。

6.3.3 不一样device_add调用结果

首先，在device_add被调用之前，有若干个非常重要的条件已经被设置了。如下：pdev->dev->kobj.kset = devices_ksetpdev->dev-.parent = &platform_buspdev->dev.bus = &platform_bus_typeset_up函数执行时，由于参数parent为&platform_bus，因此最后将设置pdev->dev->kobj.parent = platform_bus.kobj。平台设备对象的父对象为虚拟的platform设备。kobject_add函数执行时，由于参数parent的存在，将在parent对象所对应的目录下创建另一个目录。parent对象代表目录/sys/devices/下的platform，因此将在/sys/devices/platform下建立目录s3c2410-spi.0。device_create_file建立属性文件uevent。bus_add_device函数执行时，由于dev.bus 为&platform_bus_type，因此将建立三个symlink。/sys/devices/platform/s3c2410-spi.0下建立链接subsystem和bus，他们指向/sys/bus/platform。/sys/bus/platform/devices/下建立链接s3c2410-spi.0，指向/sys/devices/platform/s3c2410-spi.0。dpm_sysfs_add函数在/sys/devices/platform/s3c2410-spi.0下建立子目录power，并在该子目录下建立属性文件wakeup。执行到这里时，sysfs已将内核中新添加的SPI主控制器平台设备呈现出来了，我们来验证下。[root@yj423 s3c2410-spi.0]#pwd/sys/devices/platform/s3c2410-spi.0[root@yj423 s3c2410-spi.0]#lllrwxrwxrwx 1 root root 0 Jan 1 00:29 bus -> ../../../bus/platformlrwxrwxrwx 1 root root 0 Jan 1 00:29 driver -> ../../../bus/platform/drivers/s3c2410-spi-r--r--r-- 1 root root 4096 Jan 1 00:29 modaliasdrwxr-xr-x 2 root root 0 Jan 1 00:29 powerdrwxr-xr-x 3 root root 0 Jan 1 00:00 spi0.0drwxr-xr-x 3 root root 0 Jan 1 00:00 spi0.1lrwxrwxrwx 1 root root 0 Jan 1 00:29 spi_master:spi0 -> ../../../class/spi_master/spi0lrwxrwxrwx 1 root root 0 Jan 1 00:29 subsystem -> ../../../bus/platform-rw-r--r-- 1 root root 4096 Jan 1 00:29 uevent[root@yj423 devices]#pwd/sys/bus/platform/devices[root@yj423 devices]#ll s3c2410-spi.0lrwxrwxrwx 1 root root 0 Jan 1 00:44 s3c2410-spi.0 -> ../../../devices/platform/s3c2410-spi.0通过sysfs将设备驱动的模型层次呈现在用户空间以后，将更新内核的设备模型之间的关系，这是通过修改链表的指向来完成的。bus_attach_device函数执行时，将设备添加到总线的设备链表中，同时也会尝试绑定驱动，不过会失败。接着，由于dev->parent的存在，将SPI主控制器设备添加到父设备platform虚拟设备的儿子链表中。

7. driver举例

我们已经介绍过platform总线的注册，也讲述了SPI主控制器设备作为平台设备的注册过程，在本节，将描述SPI主控制器的platform驱动是如何注册的。

7.1 s3c24xx_spi_init

下列代码位于drivers/spi/spi_s3c24xx.c。

MODULE_ALIAS("platform:s3c2410-spi");static struct platform_driver s3c24xx_spi_driver = {.remove        = __exit_p(s3c24xx_spi_remove),.suspend    = s3c24xx_spi_suspend,.resume        = s3c24xx_spi_resume,.driver        = {.name    = "s3c2410-spi",.owner    = THIS_MODULE,},};static int __init s3c24xx_spi_init(void){return platform_driver_probe(&s3c24xx_spi_driver, s3c24xx_spi_probe);//设备不可热插拔，所以使用该函数，而不是platform_driver_register}

驱动注册通过调用platform_driver_probe来完成。注意：driver.name字段使用来匹配设备的，该字段必须和6.3节一开始给出的pdev.name字段相同。

7.2 platform_driver_probe

下列代码位于drivers/base/platform.c。

/*** platform_driver_probe - register driver for non-hotpluggable device* @drv: platform driver structure* @probe: the driver probe routine, probably from an __init section** Use this instead of platform_driver_register() when you know the device* is not hotpluggable and has already been registered, and you want to* remove its run-once probe() infrastructure from memory after the driver* has bound to the device.** One typical use for this would be with drivers for controllers integrated* into system-on-chip processors, where the controller devices have been* configured as part of board setup.** Returns zero if the driver registered and bound to a device, else returns* a negative error code and with the driver not registered.*/int __init_or_module platform_driver_probe(struct platform_driver *drv,int (*probe)(struct platform_device *)){int retval, code;/* temporary section violation during probe() */drv->probe = probe;retval = code = platform_driver_register(drv); /*注册platform驱动*//* Fixup that section violation, being paranoid about code scanning* the list of drivers in order to probe new devices.  Check to see* if the probe was successful, and make sure any forced probes of* new devices fail.*/spin_lock(&platform_bus_type.p->klist_drivers.k_lock);drv->probe = NULL;if (code == 0 && list_empty(&drv->driver.p->klist_devices.k_list))retval = -ENODEV;drv->driver.probe = platform_drv_probe_fail;spin_unlock(&platform_bus_type.p->klist_drivers.k_lock);if (code != retval)platform_driver_unregister(drv);return retval;}EXPORT_SYMBOL_GPL(platform_driver_probe);

这里的重点是platform_driver_register，由它来完成了platform驱动的注册。

7.3 platform_driver_register

/*** platform_driver_register* @drv: platform driver structure*/int platform_driver_register(struct platform_driver *drv){drv->driver.bus = &platform_bus_type;if (drv->probe)drv->driver.probe = platform_drv_probe;if (drv->remove)drv->driver.remove = platform_drv_remove;if (drv->shutdown)drv->driver.shutdown = platform_drv_shutdown;if (drv->suspend)drv->driver.suspend = platform_drv_suspend;if (drv->resume)drv->driver.resume = platform_drv_resume;return driver_register(&drv->driver); /*驱动注册*/}EXPORT_SYMBOL_GPL(platform_driver_register);

driver_register函数就是driver注册的核心函数。需要注意的是，在调用函数之前，将该驱动所挂载的总线设置为platform总线（platform_bus_type）。

7.4 driver_register

下列代码位于drivers/base/driver.c。

/*** driver_register - register driver with bus* @drv: driver to register** We pass off most of the work to the bus_add_driver() call,* since most of the things we have to do deal with the bus* structures.*/int driver_register(struct device_driver *drv){int ret;struct device_driver *other;BUG_ON(!drv->bus->p);if ((drv->bus->probe && drv->probe) ||(drv->bus->remove && drv->remove) ||(drv->bus->shutdown && drv->shutdown))printk(KERN_WARNING "Driver '%s' needs updating - please use ""bus_type methods\n", drv->name);other = driver_find(drv->name, drv->bus);/*用驱动名字来搜索在该总线上驱动是否已经存在*/if (other) {	/*存在则报错*/put_driver(other);printk(KERN_ERR "Error: Driver '%s' is already registered, ""aborting...\n", drv->name);return -EEXIST;}ret = bus_add_driver(drv);	/*将驱动添加到一个总线中*/if (ret)return ret;ret = driver_add_groups(drv, drv->groups); /*建立属性组文件*/if (ret)bus_remove_driver(drv);return ret;}EXPORT_SYMBOL_GPL(driver_register);

这里主要调用两个函数driver_find和bus_add_driver。前者将通过总线来搜索该驱动是否存在，后者将添加驱动到总线中。接下来就分析这两个函数。

7.5 driver_find

下列代码位于drivers/base/driver.c。

/*** driver_find - locate driver on a bus by its name.* @name: name of the driver.* @bus: bus to scan for the driver.** Call kset_find_obj() to iterate over list of drivers on* a bus to find driver by name. Return driver if found.** Note that kset_find_obj increments driver's reference count.*/struct device_driver *driver_find(const char *name, struct bus_type *bus){struct kobject *k = kset_find_obj(bus->p->drivers_kset, name);struct driver_private *priv;if (k) {priv = to_driver(k);return priv->driver;}return NULL;}EXPORT_SYMBOL_GPL(driver_find);

/*** 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(k);break;}}spin_unlock(&kset->list_lock);return ret;}

这里调用了kset_find_obj函数，传入的实参bus->p->drivers_kset，它对应的就是/sys/bus/platform/下的drivers目录，然后通过链表，它将搜索该目录下的所有文件，来寻找是否有名为s3c2410-spi的文件。还记得吗？ kobject就是一个文件对象。如果没有找到将返回NULL，接着将调用bus_add_driver把驱动注册进内核。

7.6 bus_add_driver

下列代码位于drivers/base/bus.c

/*** bus_add_driver - Add a driver to the bus.* @drv: driver.*/int bus_add_driver(struct device_driver *drv){struct bus_type *bus;struct driver_private *priv;int error = 0;bus = bus_get(drv->bus);	/*增加引用计数获取bus_type*/if (!bus)return -EINVAL;pr_debug("bus: '%s': add driver %s\n", bus->name, drv->name);priv = kzalloc(sizeof(*priv), GFP_KERNEL);	/*分配driver_private结构体*/if (!priv) {error = -ENOMEM;goto out_put_bus;}/*初始化内核链表*/klist_init(&priv->klist_devices, NULL, NULL);/*相互保存*/priv->driver = drv;drv->p = priv;/*设置该kobj属于那个kset*/priv->kobj.kset = bus->p->drivers_kset;error = kobject_init_and_add(&priv->kobj, &driver_ktype, NULL,	/*parent=NULL*/"%s", drv->name);	/*执行完以后，会在bus/总线名/drivers/下建立名为drv->name的目录*/if (error)goto out_unregister;if (drv->bus->p->drivers_autoprobe) {error = driver_attach(drv);	/*尝试绑定驱动和设备*/if (error)goto out_unregister;}/*添加该驱动到bus的内核链表中*/klist_add_tail(&priv->knode_bus, &bus->p->klist_drivers);module_add_driver(drv->owner, drv);/*?????????*//*创建属性，在bus/总线名/drivers/驱动名/下建立文件uevent*/error = driver_create_file(drv, &driver_attr_uevent);if (error) {printk(KERN_ERR "%s: uevent attr (%s) failed\n",__func__, drv->name);}/*利用bus->drv_attrs创建属性，位于bus/总线名/drivers/驱动名/*/error = driver_add_attrs(bus, drv);if (error) {/* How the hell do we get out of this pickle? Give up */printk(KERN_ERR "%s: driver_add_attrs(%s) failed\n",__func__, drv->name);}/*创建属性，在bus/总线名/drivers/驱动名/下建立文件bind和unbind*/error = add_bind_files(drv);if (error) {/* Ditto */printk(KERN_ERR "%s: add_bind_files(%s) failed\n",__func__, drv->name);}/*通知用户空间???*/kobject_uevent(&priv->kobj, KOBJ_ADD);return 0;out_unregister:kfree(drv->p);drv->p = NULL;kobject_put(&priv->kobj);out_put_bus:bus_put(bus);return error;}

在设置driver的kobj.kset为drivers目录所对应的kset之后，调用了kobject_init_and_add，我们来看下。

7.6.1 kobject_init_and_add

下列代码位于lib/kobject.c。

/*** 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;}EXPORT_SYMBOL_GPL(kobject_init_and_add);

该函数中调用了两个函数，这两个函数分别在6.1.2和6.2.2中讲述过，这里不再赘述。调用该函数时由于parent为NULL，但kobj.kset为drivers目录，所以将在/sys/bus/platform/drivers/下建立目录，名为s3c2410-spi。我们来验证下：[root@yj423 s3c2410-spi]#pwd/sys/bus/platform/drivers/s3c2410-spi接着由于drivers_autoprobe在bus_register执行的时候已经置1，将调用driver_attach。

7.6.2 driver_attach

下列代码位于drivers/base/dd.c。

/*** driver_attach - try to bind driver to devices.* @drv: driver.** Walk the list of devices that the bus has on it and try to* match the driver with each one.  If driver_probe_device()* returns 0 and the @dev->driver is set, we've found a* compatible pair.*/int driver_attach(struct device_driver *drv){return bus_for_each_dev(drv->bus, NULL, drv, __driver_attach);}EXPORT_SYMBOL_GPL(driver_attach);

该函数将调用bus_for_each_dev来寻找总线上的每个设备，这里的总线即为platform总线，然后尝试绑定设备。这里需要注意的是最后一个参数__driver_attach，这是一个函数名，后面将会调用它。

/*** bus_for_each_dev - device iterator.* @bus: bus type.* @start: device to start iterating from.* @data: data for the callback.* @fn: function to be called for each device.** Iterate over @bus's list of devices, and call @fn for each,* passing it @data. If @start is not NULL, we use that device to* begin iterating from.** We check the return of @fn each time. If it returns anything* other than 0, we break out and return that value.** NOTE: The device that returns a non-zero value is not retained* in any way, nor is its refcount incremented. If the caller needs* to retain this data, it should do, and increment the reference* count in the supplied callback.*/int bus_for_each_dev(struct bus_type *bus, struct device *start,void *data, int (*fn)(struct device *, void *)){struct klist_iter i;struct device *dev;int error = 0;if (!bus)return -EINVAL;klist_iter_init_node(&bus->p->klist_devices, &i,(start ? &start->p->knode_bus : NULL));while ((dev = next_device(&i)) && !error)error = fn(dev, data);klist_iter_exit(&i);return error;}EXPORT_SYMBOL_GPL(bus_for_each_dev);

通过klist将遍历该总线上的所有设备，并为其调用__driver_attach函数。

static int __driver_attach(struct device *dev, void *data){struct device_driver *drv = data;/** Lock device and try to bind to it. We drop the error* here and always return 0, because we need to keep trying* to bind to devices and some drivers will return an error* simply if it didn't support the device.** driver_probe_device() will spit a warning if there* is an error.*/if (!driver_match_device(drv, dev))return 0;if (dev->parent)	/* Needed for USB */down(&dev->parent->sem);down(&dev->sem);if (!dev->driver)driver_probe_device(drv, dev);up(&dev->sem);if (dev->parent)up(&dev->parent->sem);return 0;}

首先调用了driver_match_device函数，该函数进会进行匹配，如果匹配成功将返回1。我们看下这个函数：

static inline int driver_match_device(struct device_driver *drv,struct device *dev){return drv->bus->match ? drv->bus->match(dev, drv) : 1;}

这里直接调用了platform总线的match方法，我们来看下这个方法。

/*** platform_match - bind platform device to platform driver.* @dev: device.* @drv: driver.** Platform device IDs are assumed to be encoded like this:* "<name><instance>", where <name> is a short description of the type of* device, like "pci" or "floppy", and <instance> is the enumerated* instance of the device, like '0' or '42'.  Driver IDs are simply* "<name>".  So, extract the <name> from the platform_device structure,* and compare it against the name of the driver. Return whether they match* or not.*/static int platform_match(struct device *dev, struct device_driver *drv){struct platform_device *pdev = to_platform_device(dev);struct platform_driver *pdrv = to_platform_driver(drv);/* match against the id table first */if (pdrv->id_table)return platform_match_id(pdrv->id_table, pdev) != NULL;/* fall-back to driver name match */return (strcmp(pdev->name, drv->name) == 0);}

该方法的核心其实就是使用stcmp进行字符匹配，判断pdev->name和drv->name是否相等。在本例中两者同为s3c2410-spi。因此匹配完成，返回1。返回后，由于dev->driver为NULL，将调用driver_probe_device函数。我们来看下：

/*** driver_probe_device - attempt to bind device & driver together* @drv: driver to bind a device to* @dev: device to try to bind to the driver** This function returns -ENODEV if the device is not registered,* 1 if the device is bound sucessfully and 0 otherwise.** This function must be called with @dev->sem held.  When called for a* USB interface, @dev->parent->sem must be held as well.*/int driver_probe_device(struct device_driver *drv, struct device *dev){int ret = 0;if (!device_is_registered(dev))return -ENODEV;pr_debug("bus: '%s': %s: matched device %s with driver %s\n",drv->bus->name, __func__, dev_name(dev), drv->name);ret = really_probe(dev, drv);return ret;}static inline int device_is_registered(struct device *dev){return dev->kobj.state_in_sysfs;}

该函数将调用really_probe来绑定设备和它的驱动。

static int really_probe(struct device *dev, struct device_driver *drv){int ret = 0;atomic_inc(&probe_count);pr_debug("bus: '%s': %s: probing driver %s with device %s\n",drv->bus->name, __func__, drv->name, dev_name(dev));WARN_ON(!list_empty(&dev->devres_head));dev->driver = drv;if (driver_sysfs_add(dev)) {	/*创建两个symlink，更新sysfs*/printk(KERN_ERR "%s: driver_sysfs_add(%s) failed\n",__func__, dev_name(dev));goto probe_failed;}if (dev->bus->probe) {ret = dev->bus->probe(dev);/*调用总线的probe方法*/if (ret)goto probe_failed;} else if (drv->probe) {ret = drv->probe(dev);	/*调用驱动的probe方法*/if (ret)goto probe_failed;}driver_bound(dev);              /*绑定设备和驱动*/ret = 1;pr_debug("bus: '%s': %s: bound device %s to driver %s\n",drv->bus->name, __func__, dev_name(dev), drv->name);goto done;probe_failed:devres_release_all(dev);driver_sysfs_remove(dev);dev->driver = NULL;if (ret != -ENODEV && ret != -ENXIO) {/* driver matched but the probe failed */printk(KERN_WARNING"%s: probe of %s failed with error %d\n",drv->name, dev_name(dev), ret);}/** Ignore errors returned by ->probe so that the next driver can try* its luck.*/ret = 0;done:atomic_dec(&probe_count);wake_up(&probe_waitqueue);return ret;}

在这个函数中调用4个函数。第一个函数driver_sysfs_add将更新sysfs。

static int driver_sysfs_add(struct device *dev){int ret;/* 在/sys/bus/XXX/drivers/XXX目录下建立symlink，链接名为kobj->name，链接指向/sys/devices/platform/XXX */ret = sysfs_create_link(&dev->driver->p->kobj, &dev->kobj,kobject_name(&dev->kobj));if (ret == 0) {/* 在/sys/devices/platform/XXX/下建立symlink，链接名为driver，指向/sys/bus/xxx/drivers目录下的某个目录*/ret = sysfs_create_link(&dev->kobj, &dev->driver->p->kobj,"driver");if (ret)sysfs_remove_link(&dev->driver->p->kobj,kobject_name(&dev->kobj));}return ret;}

执行完以后，建立了两个链接。在/sys/bus/platform/drivers/s3c2410-spi下建立链接，指向/sys/devices/platform/s3c2410-spi.0在/sys/devices/platform/s3c2410-spi.0下建立链接，指向/sys/devices/platform/s3c2410-spi.0。这样就在用户空间呈现出驱动和设备的关系了。我们来验证下。[root@yj423 s3c2410-spi]#pwd/sys/bus/platform/drivers/s3c2410-spi[root@yj423 s3c2410-spi]#ll s3c2410-spi.0lrwxrwxrwx 1 root root 0 Jan 1 02:28 s3c2410-spi.0 -> ../../../../devices/platform/s3c2410-spi.0[root@yj423 s3c2410-spi.0]#pwd/sys/devices/platform/s3c2410-spi.0[root@yj423 s3c2410-spi.0]#ll driverlrwxrwxrwx 1 root root 0 Jan 1 02:26 driver -> ../../../bus/platform/drivers/s3c2410-spi第2个函数执行总线的probe方法，由于platform总线没有提供probe方法，因此不执行。第3个函数执行驱动的probe方法，驱动提供了probe，因此调用它，该函数的细节超过了本文的讨论内容，所以略过。第4个函数执行driver_bound，用来绑定设备和驱动，来看下这个函数。

static void driver_bound(struct device *dev){if (klist_node_attached(&dev->p->knode_driver)) {printk(KERN_WARNING "%s: device %s already bound\n",__func__, kobject_name(&dev->kobj));return;}pr_debug("driver: '%s': %s: bound to device '%s'\n", dev_name(dev),__func__, dev->driver->name);if (dev->bus)blocking_notifier_call_chain(&dev->bus->p->bus_notifier,BUS_NOTIFY_BOUND_DRIVER, dev);klist_add_tail(&dev->p->knode_driver, &dev->driver->p->klist_devices);}

其实，所谓的绑定，就是将设备的驱动节点添加到驱动支持的设备链表中。至此，通过内核链表，这个platform device 和platform driver 已经绑定完成，将继续遍历内核链表尝试匹配和绑定，直到链表结束。在driver_attach执行完毕以后，bus_add_driver函数还有些剩余工作要完成。首先，将驱动添加到总线的驱动列表中。接着，如果定义了驱动属性文件，则创建。最后，在/sys/bus/platform/drivers/s3c2410-spi/下建立属性文件uevent，并在同一目录下建立文件bind和unbind。我们来验证下：[root@yj423 s3c2410-spi]#pwd/sys/bus/platform/drivers/s3c2410-spi[root@yj423 s3c2410-spi]#lsbind s3c2410-spi.0 uevent unbind

7.7 小结

在本节中，我们看到了platform driver是如何注册到内核中，在注册过程中，通过更新了sysfs，向用户空间展示总线，设备和驱动之间的关系。同时，还更新了链表的指向，在内核中体现了同样的关系。最后以platform driver的注册过程结束本章。

8. sysfs底层函数

下面讲述的内容将基于VFS，有关VFS的基本内容超过本文的范围，请参考<<深入理解Linux内核>>一书的第12章。在前面讲述的过程中，我们知道设备驱动模型是如何通过kobject将总线，设备和驱动间的层次关系在用户空间呈现出来的。事实上，就是通过目录，文件和symlink来呈现相互之间的关系。在前面的叙述中，我们并没有对目录，文件和symlink的创建进行 讲解，本章就对这些底层函数进行讲解。在讲解这些函数之前，我们先来看下，sysfs文件系统是如何注册的。

8.1 注册sysfs文件系统

sysfs文件系统的注册是调用sysfs_init函数来完成的，该函数在内核启动阶段被调用，我们来看下大致函数调用流程，这里不作分析。start_kernel( ) -> vfs_caches_init( ) -> mnt_init( ) -> mnt_init( ) -> sysfs_init( )。

int __init sysfs_init(void){int err = -ENOMEM;/*建立cache,名字为sysfs_dir_cache*/sysfs_dir_cachep = kmem_cache_create("sysfs_dir_cache",sizeof(struct sysfs_dirent),0, 0, NULL);if (!sysfs_dir_cachep)goto out;err = sysfs_inode_init();if (err)goto out_err;/*注册文件系统*/err = register_filesystem(&sysfs_fs_type);if (!err) {/*注册成功，加载文件系统*/sysfs_mount = kern_mount(&sysfs_fs_type);if (IS_ERR(sysfs_mount)) {printk(KERN_ERR "sysfs: could not mount!\n");err = PTR_ERR(sysfs_mount);sysfs_mount = NULL;unregister_filesystem(&sysfs_fs_type);goto out_err;}} elsegoto out_err;out:return err;out_err:kmem_cache_destroy(sysfs_dir_cachep);sysfs_dir_cachep = NULL;goto out;}static struct file_system_type sysfs_fs_type = {.name        = "sysfs",.get_sb        = sysfs_get_sb,.kill_sb    = kill_anon_super,};

8.1.1 register_filesystem

下列代码位于fs/filesystems.c。

/***	register_filesystem - register a new filesystem*	@fs: the file system structure**	Adds the file system passed to the list of file systems the kernel*	is aware of for mount and other syscalls. Returns 0 on success,*	or a negative errno code on an error.**	The &struct file_system_type that is passed is linked into the kernel*	structures and must not be freed until the file system has been*	unregistered.*/int register_filesystem(struct file_system_type * fs){int res = 0;struct file_system_type ** p;BUG_ON(strchr(fs->name, '.'));if (fs->next)return -EBUSY;INIT_LIST_HEAD(&fs->fs_supers);write_lock(&file_systems_lock);p = find_filesystem(fs->name, strlen(fs->name));	/*查找要住的文件是同是否存在，返回位置*/if (*p)res = -EBUSY;	/*该文件系统已存在，返回error*/else*p = fs;		/*将新的文件系统加入到链表中*/write_unlock(&file_systems_lock);return res;}

static struct file_system_type **find_filesystem(const char *name, unsigned len){struct file_system_type **p;for (p=&file_systems; *p; p=&(*p)->next)if (strlen((*p)->name) == len &&strncmp((*p)->name, name, len) == 0)break;return p;}

该函数将调用函数file_system_type，此函数根据name字段（sysfs）来查找要注册的文件系统是否已经存在。如果不存在，表示还未注册，则将新的fs添加到链表中，链表的第一项为全局变量file_systems。该全局变量为单项链表，所有已注册的文件系统都被插入到这个链表当中。

8.1.2 kern_mount函数

下列代码位于include/linux/fs.h

#define kern_mount(type) kern_mount_data(type, NULL)

下列代码位于fs/sysfs/mount.c

struct vfsmount *kern_mount_data(struct file_system_type *type, void *data){return vfs_kern_mount(type, MS_KERNMOUNT, type->name, data);}EXPORT_SYMBOL_GPL(kern_mount_data);

kern_mount实际上最后是调用了vfs_kern_mount函数。我们来看下：

struct vfsmount *vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void *data){struct vfsmount *mnt;char *secdata = NULL;int error;if (!type)return ERR_PTR(-ENODEV);error = -ENOMEM;mnt = alloc_vfsmnt(name);	/*分配struct vfsmount*/if (!mnt)goto out;if (data && !(type->fs_flags & FS_BINARY_MOUNTDATA)) {secdata = alloc_secdata();if (!secdata)goto out_mnt;error = security_sb_copy_data(data, secdata);if (error)goto out_free_secdata;}/*get_sb方法，分配superblock对象，并初始化*/error = type->get_sb(type, flags, name, data, mnt);if (error < 0)goto out_free_secdata;BUG_ON(!mnt->mnt_sb);error = security_sb_kern_mount(mnt->mnt_sb, flags, secdata);if (error)goto out_sb;mnt->mnt_mountpoint = mnt->mnt_root;/*设置挂载点的dentry*/mnt->mnt_parent = mnt;		    /*设置所挂载的fs为自己本身*/up_write(&mnt->mnt_sb->s_umount);free_secdata(secdata);return mnt;out_sb:dput(mnt->mnt_root);deactivate_locked_super(mnt->mnt_sb);out_free_secdata:free_secdata(secdata);out_mnt:free_vfsmnt(mnt);out:return ERR_PTR(error);}

该函数在首先调用alloc_vfsmnt来分配struct vfsmount结构，并做了一些初试化工作。下列函数位于fs/super.c

struct vfsmount *alloc_vfsmnt(const char *name){struct vfsmount *mnt = kmem_cache_zalloc(mnt_cache, GFP_KERNEL);if (mnt) {int err;err = mnt_alloc_id(mnt);	/*设置mnt->mnt_id*/if (err)goto out_free_cache;if (name) {mnt->mnt_devname = kstrdup(name, GFP_KERNEL); /*拷贝name，并赋值*/if (!mnt->mnt_devname)goto out_free_id;}atomic_set(&mnt->mnt_count, 1);INIT_LIST_HEAD(&mnt->mnt_hash);INIT_LIST_HEAD(&mnt->mnt_child);INIT_LIST_HEAD(&mnt->mnt_mounts);INIT_LIST_HEAD(&mnt->mnt_list);INIT_LIST_HEAD(&mnt->mnt_expire);INIT_LIST_HEAD(&mnt->mnt_share);INIT_LIST_HEAD(&mnt->mnt_slave_list);INIT_LIST_HEAD(&mnt->mnt_slave);atomic_set(&mnt->__mnt_writers, 0);}return mnt;out_free_id:mnt_free_id(mnt);out_free_cache:kmem_cache_free(mnt_cache, mnt);return NULL;}

分配好结构体以后，由于参数data为NULL，将直接调用文件系统类型提供的get_sb方法，该方法就是函数sysfs_get_sb。我们来看下：下列函数位于fs/sysfs/mount.c。

static int sysfs_get_sb(struct file_system_type *fs_type,int flags, const char *dev_name, void *data, struct vfsmount *mnt){return get_sb_single(fs_type, flags, data, sysfs_fill_super, mnt);}

这里直接调用了get_sb_single函数，注意这里的第4个实参sysfs_fill_super，该参数是函数名，后面将会调用该函数。该函数将分配sysfs文件系统的superblock，获取文件系统根目录的inode和dentry。该函数的执行过程相当复杂，在下一节单独讲述。

8.2 get_sb_single函数

下列函数位于fs/sysfs/mount.c。

int get_sb_single(struct file_system_type *fs_type,int flags, void *data,int (*fill_super)(struct super_block *, void *, int),struct vfsmount *mnt){struct super_block *s;int error;/*查找或者创建super_block*/s = sget(fs_type, compare_single, set_anon_super, NULL);if (IS_ERR(s))return PTR_ERR(s);if (!s->s_root) {		/*没有根目录dentry*/s->s_flags = flags;/*获取root( / )的 inode和dentry*/error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);if (error) {deactivate_locked_super(s);return error;}s->s_flags |= MS_ACTIVE;}do_remount_sb(s, flags, data, 0);simple_set_mnt(mnt, s);	/*设置vfsmount的superblock和根dentry*/return 0;}EXPORT_SYMBOL(get_sb_single);

8.2.1 sget函数

首先调用了sget函数来查找是否下列函数位于fs/super.c。

/***	sget	-	find or create a superblock*	@type:	filesystem type superblock should belong to*	@test:	comparison callback*	@set:	setup callback*	@data:	argument to each of them*/struct super_block *sget(struct file_system_type *type,int (*test)(struct super_block *,void *),int (*set)(struct super_block *,void *),void *data){struct super_block *s = NULL;struct super_block *old;int err;retry:spin_lock(&sb_lock);if (test) {/*遍历所有属于该文件系统的super_block*/list_for_each_entry(old, &type->fs_supers, s_instances) {if (!test(old, data))continue;if (!grab_super(old))goto retry;if (s) {up_write(&s->s_umount);destroy_super(s);}return old;}}if (!s) {spin_unlock(&sb_lock);s = alloc_super(type);	/*创建新的super_block并初始化*/if (!s)return ERR_PTR(-ENOMEM);goto retry;}err = set(s, data);		/*设置s->s_dev */if (err) {spin_unlock(&sb_lock);up_write(&s->s_umount);destroy_super(s);return ERR_PTR(err);}s->s_type = type;strlcpy(s->s_id, type->name, sizeof(s->s_id));	/*拷贝name*/list_add_tail(&s->s_list, &super_blocks);		/*将新的super_block添加到链表头super_blocks中*/list_add(&s->s_instances, &type->fs_supers);	/*将新的super_block添加到相应的文件系统类型的链表中*/spin_unlock(&sb_lock);get_filesystem(type);return s;}EXPORT_SYMBOL(sget);

该函数将遍历属于sysfs文件系统的所有superblock，本例中由于之前没有任何superblock创建，遍历立即结束。然后调用alloc_super函数来创建新的struct super_block。下列函数位于fs/super.c。

/***	alloc_super	-	create new superblock*	@type:	filesystem type superblock should belong to**	Allocates and initializes a new &struct super_block.  alloc_super()*	returns a pointer new superblock or %NULL if allocation had failed.*/static struct super_block *alloc_super(struct file_system_type *type){struct super_block *s = kzalloc(sizeof(struct super_block),  GFP_USER);/*分配并清0super_block*/static struct super_operations default_op;if (s) {if (security_sb_alloc(s)) {kfree(s);s = NULL;goto out;}INIT_LIST_HEAD(&s->s_dirty);INIT_LIST_HEAD(&s->s_io);INIT_LIST_HEAD(&s->s_more_io);INIT_LIST_HEAD(&s->s_files);INIT_LIST_HEAD(&s->s_instances);INIT_HLIST_HEAD(&s->s_anon);INIT_LIST_HEAD(&s->s_inodes);INIT_LIST_HEAD(&s->s_dentry_lru);INIT_LIST_HEAD(&s->s_async_list);init_rwsem(&s->s_umount);mutex_init(&s->s_lock);lockdep_set_class(&s->s_umount, &type->s_umount_key);/** The locking rules for s_lock are up to the* filesystem. For example ext3fs has different* lock ordering than usbfs:*/lockdep_set_class(&s->s_lock, &type->s_lock_key);/** sget() can have s_umount recursion.** When it cannot find a suitable sb, it allocates a new* one (this one), and tries again to find a suitable old* one.** In case that succeeds, it will acquire the s_umount* lock of the old one. Since these are clearly distrinct* locks, and this object isn't exposed yet, there's no* risk of deadlocks.** Annotate this by putting this lock in a different* subclass.*/down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);s->s_count = S_BIAS;atomic_set(&s->s_active, 1);mutex_init(&s->s_vfs_rename_mutex);mutex_init(&s->s_dquot.dqio_mutex);mutex_init(&s->s_dquot.dqonoff_mutex);init_rwsem(&s->s_dquot.dqptr_sem);init_waitqueue_head(&s->s_wait_unfrozen);s->s_maxbytes = MAX_NON_LFS;s->dq_op = sb_dquot_ops;s->s_qcop = sb_quotactl_ops;s->s_op = &default_op;s->s_time_gran = 1000000000;}out:return s;}

分配完以后，调用作为参数传入的函数指针set，也就是set_anon_super函数，该函数用来设置s->s_dev。下列函数位于fs/super.c。

int set_anon_super(struct super_block *s, void *data){int dev;int error;retry:if (ida_pre_get(&unnamed_dev_ida, GFP_ATOMIC) == 0)/*分配ID号*/return -ENOMEM;spin_lock(&unnamed_dev_lock);error = ida_get_new(&unnamed_dev_ida, &dev);/*获取ID号，保存在dev中*/spin_unlock(&unnamed_dev_lock);if (error == -EAGAIN)/* We raced and lost with another CPU. */goto retry;else if (error)return -EAGAIN;if ((dev & MAX_ID_MASK) == (1 << MINORBITS)) {spin_lock(&unnamed_dev_lock);ida_remove(&unnamed_dev_ida, dev);spin_unlock(&unnamed_dev_lock);return -EMFILE;}s->s_dev = MKDEV(0, dev & MINORMASK);	/*构建设备号*/return 0;}

8.2.2 sysfs_fill_super函数

分配了super_block之后，将判断该super_block是否有root dentry。本例中，显然没有。然后调用形参fill_super指向的函数，也就是sysfs_fill_super函数。下列函数位于fs/sysfs/mount.c。

struct super_block * sysfs_sb = NULL;static int sysfs_fill_super(struct super_block *sb, void *data, int silent){struct inode *inode;struct dentry *root;sb->s_blocksize = PAGE_CACHE_SIZE;	/*4KB*/sb->s_blocksize_bits = PAGE_CACHE_SHIFT; /*4KB*/sb->s_magic = SYSFS_MAGIC;			/*0x62656572*/sb->s_op = &sysfs_ops;sb->s_time_gran = 1;sysfs_sb = sb;		/*sysfs_sb即为sysfs的super_block*//* get root inode, initialize and unlock it */mutex_lock(&sysfs_mutex);inode = sysfs_get_inode(&sysfs_root); /*sysfs_root即为sysfs所在的根目录的dirent，，获取inode*/mutex_unlock(&sysfs_mutex);if (!inode) {pr_debug("sysfs: could not get root inode\n");return -ENOMEM;}/* instantiate and link root dentry */root = d_alloc_root(inode);	/*为获得的根inode分配root(/) dentry*/if (!root) {pr_debug("%s: could not get root dentry!\n",__func__);iput(inode);return -ENOMEM;}root->d_fsdata = &sysfs_root;sb->s_root = root;   /*保存superblock的根dentry*/return 0;}struct sysfs_dirent sysfs_root = {    /*sysfs_root即为sysfs所在的根目录的dirent*/.s_name        = "",.s_count    = ATOMIC_INIT(1),.s_flags    = SYSFS_DIR,.s_mode        = S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO,.s_ino        = 1,};

在设置了一些字段后，设置了sysfs_sb这个全局变量，该全局变量表示的就是sysfs的super_block。随后，调用了sysfs_get_inode函数，来获取sysfs的根目录的dirent。该函数的参数sysfs_root为全局变量，表示sysfs的根目录的sysfs_dirent。我们看些这个sysfs_dirent数据结构：

/** sysfs_dirent - the building block of sysfs hierarchy.  Each and* every sysfs node is represented by single sysfs_dirent.** As long as s_count reference is held, the sysfs_dirent itself is* accessible.  Dereferencing s_elem or any other outer entity* requires s_active reference.*/struct sysfs_dirent {atomic_t		s_count;atomic_t		s_active;struct sysfs_dirent	*s_parent;struct sysfs_dirent	*s_sibling;const char		*s_name;union {struct sysfs_elem_dir		s_dir;struct sysfs_elem_symlink	s_symlink;struct sysfs_elem_attr		s_attr;struct sysfs_elem_bin_attr	s_bin_attr;};unsigned int		s_flags;ino_t			s_ino;umode_t			s_mode;struct iattr		*s_iattr;};

其中比较关键的就是那个联合体，针对不同的形式（目录，symlink，属性文件和可执行文件）将使用不同的数据结构。另外，sysfs_dirent将最为dentry的fs专有数据被保存下来，这一点会在下面中看到。接着，在来看下sysfs_get_inode函数：下列函数位于fs/sysfs/inode.c。

/***	sysfs_get_inode - get inode for sysfs_dirent*	@sd: sysfs_dirent to allocate inode for**	Get inode for @sd.  If such inode doesn't exist, a new inode*	is allocated and basics are initialized.  New inode is*	returned locked.**	LOCKING:*	Kernel thread context (may sleep).**	RETURNS:*	Pointer to allocated inode on success, NULL on failure.*/struct inode * sysfs_get_inode(struct sysfs_dirent *sd){struct inode *inode;inode = iget_locked(sysfs_sb, sd->s_ino);	/*在inode cache查找inode是否存在，不存在侧创建一个*/if (inode && (inode->i_state & I_NEW))		/*如果是新创建的inode，则包含I_NEW*/sysfs_init_inode(sd, inode);return inode;}/*** iget_locked - obtain an inode from a mounted file system* @sb:        super block of file system* @ino:    inode number to get** iget_locked() uses ifind_fast() to search for the inode specified by @ino in* the inode cache and if present it is returned with an increased reference* count. This is for file systems where the inode number is sufficient for* unique identification of an inode.** If the inode is not in cache, get_new_inode_fast() is called to allocate a* new inode and this is returned locked, hashed, and with the I_NEW flag set.* The file system gets to fill it in before unlocking it via* unlock_new_inode().*/struct inode *iget_locked(struct super_block *sb, unsigned long ino){struct hlist_head *head = inode_hashtable + hash(sb, ino);struct inode *inode;inode = ifind_fast(sb, head, ino);/*在inode cache查找该inode*/if (inode)return inode;         /*找到了该inode*//** get_new_inode_fast() will do the right thing, re-trying the search* in case it had to block at any point.*/return get_new_inode_fast(sb, head, ino);    /*分配一个新的inode*/}EXPORT_SYMBOL(iget_locked);static void sysfs_init_inode(struct sysfs_dirent *sd, struct inode *inode){struct bin_attribute *bin_attr;inode->i_private = sysfs_get(sd);inode->i_mapping->a_ops = &sysfs_aops;inode->i_mapping->backing_dev_info = &sysfs_backing_dev_info;inode->i_op = &sysfs_inode_operations;inode->i_ino = sd->s_ino;lockdep_set_class(&inode->i_mutex, &sysfs_inode_imutex_key);if (sd->s_iattr) {/* sysfs_dirent has non-default attributes* get them for the new inode from persistent copy* in sysfs_dirent*/set_inode_attr(inode, sd->s_iattr);} elseset_default_inode_attr(inode, sd->s_mode);/*设置inode属性*//* initialize inode according to type */switch (sysfs_type(sd)) {case SYSFS_DIR:inode->i_op = &sysfs_dir_inode_operations;inode->i_fop = &sysfs_dir_operations;inode->i_nlink = sysfs_count_nlink(sd);break;case SYSFS_KOBJ_ATTR:inode->i_size = PAGE_SIZE;inode->i_fop = &sysfs_file_operations;break;case SYSFS_KOBJ_BIN_ATTR:bin_attr = sd->s_bin_attr.bin_attr;inode->i_size = bin_attr->size;inode->i_fop = &bin_fops;break;case SYSFS_KOBJ_LINK:inode->i_op = &sysfs_symlink_inode_operations;break;default:BUG();}unlock_new_inode(inode);}

该函数首先调用了，iget_locked来查找该inode是否已存在，如果不存在则创建。如果是新创建的inode，则对inode进行初始化。再获取了根目录的inode和sysfs_dirent后，调用d_alloc_root来获得dirent。

/*** d_alloc_root - allocate root dentry* @root_inode: inode to allocate the root for** Allocate a root ("/") dentry for the inode given. The inode is* instantiated and returned. %NULL is returned if there is insufficient* memory or the inode passed is %NULL.*/struct dentry * d_alloc_root(struct inode * root_inode){struct dentry *res = NULL;if (root_inode) {static const struct qstr name = { .name = "/", .len = 1 };res = d_alloc(NULL, &name);	/*分配struct dentry，没有父dentry*/if (res) {res->d_sb = root_inode->i_sb;res->d_parent = res;d_instantiated_instantiate(res, root_inode); /*绑定inode和dentry之间的关系*/}}return res;}/*** d_alloc    -    allocate a dcache entry* @parent: parent of entry to allocate* @name: qstr of the name** Allocates a dentry. It returns %NULL if there is insufficient memory* available. On a success the dentry is returned. The name passed in is* copied and the copy passed in may be reused after this call.*/struct dentry *d_alloc(struct dentry * parent, const struct qstr *name){struct dentry *dentry;char *dname;dentry = kmem_cache_alloc(dentry_cache, GFP_KERNEL);/*分配struct dentry*/if (!dentry)return NULL;if (name->len > DNAME_INLINE_LEN-1) {dname = kmalloc(name->len + 1, GFP_KERNEL);if (!dname) {kmem_cache_free(dentry_cache, dentry);return NULL;}} else  {dname = dentry->d_iname;}dentry->d_name.name = dname;dentry->d_name.len = name->len;dentry->d_name.hash = name->hash;memcpy(dname, name->name, name->len);dname[name->len] = 0;atomic_set(&dentry->d_count, 1);dentry->d_flags = DCACHE_UNHASHED;spin_lock_init(&dentry->d_lock);dentry->d_inode = NULL;dentry->d_parent = NULL;dentry->d_sb = NULL;dentry->d_op = NULL;dentry->d_fsdata = NULL;dentry->d_mounted = 0;INIT_HLIST_NODE(&dentry->d_hash);INIT_LIST_HEAD(&dentry->d_lru);INIT_LIST_HEAD(&dentry->d_subdirs);INIT_LIST_HEAD(&dentry->d_alias);if (parent) {    /*有父目录，则设置指针来表示关系*/dentry->d_parent = dget(parent);dentry->d_sb = parent->d_sb;  /*根dentry的父对象为自己*/} else {INIT_LIST_HEAD(&dentry->d_u.d_child);}spin_lock(&dcache_lock);if (parent)        /*有父目录，则添加到父目录的儿子链表中*/list_add(&dentry->d_u.d_child, &parent->d_subdirs);dentry_stat.nr_dentry++;spin_unlock(&dcache_lock);return dentry;}/*** d_instantiate - fill in inode information for a dentry* @entry: dentry to complete* @inode: inode to attach to this dentry** Fill in inode information in the entry.** This turns negative dentries into productive full members* of society.** NOTE! This assumes that the inode count has been incremented* (or otherwise set) by the caller to indicate that it is now* in use by the dcache.*/void d_instantiate(struct dentry *entry, struct inode * inode){BUG_ON(!list_empty(&entry->d_alias));spin_lock(&dcache_lock);__d_instantiate(entry, inode);spin_unlock(&dcache_lock);security_d_instantiate(entry, inode);}/* the caller must hold dcache_lock */static void __d_instantiate(struct dentry *dentry, struct inode *inode){if (inode)list_add(&dentry->d_alias, &inode->i_dentry);/*将dentry添加到inode的链表中*/dentry->d_inode = inode;        /*保存dentry对应的inode*/fsnotify_d_instantiate(dentry, inode);}

该函数首先调用了d_alloc来创建struct dentry，参数parent为NULL，既然是为根( / )建立dentry，自然没有父对象。接着调用d_instantiate来绑定inode和dentry之间的关系。在sysfs_fill_super函数执行的最后，将sysfs_root保存到了dentry->d_fsdata。可见，在sysfs中用sysfs_dirent来表示目录，但是对于VFS，还是要使用dentry来表示目录。

8.2.3 do_remount_sb

下列代码位于fs/super.c。

/***	do_remount_sb - asks filesystem to change mount options.*	@sb:	superblock in question*	@flags:	numeric part of options*	@data:	the rest of options*      @force: whether or not to force the change**	Alters the mount options of a mounted file system.*/int do_remount_sb(struct super_block *sb, int flags, void *data, int force){int retval;int remount_rw;#ifdef CONFIG_BLOCKif (!(flags & MS_RDONLY) && bdev_read_only(sb->s_bdev))return -EACCES;#endifif (flags & MS_RDONLY)acct_auto_close(sb);shrink_dcache_sb(sb);fsync_super(sb);/* If we are remounting RDONLY and current sb is read/write,make sure there are no rw files opened */if ((flags & MS_RDONLY) && !(sb->s_flags & MS_RDONLY)) {if (force)mark_files_ro(sb);else if (!fs_may_remount_ro(sb))return -EBUSY;retval = vfs_dq_off(sb, 1);if (retval < 0 && retval != -ENOSYS)return -EBUSY;}remount_rw = !(flags & MS_RDONLY) && (sb->s_flags & MS_RDONLY);if (sb->s_op->remount_fs) {lock_super(sb);retval = sb->s_op->remount_fs(sb, &flags, data);unlock_super(sb);if (retval)return retval;}sb->s_flags = (sb->s_flags & ~MS_RMT_MASK) | (flags & MS_RMT_MASK);if (remount_rw)vfs_dq_quota_on_remount(sb);return 0;}

这个函数用来修改挂在选项，这个函数就不分析了，不是重点。

8.2.4simple_set_mnt

下列函数位于fs/namespace.c。

void simple_set_mnt(struct vfsmount *mnt, struct super_block *sb){mnt->mnt_sb = sb;mnt->mnt_root = dget(sb->s_root);}

该函数设置了vfsmount的superblock和根dentry。

8.2.5 小结

这里，对sysfs的注册过程做一个总结。sysfs_init函数调用过程示意图如下：在整个过程中，先后使用和创建了许多struct第一，根据file_system_type表示的sysfs文件系统的类型注册了sysfs。第二，建立了vfsmount。第三，创建了超级块super_block。第四，根据sysfs_dirent表示的根目录，建立了inode。最后，根据刚才建立的inode创建了dentry。除了sysfs_dirent，其他5个结构体都是VFS中基本的数据结构，而sysfs_dirent则是特定于sysfs文件系统的数据结构。

8.3 创建目录

在前面的描述中，使用sysfs_create_dir在sysfs下建立一个目录。我们来看下这个函数是如何来建立目录的。下列代码位于fs/sysfs/dir.c。

/***	sysfs_create_dir - create a directory for an object.*	@kobj:		object we're creating directory for.*/int sysfs_create_dir(struct kobject * kobj){struct sysfs_dirent *parent_sd, *sd;int error = 0;BUG_ON(!kobj);if (kobj->parent)	/*如果有parent，获取parent对应的sys目录*/parent_sd = kobj->parent->sd;else				/*没有则是在sys根目录*/parent_sd = &sysfs_root;error = create_dir(kobj, parent_sd, kobject_name(kobj), &sd);if (!error)kobj->sd = sd;return error;}

函数中，首先获取待建目录的父sysfs_dirent，然后将它作为参数 来调用create_dir函数。很明显，就是要在父sysfs_dirent下建立新的sysfs_dirent，新建立的sysfs_dirent将保存到参数sd中。下列代码位于fs/sysfs/dir.c。

static int create_dir(struct kobject *kobj, struct sysfs_dirent *parent_sd,const char *name, struct sysfs_dirent **p_sd){umode_t mode = S_IFDIR| S_IRWXU | S_IRUGO | S_IXUGO;struct sysfs_addrm_cxt acxt;struct sysfs_dirent *sd;int rc;/* allocate */	/*分配sysfs_dirent并初始化*/sd = sysfs_new_dirent(name, mode, SYSFS_DIR);if (!sd)return -ENOMEM;sd->s_dir.kobj = kobj;	 /*保存kobject对象*//* link in */sysfs_addrm_start(&acxt, parent_sd);/*寻找父sysfs_dirent对应的inode*/rc = sysfs_add_one(&acxt, sd);	/*检查父sysfs_dirent下是否已有有该sysfs_dirent，没有则添加到父sysfs_dirent中*/sysfs_addrm_finish(&acxt);		/*收尾工作*/if (rc == 0)		/*rc为0表示创建成功*/*p_sd = sd;elsesysfs_put(sd);	/*增加引用计数*/return rc;}

这里要注意一下mode变量，改变了使用了宏定义SYSFS_DIR，这个就表示要创建的是一个目录。mode还有几个宏定义可以使用，如下：

#define SYSFS_KOBJ_ATTR			0x0002#define SYSFS_KOBJ_BIN_ATTR		0x0004#define SYSFS_KOBJ_LINK			0x0008#define SYSFS_COPY_NAME			(SYSFS_DIR | SYSFS_KOBJ_LINK)

8.3.1 sysfs_new_dirent

在create_dir函数中，首先调用了sysfs_new_dirent来建立一个新的sysfs_dirent结构体。下列代码位于fs/sysfs/dir.c。

struct sysfs_dirent *sysfs_new_dirent(const char *name, umode_t mode, int type){char *dup_name = NULL;struct sysfs_dirent *sd;if (type & SYSFS_COPY_NAME) {name = dup_name = kstrdup(name, GFP_KERNEL);if (!name)return NULL;}/*分配sysfs_dirent并清0*/sd = kmem_cache_zalloc(sysfs_dir_cachep, GFP_KERNEL);if (!sd)goto err_out1;if (sysfs_alloc_ino(&sd->s_ino))	/*分配ID号*/goto err_out2;atomic_set(&sd->s_count, 1);atomic_set(&sd->s_active, 0);sd->s_name = name;sd->s_mode = mode;sd->s_flags = type;return sd;err_out2:kmem_cache_free(sysfs_dir_cachep, sd);err_out1:kfree(dup_name);return NULL;}

8.3.2 有关sysfs_dirent中的联合体

分配了sysfs_dirent后，设置了该结构中的联合体数据。先来看下联合体中的四个数据结构。

/* type-specific structures for sysfs_dirent->s_* union members */struct sysfs_elem_dir {struct kobject		*kobj;/* children list starts here and goes through sd->s_sibling */struct sysfs_dirent	*children;};struct sysfs_elem_symlink {struct sysfs_dirent    *target_sd;};struct sysfs_elem_attr {struct attribute    *attr;struct sysfs_open_dirent *open;};struct sysfs_elem_bin_attr {struct bin_attribute    *bin_attr;struct hlist_head    buffers;};

根据sysfs_dirent所代表的类型不同，也就是目录，synlink，属性文件和bin文件，将分别使用该联合体中相应的struct。在本例中要创建的是目录，自然使用sysfs_elem_dir结构体，然后保存了kobject对象。在8.4和8.5中我们将分别看到sysfs_elem_attr和sysfs_elem_symlink的使用。

8.3.3 sysfs_addrm_start

在获取了父sysfs_dirent，调用sysfs_addrm_start来获取与之对应的inode。下列代码位于fs/sysfs/dir.c。

/***	sysfs_addrm_start - prepare for sysfs_dirent add/remove*	@acxt: pointer to sysfs_addrm_cxt to be used*	@parent_sd: parent sysfs_dirent**	This function is called when the caller is about to add or*	remove sysfs_dirent under @parent_sd.  This function acquires*	sysfs_mutex, grabs inode for @parent_sd if available and lock*	i_mutex of it.  @acxt is used to keep and pass context to*	other addrm functions.**	LOCKING:*	Kernel thread context (may sleep).  sysfs_mutex is locked on*	return.  i_mutex of parent inode is locked on return if*	available.*/void sysfs_addrm_start(struct sysfs_addrm_cxt *acxt,struct sysfs_dirent *parent_sd){struct inode *inode;memset(acxt, 0, sizeof(*acxt));acxt->parent_sd = parent_sd;/* Lookup parent inode.  inode initialization is protected by* sysfs_mutex, so inode existence can be determined by* looking up inode while holding sysfs_mutex.*/mutex_lock(&sysfs_mutex);/*根据parent_sd来寻找父inode*/inode = ilookup5(sysfs_sb, parent_sd->s_ino, sysfs_ilookup_test,parent_sd);if (inode) {WARN_ON(inode->i_state & I_NEW);/* parent inode available */acxt->parent_inode = inode;		/*保存找到的父inode*//* sysfs_mutex is below i_mutex in lock hierarchy.* First, trylock i_mutex.  If fails, unlock* sysfs_mutex and lock them in order.*/if (!mutex_trylock(&inode->i_mutex)) {mutex_unlock(&sysfs_mutex);mutex_lock(&inode->i_mutex);mutex_lock(&sysfs_mutex);}}}/** Context structure to be used while adding/removing nodes.*/struct sysfs_addrm_cxt {struct sysfs_dirent    *parent_sd;struct inode        *parent_inode;struct sysfs_dirent    *removed;int            cnt;};

注意形参sysfs_addrm_cxt，该结构作用是临时存放数据。

8.3.4 sysfs_add_one

下列代码位于fs/sysfs/dir.c。

/***	sysfs_add_one - add sysfs_dirent to parent*	@acxt: addrm context to use*	@sd: sysfs_dirent to be added**	Get @acxt->parent_sd and set sd->s_parent to it and increment*	nlink of parent inode if @sd is a directory and link into the*	children list of the parent.**	This function should be called between calls to*	sysfs_addrm_start() and sysfs_addrm_finish() and should be*	passed the same @acxt as passed to sysfs_addrm_start().**	LOCKING:*	Determined by sysfs_addrm_start().**	RETURNS:*	0 on success, -EEXIST if entry with the given name already*	exists.*/int sysfs_add_one(struct sysfs_addrm_cxt *acxt, struct sysfs_dirent *sd){int ret;ret = __sysfs_add_one(acxt, sd);if (ret == -EEXIST) {char *path = kzalloc(PATH_MAX, GFP_KERNEL);WARN(1, KERN_WARNING"sysfs: cannot create duplicate filename '%s'\n",(path == NULL) ? sd->s_name :strcat(strcat(sysfs_pathname(acxt->parent_sd, path), "/"),sd->s_name));kfree(path);}return ret;}/***    __sysfs_add_one - add sysfs_dirent to parent without warning*    @acxt: addrm context to use*    @sd: sysfs_dirent to be added**    Get @acxt->parent_sd and set sd->s_parent to it and increment*    nlink of parent inode if @sd is a directory and link into the*    children list of the parent.**    This function should be called between calls to*    sysfs_addrm_start() and sysfs_addrm_finish() and should be*    passed the same @acxt as passed to sysfs_addrm_start().**    LOCKING:*    Determined by sysfs_addrm_start().**    RETURNS:*    0 on success, -EEXIST if entry with the given name already*    exists.*/int __sysfs_add_one(struct sysfs_addrm_cxt *acxt, struct sysfs_dirent *sd){/*查找该parent_sd下有无将要建立的sd,没有返回NULL*/if (sysfs_find_dirent(acxt->parent_sd, sd->s_name))return -EEXIST;sd->s_parent = sysfs_get(acxt->parent_sd);    /*设置父sysfs_dirent，增加父sysfs_dirent的引用计数*/if (sysfs_type(sd) == SYSFS_DIR && acxt->parent_inode)    /*如果要创建的是目录或文件，并且有父inode*/inc_nlink(acxt->parent_inode);    /*inode->i_nlink加1*/acxt->cnt++;sysfs_link_sibling(sd);return 0;}/***    sysfs_find_dirent - find sysfs_dirent with the given name*    @parent_sd: sysfs_dirent to search under*    @name: name to look for**    Look for sysfs_dirent with name @name under @parent_sd.**    LOCKING:*    mutex_lock(sysfs_mutex)**    RETURNS:*    Pointer to sysfs_dirent if found, NULL if not.*/struct sysfs_dirent *sysfs_find_dirent(struct sysfs_dirent *parent_sd,const unsigned char *name){struct sysfs_dirent *sd;for (sd = parent_sd->s_dir.children; sd; sd = sd->s_sibling)if (!strcmp(sd->s_name, name))return sd;return NULL;}/***    sysfs_link_sibling - link sysfs_dirent into sibling list*    @sd: sysfs_dirent of interest**    Link @sd into its sibling list which starts from*    sd->s_parent->s_dir.children.**    Locking:*    mutex_lock(sysfs_mutex)*/static void sysfs_link_sibling(struct sysfs_dirent *sd){struct sysfs_dirent *parent_sd = sd->s_parent;struct sysfs_dirent **pos;BUG_ON(sd->s_sibling);/* Store directory entries in order by ino.  This allows* readdir to properly restart without having to add a* cursor into the s_dir.children list.*//*children链表根据s_ino按升序排列，现在将sd插入到正确的儿子链表中*/for (pos = &parent_sd->s_dir.children; *pos; pos = &(*pos)->s_sibling) {if (sd->s_ino < (*pos)->s_ino)break;}/*插入链表*/sd->s_sibling = *pos;*pos = sd;}

该函数直接调用了__sysfs_add_one，后者先调用sysfs_find_dirent来查找该parent_sd下有无该的sysfs_dirent，如果没有，则设置创建好的新的sysfs_dirent的s_parent字段。也就是将新的sysfs_dirent添加到父sys_dirent中。接着调用sysfs_link_sibling函数，将新建的sysfs_dirent添加到sd->s_parent->s_dir.children链表中。

8.3.5 sysfs_addrm_finish

下列代码位于fs/sysfs/dir.c。

/***	sysfs_addrm_finish - finish up sysfs_dirent add/remove*	@acxt: addrm context to finish up**	Finish up sysfs_dirent add/remove.  Resources acquired by*	sysfs_addrm_start() are released and removed sysfs_dirents are*	cleaned up.  Timestamps on the parent inode are updated.**	LOCKING:*	All mutexes acquired by sysfs_addrm_start() are released.*/void sysfs_addrm_finish(struct sysfs_addrm_cxt *acxt){/* release resources acquired by sysfs_addrm_start() */mutex_unlock(&sysfs_mutex);if (acxt->parent_inode) {struct inode *inode = acxt->parent_inode;/* if added/removed, update timestamps on the parent */if (acxt->cnt)inode->i_ctime = inode->i_mtime = CURRENT_TIME;/*更新父inode的时间*/mutex_unlock(&inode->i_mutex);iput(inode);}/* kill removed sysfs_dirents */while (acxt->removed) {struct sysfs_dirent *sd = acxt->removed;acxt->removed = sd->s_sibling;sd->s_sibling = NULL;sysfs_drop_dentry(sd);sysfs_deactivate(sd);unmap_bin_file(sd);sysfs_put(sd);}}

该函数结束了添加sysfs_dirent的工作，这个就不多做说明了。至此，添加一个目录的工作已经完成了，添加目录的工作其实就是创建了一个新的sysfs_dirent，并把它添加到父sysfs_dirent中。下面我们看下如何添加属性文件。

8.4 创建属性文件

添加属性文件使用sysfs_create_file函数。下列函数位于fs/sysfs/file.c。

/***	sysfs_create_file - create an attribute file for an object.*	@kobj:	object we're creating for.*	@attr:	attribute descriptor.*/int sysfs_create_file(struct kobject * kobj, const struct attribute * attr){BUG_ON(!kobj || !kobj->sd || !attr);return sysfs_add_file(kobj->sd, attr, SYSFS_KOBJ_ATTR);}int sysfs_add_file(struct sysfs_dirent *dir_sd, const struct attribute *attr,int type){return sysfs_add_file_mode(dir_sd, attr, type, attr->mode);}int sysfs_add_file_mode(struct sysfs_dirent *dir_sd,const struct attribute *attr, int type, mode_t amode){umode_t mode = (amode & S_IALLUGO) | S_IFREG;struct sysfs_addrm_cxt acxt;struct sysfs_dirent *sd;int rc;/*分配sysfs_dirent并初始化*/sd = sysfs_new_dirent(attr->name, mode, type);if (!sd)return -ENOMEM;sd->s_attr.attr = (void *)attr;sysfs_addrm_start(&acxt, dir_sd);    /*寻找父sysfs_dirent对应的inode*/rc = sysfs_add_one(&acxt, sd);        /*检查父sysfs_dirent下是否已有有该sysfs_dirent，没有则创建*/sysfs_addrm_finish(&acxt);            /*收尾工作*/if (rc)            /*0表示创建成功*/sysfs_put(sd);return rc;}

sysfs_create_file用参数SYSFS_KOBJ_ATTR（表示建立属性文件）来调用了sysfs_add_file，后者又直接调用了sysfs_add_file_mode。sysfs_add_file_mode函数的执行和8.3节的create_dir函数非常类似，只不过它并没有保存kobject对象，也就是说该sysfs_dirent并没有一个对应的kobject对象。需要注意的是，这里是建立属性文件，因此使用了联合体中的结构体s_attr。

8.5 创建symlink

最后，来看下symlink的建立。

/***	sysfs_create_link - create symlink between two objects.*	@kobj:	object whose directory we're creating the link in.*	@target:	object we're pointing to.*	@name:		name of the symlink.*/int sysfs_create_link(struct kobject *kobj, struct kobject *target,const char *name){return sysfs_do_create_link(kobj, target, name, 1);}static int sysfs_do_create_link(struct kobject *kobj, struct kobject *target,const char *name, int warn){struct sysfs_dirent *parent_sd = NULL;struct sysfs_dirent *target_sd = NULL;struct sysfs_dirent *sd = NULL;struct sysfs_addrm_cxt acxt;int error;BUG_ON(!name);if (!kobj)    /*kobj为空，表示在sysyfs跟目录下建立symlink*/parent_sd = &sysfs_root;else        /*有父sysfs_dirent*/parent_sd = kobj->sd;error = -EFAULT;if (!parent_sd)goto out_put;/* target->sd can go away beneath us but is protected with* sysfs_assoc_lock.  Fetch target_sd from it.*/spin_lock(&sysfs_assoc_lock);if (target->sd)target_sd = sysfs_get(target->sd);    、/*获取目标对象的sysfs_dirent*/spin_unlock(&sysfs_assoc_lock);error = -ENOENT;if (!target_sd)goto out_put;error = -ENOMEM;/*分配sysfs_dirent并初始化*/sd = sysfs_new_dirent(name, S_IFLNK|S_IRWXUGO, SYSFS_KOBJ_LINK);if (!sd)goto out_put;sd->s_symlink.target_sd = target_sd;/*保存目标sysfs_dirent*/target_sd = NULL;    /* reference is now owned by the symlink */sysfs_addrm_start(&acxt, parent_sd);/*寻找父sysfs_dirent对应的inode*/if (warn)error = sysfs_add_one(&acxt, sd);/*检查父sysfs_dirent下是否已有有该sysfs_dirent，没有则创建*/elseerror = __sysfs_add_one(&acxt, sd);sysfs_addrm_finish(&acxt);            /*收尾工作*/if (error)goto out_put;return 0;out_put:sysfs_put(target_sd);sysfs_put(sd);return error;}

这个函数的执行也和8.3节的create_dir函数非常类似。其次，symlink同样没有对应的kobject对象。因为sysfs_dirent表示的是symlink，这里使用了联合体中的s_symlink。同时设置了s_symlink.target_sd指向的目标sysfs_dirent为参数targed_sd。

8.6 小结

本节首先对syfs这一特殊的文件系统的注册过程进行了分析。接着对目录，属性文件和symlink的建立进行了分析。这三者的建立过程基本一致，但是目录有kobject对象，而剩余两个没有。其次，这三者的每个sysfs_dirent中，都使用了自己的联合体数据。

9 总结

本文首先对sysfs的核心数据kobject，kset等数据结构做出了分析，正是通过它们才能向用户空间呈现出设备驱动模型。接着，以/sys/bus目录的建立为例，来说明如何通过kobject和kset来建立该bus目录。随后，介绍了驱动模型中表示总线，设备和驱动的三个数据结构。然后，介绍了platform总线(bus/platform)的注册，再介绍了虚拟的platform设备(devices/platform)的添加过程。之后 ，以spi主控制器的platform设备为例，介绍了该platform设备和相应的驱动的注册过程。最后，介绍了底层sysfs文件系统的注册过程和如何建立目录，属性文件和symlink的过程。更新说明：2012.09.14 在6.2.9中，添加分析 bus_for_each_drv。