sysfs分析

 

 

 

Linux设备模型——设备驱动模型和sysfs文件系统解读

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]#ls
block     class     devices   fs        module
bus       dev       firmware  kernel    power

block：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。

 

<strong><span style="font-family:System;font-size:12px;">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;
}；</span></strong>

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]#ls
devices            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);
    else
        dev_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_kset

pdev->dev-.parent = &platform_bus

pdev->dev.bus = &platform_bus_type

set_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]#ll
lrwxrwxrwx    1 root     root             0 Jan  1 00:29 bus -> ../../../bus/platform
lrwxrwxrwx    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 modalias
drwxr-xr-x    2 root     root             0 Jan  1 00:29 power
drwxr-xr-x    3 root     root             0 Jan  1 00:00 spi0.0
drwxr-xr-x    3 root     root             0 Jan  1 00:00 spi0.1
lrwxrwxrwx    1 root     root             0 Jan  1 00:29 spi_master:spi0 -> ../../../class/spi_master/spi0
lrwxrwxrwx    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.0
lrwxrwxrwx    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.0
lrwxrwxrwx    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 driver
lrwxrwxrwx    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]#ls
bind           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;
        }
    } else
        goto 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);
    } else
        set_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_BLOCK
    if (!(flags & MS_RDONLY) && bdev_read_only(sb->s_bdev))
        return -EACCES;
#endif
    if (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;
    else
        sysfs_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*/
    else
        error = __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。