作者：刘昊昱 

博客：http://blog.csdn.net/liuhaoyutz

内核版本：3.10.1

一、device定义

device结构体定义在include/linux/device.h文件中：

 598/**
 599 * struct device - The basic device structure
 600 * @parent: The device's "parent" device, the device to which it is attached.
 601 *      In most cases, a parent device is some sort of bus or host
 602 *      controller. If parent is NULL, the device, is a top-level device,
 603 *      which is not usually what you want.
 604 * @p:      Holds the private data of the driver core portions of the device.
 605 *      See the comment of the struct device_private for detail.
 606 * @kobj:   A top-level, abstract class from which other classes are derived.
 607 * @init_name:  Initial name of the device.
 608 * @type:   The type of device.
 609 *      This identifies the device type and carries type-specific
 610 *      information.
 611 * @mutex:  Mutex to synchronize calls to its driver.
 612 * @bus:    Type of bus device is on.
 613 * @driver: Which driver has allocated this
 614 * @platform_data: Platform data specific to the device.
 615 *      Example: For devices on custom boards, as typical of embedded
 616 *      and SOC based hardware, Linux often uses platform_data to point
 617 *      to board-specific structures describing devices and how they
 618 *      are wired.  That can include what ports are available, chip
 619 *      variants, which GPIO pins act in what additional roles, and so
 620 *      on.  This shrinks the "Board Support Packages" (BSPs) and
 621 *      minimizes board-specific #ifdefs in drivers.
 622 * @power:  For device power management.
 623 *      See Documentation/power/devices.txt for details.
 624 * @pm_domain:  Provide callbacks that are executed during system suspend,
 625 *      hibernation, system resume and during runtime PM transitions
 626 *      along with subsystem-level and driver-level callbacks.
 627 * @pins:   For device pin management.
 628 *      See Documentation/pinctrl.txt for details.
 629 * @numa_node:  NUMA node this device is close to.
 630 * @dma_mask:   Dma mask (if dma'ble device).
 631 * @coherent_dma_mask: Like dma_mask, but for alloc_coherent mapping as not all
 632 *      hardware supports 64-bit addresses for consistent allocations
 633 *      such descriptors.
 634 * @dma_parms:  A low level driver may set these to teach IOMMU code about
 635 *      segment limitations.
 636 * @dma_pools:  Dma pools (if dma'ble device).
 637 * @dma_mem:    Internal for coherent mem override.
 638 * @archdata:   For arch-specific additions.
 639 * @of_node:    Associated device tree node.
 640 * @acpi_node:  Associated ACPI device node.
 641 * @devt:   For creating the sysfs "dev".
 642 * @id:     device instance
 643 * @devres_lock: Spinlock to protect the resource of the device.
 644 * @devres_head: The resources list of the device.
 645 * @knode_class: The node used to add the device to the class list.
 646 * @class:  The class of the device.
 647 * @groups: Optional attribute groups.
 648 * @release:    Callback to free the device after all references have
 649 *      gone away. This should be set by the allocator of the
 650 *      device (i.e. the bus driver that discovered the device).
 651 *
 652 * At the lowest level, every device in a Linux system is represented by an
 653 * instance of struct device. The device structure contains the information
 654 * that the device model core needs to model the system. Most subsystems,
 655 * however, track additional information about the devices they host. As a
 656 * result, it is rare for devices to be represented by bare device structures;
 657 * instead, that structure, like kobject structures, is usually embedded within
 658 * a higher-level representation of the device.
 659 */
 660struct device {
 661    struct device       *parent;
 662
 663    struct device_private   *p;
 664
 665    struct kobject kobj;
 666    const char      *init_name; /* initial name of the device */
 667    const struct device_type *type;
 668
 669    struct mutex        mutex;  /* mutex to synchronize calls to
 670                     * its driver.
 671                     */
 672
 673    struct bus_type *bus;       /* type of bus device is on */
 674    struct device_driver *driver;   /* which driver has allocated this
 675                       device */
 676    void        *platform_data; /* Platform specific data, device
 677                       core doesn't touch it */
 678    struct dev_pm_info  power;
 679    struct dev_pm_domain    *pm_domain;
 680
 681#ifdef CONFIG_PINCTRL
 682    struct dev_pin_info *pins;
 683#endif
 684
 685#ifdef CONFIG_NUMA
 686    int     numa_node;  /* NUMA node this device is close to */
 687#endif
 688    u64     *dma_mask;  /* dma mask (if dma'able device) */
 689    u64     coherent_dma_mask;/* Like dma_mask, but for
 690                         alloc_coherent mappings as
 691                         not all hardware supports
 692                         64 bit addresses for consistent
 693                         allocations such descriptors. */
 694
 695    struct device_dma_parameters *dma_parms;
 696
 697    struct list_head    dma_pools;  /* dma pools (if dma'ble) */
 698
 699    struct dma_coherent_mem *dma_mem; /* internal for coherent mem
 700                         override */
 701#ifdef CONFIG_CMA
 702    struct cma *cma_area;       /* contiguous memory area for dma
 703                       allocations */
 704#endif
 705    /* arch specific additions */
 706    struct dev_archdata archdata;
 707
 708    struct device_node  *of_node; /* associated device tree node */
 709    struct acpi_dev_node    acpi_node; /* associated ACPI device node */
 710
 711    dev_t           devt;   /* dev_t, creates the sysfs "dev" */
 712    u32         id; /* device instance */
 713
 714    spinlock_t      devres_lock;
 715    struct list_head    devres_head;
 716
 717    struct klist_node   knode_class;
 718    struct class        *class;
 719    const struct attribute_group **groups;  /* optional groups */
 720
 721    void    (*release)(struct device *dev);
 722    struct iommu_group  *iommu_group;
 723};

我们来看一下device结构体的几个重要成员变量。

parent，device的父设备，即device依附的设备，通常父设备是某种类型的bus或host controller。如果parent的值是NULL，那么device将是一个顶层设备，这通常不是我们想要的。

p，device的私有数据，它是struct device_private结构体变量，这个结构体定义在drivers/base/base.h文件中，其内容如下：

 55/**
 56 * struct device_private - structure to hold the private to the driver core portions of the device structure.
 57 *
 58 * @klist_children - klist containing all children of this device
 59 * @knode_parent - node in sibling list
 60 * @knode_driver - node in driver list
 61 * @knode_bus - node in bus list
 62 * @deferred_probe - entry in deferred_probe_list which is used to retry the
 63 *  binding of drivers which were unable to get all the resources needed by
 64 *  the device; typically because it depends on another driver getting
 65 *  probed first.
 66 * @driver_data - private pointer for driver specific info.  Will turn into a
 67 * list soon.
 68 * @device - pointer back to the struct class that this structure is
 69 * associated with.
 70 *
 71 * Nothing outside of the driver core should ever touch these fields.
 72 */
 73struct device_private {
 74    struct klist klist_children;
 75    struct klist_node knode_parent;
 76    struct klist_node knode_driver;
 77    struct klist_node knode_bus;
 78    struct list_head deferred_probe;
 79    void *driver_data;
 80    struct device *device;
 81};

kobj，device对应的kobject。

init_name，device的名字。

type，device类型。

bus，device所属的bus。

driver，device所关联的device_driver。

platform_data，平台相关数据。

二、devics相关sysfs结构初始化

Linux设备模型中，device相关的sysfs结构的初始化是由devices_init 函数完成的，该函数定义在drivers/base/core.c文件中，其内容如下：

1395int __init devices_init(void)
1396{
1397    devices_kset = kset_create_and_add("devices", &device_uevent_ops, NULL);
1398    if (!devices_kset)
1399        return -ENOMEM;
1400    dev_kobj = kobject_create_and_add("dev", NULL);
1401    if (!dev_kobj)
1402        goto dev_kobj_err;
1403    sysfs_dev_block_kobj = kobject_create_and_add("block", dev_kobj);
1404    if (!sysfs_dev_block_kobj)
1405        goto block_kobj_err;
1406    sysfs_dev_char_kobj = kobject_create_and_add("char", dev_kobj);
1407    if (!sysfs_dev_char_kobj)
1408        goto char_kobj_err;
1409
1410    return 0;
1411
1412 char_kobj_err:
1413    kobject_put(sysfs_dev_block_kobj);
1414 block_kobj_err:
1415    kobject_put(dev_kobj);
1416 dev_kobj_err:
1417    kset_unregister(devices_kset);
1418    return -ENOMEM;
1419}

1397行，创建devices_kset，它是所有device的集合，它的名字是“devices”，对应/sys/devices目录。

1400行，创建dev_kobj，它的名字是“dev”，对应/sys/dev目录。

1403行，创建sysfs_dev_block_kobj，它的名字是“block”，对应/sys/dev/block目录。

1406行，创建sysfs_dev_char_kobj，它的名字是“char”，对应/sys/dev/char目录。

三、device的注册

注册一个device是通过调用device_register函数完成的，该函数定义在drivers/base/core.c文件中，其内容如下：

1147/**
1148 * device_register - register a device with the system.
1149 * @dev: pointer to the device structure
1150 *
1151 * This happens in two clean steps - initialize the device
1152 * and add it to the system. The two steps can be called
1153 * separately, but this is the easiest and most common.
1154 * I.e. you should only call the two helpers separately if
1155 * have a clearly defined need to use and refcount the device
1156 * before it is added to the hierarchy.
1157 *
1158 * For more information, see the kerneldoc for device_initialize()
1159 * and device_add().
1160 *
1161 * NOTE: _Never_ directly free @dev after calling this function, even
1162 * if it returned an error! Always use put_device() to give up the
1163 * reference initialized in this function instead.
1164 */
1165int device_register(struct device *dev)
1166{
1167    device_initialize(dev);
1168    return device_add(dev);
1169}

1167行，调用device_initialize函数对device结构体进行初始化。该函数定义在drivers/base/core.c文件中，其内容如下：

 675/**
 676 * device_initialize - init device structure.
 677 * @dev: device.
 678 *
 679 * This prepares the device for use by other layers by initializing
 680 * its fields.
 681 * It is the first half of device_register(), if called by
 682 * that function, though it can also be called separately, so one
 683 * may use @dev's fields. In particular, get_device()/put_device()
 684 * may be used for reference counting of @dev after calling this
 685 * function.
 686 *
 687 * All fields in @dev must be initialized by the caller to 0, except
 688 * for those explicitly set to some other value.  The simplest
 689 * approach is to use kzalloc() to allocate the structure containing
 690 * @dev.
 691 *
 692 * NOTE: Use put_device() to give up your reference instead of freeing
 693 * @dev directly once you have called this function.
 694 */
 695void device_initialize(struct device *dev)
 696{
 697    dev->kobj.kset = devices_kset;
 698    kobject_init(&dev->kobj, &device_ktype);
 699    INIT_LIST_HEAD(&dev->dma_pools);
 700    mutex_init(&dev->mutex);
 701    lockdep_set_novalidate_class(&dev->mutex);
 702    spin_lock_init(&dev->devres_lock);
 703    INIT_LIST_HEAD(&dev->devres_head);
 704    device_pm_init(dev);
 705    set_dev_node(dev, -1);
 706}

该函数对device结构体的部分成员进行了初始化。

1168行，调用device_add函数，将device添加到系统中，该函数定义在drivers/base/core.c文件中，其内容如下：

 980/**
 981 * device_add - add device to device hierarchy.
 982 * @dev: device.
 983 *
 984 * This is part 2 of device_register(), though may be called
 985 * separately _iff_ device_initialize() has been called separately.
 986 *
 987 * This adds @dev to the kobject hierarchy via kobject_add(), adds it
 988 * to the global and sibling lists for the device, then
 989 * adds it to the other relevant subsystems of the driver model.
 990 *
 991 * Do not call this routine or device_register() more than once for
 992 * any device structure.  The driver model core is not designed to work
 993 * with devices that get unregistered and then spring back to life.
 994 * (Among other things, it's very hard to guarantee that all references
 995 * to the previous incarnation of @dev have been dropped.)  Allocate
 996 * and register a fresh new struct device instead.
 997 *
 998 * NOTE: _Never_ directly free @dev after calling this function, even
 999 * if it returned an error! Always use put_device() to give up your
1000 * reference instead.
1001 */
1002int device_add(struct device *dev)
1003{
1004    struct device *parent = NULL;
1005    struct kobject *kobj;
1006    struct class_interface *class_intf;
1007    int error = -EINVAL;
1008
1009    dev = get_device(dev);
1010    if (!dev)
1011        goto done;
1012
1013    if (!dev->p) {
1014        error = device_private_init(dev);
1015        if (error)
1016            goto done;
1017    }
1018
1019    /*
1020     * for statically allocated devices, which should all be converted
1021     * some day, we need to initialize the name. We prevent reading back
1022     * the name, and force the use of dev_name()
1023     */
1024    if (dev->init_name) {
1025        dev_set_name(dev, "%s", dev->init_name);
1026        dev->init_name = NULL;
1027    }
1028
1029    /* subsystems can specify simple device enumeration */
1030    if (!dev_name(dev) && dev->bus && dev->bus->dev_name)
1031        dev_set_name(dev, "%s%u", dev->bus->dev_name, dev->id);
1032
1033    if (!dev_name(dev)) {
1034        error = -EINVAL;
1035        goto name_error;
1036    }
1037
1038    pr_debug("device: '%s': %s\n", dev_name(dev), __func__);
1039
1040    parent = get_device(dev->parent);
1041    kobj = get_device_parent(dev, parent);
1042    if (kobj)
1043        dev->kobj.parent = kobj;
1044
1045    /* use parent numa_node */
1046    if (parent)
1047        set_dev_node(dev, dev_to_node(parent));
1048
1049    /* first, register with generic layer. */
1050    /* we require the name to be set before, and pass NULL */
1051    error = kobject_add(&dev->kobj, dev->kobj.parent, NULL);
1052    if (error)
1053        goto Error;
1054
1055    /* notify platform of device entry */
1056    if (platform_notify)
1057        platform_notify(dev);
1058
1059    error = device_create_file(dev, &uevent_attr);
1060    if (error)
1061        goto attrError;
1062
1063    if (MAJOR(dev->devt)) {
1064        error = device_create_file(dev, &devt_attr);
1065        if (error)
1066            goto ueventattrError;
1067
1068        error = device_create_sys_dev_entry(dev);
1069        if (error)
1070            goto devtattrError;
1071
1072        devtmpfs_create_node(dev);
1073    }
1074
1075    error = device_add_class_symlinks(dev);
1076    if (error)
1077        goto SymlinkError;
1078    error = device_add_attrs(dev);
1079    if (error)
1080        goto AttrsError;
1081    error = bus_add_device(dev);
1082    if (error)
1083        goto BusError;
1084    error = dpm_sysfs_add(dev);
1085    if (error)
1086        goto DPMError;
1087    device_pm_add(dev);
1088
1089    /* Notify clients of device addition.  This call must come
1090     * after dpm_sysfs_add() and before kobject_uevent().
1091     */
1092    if (dev->bus)
1093        blocking_notifier_call_chain(&dev->bus->p->bus_notifier,
1094                         BUS_NOTIFY_ADD_DEVICE, dev);
1095
1096    kobject_uevent(&dev->kobj, KOBJ_ADD);
1097    bus_probe_device(dev);
1098    if (parent)
1099        klist_add_tail(&dev->p->knode_parent,
1100                   &parent->p->klist_children);
1101
1102    if (dev->class) {
1103        mutex_lock(&dev->class->p->mutex);
1104        /* tie the class to the device */
1105        klist_add_tail(&dev->knode_class,
1106                   &dev->class->p->klist_devices);
1107
1108        /* notify any interfaces that the device is here */
1109        list_for_each_entry(class_intf,
1110                    &dev->class->p->interfaces, node)
1111            if (class_intf->add_dev)
1112                class_intf->add_dev(dev, class_intf);
1113        mutex_unlock(&dev->class->p->mutex);
1114    }
1115done:
1116    put_device(dev);
1117    return error;
1118 DPMError:
1119    bus_remove_device(dev);
1120 BusError:
1121    device_remove_attrs(dev);
1122 AttrsError:
1123    device_remove_class_symlinks(dev);
1124 SymlinkError:
1125    if (MAJOR(dev->devt))
1126        devtmpfs_delete_node(dev);
1127    if (MAJOR(dev->devt))
1128        device_remove_sys_dev_entry(dev);
1129 devtattrError:
1130    if (MAJOR(dev->devt))
1131        device_remove_file(dev, &devt_attr);
1132 ueventattrError:
1133    device_remove_file(dev, &uevent_attr);
1134 attrError:
1135    kobject_uevent(&dev->kobj, KOBJ_REMOVE);
1136    kobject_del(&dev->kobj);
1137 Error:
1138    cleanup_device_parent(dev);
1139    if (parent)
1140        put_device(parent);
1141name_error:
1142    kfree(dev->p);
1143    dev->p = NULL;
1144    goto done;
1145}

1009行，调用get_device增加device（其实是device.kobj）的引用计数。

1013-1017行，如果dev->p是NULL，则调用device_private_init函数为dev->p分配内存空间并进行初始化，该函数定义在drivers/base/core.c文件中，其内容如下：

 968int device_private_init(struct device *dev)
 969{
 970    dev->p = kzalloc(sizeof(*dev->p), GFP_KERNEL);
 971    if (!dev->p)
 972        return -ENOMEM;
 973    dev->p->device = dev;
 974    klist_init(&dev->p->klist_children, klist_children_get,
 975           klist_children_put);
 976    INIT_LIST_HEAD(&dev->p->deferred_probe);
 977    return 0;
 978}

1024-1027行，如果dev->init_name不为空，则调用dev_set_name设置device的名字（其实是设置device.kobj的名字）。

1030-1031行，如果device的名字为空，并且dev->bus->dev_name不为空，则用dev->bus->dev_name作为device的名字。

1033-1036行，如果device的名字仍然为空，则出错退出。

1040行，取得dev->parent，保存在parent变量中。

1041行，调用get_device_parent函数，该函数的作用是取得dev->kobj.parent，即device对应的kobject的parent kobject。该函数定义在drivers/base/core.c文件中，其内容如下：

 769static struct kobject *get_device_parent(struct device *dev,
 770                     struct device *parent)
 771{
 772    if (dev->class) {
 773        static DEFINE_MUTEX(gdp_mutex);
 774        struct kobject *kobj = NULL;
 775        struct kobject *parent_kobj;
 776        struct kobject *k;
 777
 778#ifdef CONFIG_BLOCK
 779        /* block disks show up in /sys/block */
 780        if (sysfs_deprecated && dev->class == &block_class) {
 781            if (parent && parent->class == &block_class)
 782                return &parent->kobj;
 783            return &block_class.p->subsys.kobj;
 784        }
 785#endif
 786
 787        /*
 788         * If we have no parent, we live in "virtual".
 789         * Class-devices with a non class-device as parent, live
 790         * in a "glue" directory to prevent namespace collisions.
 791         */
 792        if (parent == NULL)
 793            parent_kobj = virtual_device_parent(dev);
 794        else if (parent->class && !dev->class->ns_type)
 795            return &parent->kobj;
 796        else
 797            parent_kobj = &parent->kobj;
 798
 799        mutex_lock(&gdp_mutex);
 800
 801        /* find our class-directory at the parent and reference it */
 802        spin_lock(&dev->class->p->glue_dirs.list_lock);
 803        list_for_each_entry(k, &dev->class->p->glue_dirs.list, entry)
 804            if (k->parent == parent_kobj) {
 805                kobj = kobject_get(k);
 806                break;
 807            }
 808        spin_unlock(&dev->class->p->glue_dirs.list_lock);
 809        if (kobj) {
 810            mutex_unlock(&gdp_mutex);
 811            return kobj;
 812        }
 813
 814        /* or create a new class-directory at the parent device */
 815        k = class_dir_create_and_add(dev->class, parent_kobj);
 816        /* do not emit an uevent for this simple "glue" directory */
 817        mutex_unlock(&gdp_mutex);
 818        return k;
 819    }
 820
 821    /* subsystems can specify a default root directory for their devices */
 822    if (!parent && dev->bus && dev->bus->dev_root)
 823        return &dev->bus->dev_root->kobj;
 824
 825    if (parent)
 826        return &parent->kobj;
 827    return NULL;
 828}

这个函数功能如下：

1、如果dev->class不为NULL，并且dev->parent为NULL

这种情况下系统将为dev->kobj.parent建立一个虚拟上层对象“virtual”，如此，将dev对象加入系统将会在/sys/devices/virtual中产生一个新的目录/sys/devices/virtual/dev->init_name。

2、如果dev->class不为NULL，并且dev->parent也不为NULL

这种情况下，要看dev->parent->class是否为空，如果不为空，则dev->kobj.parent为dev->parent->kobj，即dev父设备的内嵌kobject。

如果dev->parent->class为空，则内核需要在dev->class->p->class_dirs.list中查找是否有满足如下条件的kobject对象k：k->parent等于&parent->kobj。如果找到满足条件的k，那么dev->kobj.parent就是dev->parent->kobj，即dev父设备的内嵌kobject。如果没有满足条件的k，那么就需要调用class_dir_create_and_add函数，重新生成一个kobject对象作为dev->kobj的父kobject。

3、如果dev->class为NULL，并且dev->parent不为NULL

这种情况下，dev->kobj.parent为dev->parent->kobj，即dev父设备的内嵌kobject。也就是说dev对象对应的目录将建立在dev->parent->kobj对应的目录下。

4、如果dev->class为NULL，并且dev->parent也为NULL

这种情况下，如果dev->bus->dev_root不为NULL，则指定dev->kobj.parent为dev->bus->dev_root->kobj。

我们回到device_add函数：

1042-1043行，如果通过get_device_parent函数取得了父kobject，则用它来设置dev->kobj.parent。

1051行，调用kobject_add将dev->kobj注册到sysfs系统中。

1059行，调用device_create_file创建uevent属性文件。uevent_attr定义在drivers/base/core.c文件中，其内容如下：

 403static struct device_attribute uevent_attr =
 404    __ATTR(uevent, S_IRUGO | S_IWUSR, show_uevent, store_uevent);

device_attribute结构体定义在include/linux/device.h文件中，其内容如下：

 478/* interface for exporting device attributes */
 479struct device_attribute {
 480    struct attribute    attr;
 481    ssize_t (*show)(struct device *dev, struct device_attribute *attr,
 482            char *buf);
 483    ssize_t (*store)(struct device *dev, struct device_attribute *attr,
 484             const char *buf, size_t count);
 485};

device_create_file定义在drivers/base/core.c文件中，其内容如下：

 563/**
 564 * device_create_file - create sysfs attribute file for device.
 565 * @dev: device.
 566 * @attr: device attribute descriptor.
 567 */
 568int device_create_file(struct device *dev,
 569               const struct device_attribute *attr)
 570{
 571    int error = 0;
 572
 573    if (dev) {
 574        WARN(((attr->attr.mode & S_IWUGO) && !attr->store),
 575            "Attribute %s: write permission without 'store'\n",
 576            attr->attr.name);
 577        WARN(((attr->attr.mode & S_IRUGO) && !attr->show),
 578            "Attribute %s: read permission without 'show'\n",
 579            attr->attr.name);
 580        error = sysfs_create_file(&dev->kobj, &attr->attr);
 581    }
 582
 583    return error;
 584}

1063行，通过MAJOR(dev->devt)取得设备对应的主设备号。

1064行，如果设备的主设备号不为0，调用device_create_file创建dev属性文件。devt_attr定义在drivers/base/core.c文件中，其内容如下：

 557static struct device_attribute devt_attr =
 558    __ATTR(dev, S_IRUGO, show_dev, NULL);

1068行，调用device_create_sys_dev_entry(dev)函数，该函数定义在drivers/base/core.c文件中，其内容如下：

 943static int device_create_sys_dev_entry(struct device *dev)
 944{
 945    struct kobject *kobj = device_to_dev_kobj(dev);
 946    int error = 0;
 947    char devt_str[15];
 948
 949    if (kobj) {
 950        format_dev_t(devt_str, dev->devt);
 951        error = sysfs_create_link(kobj, &dev->kobj, devt_str);
 952    }
 953
 954    return error;
 955}

945行，调用device_to_dev_kobj(dev)，该函数定义在drivers/base/core.c文件中，其内容如下：

 920/**
 921 * device_to_dev_kobj - select a /sys/dev/ directory for the device
 922 * @dev: device
 923 *
 924 * By default we select char/ for new entries.  Setting class->dev_obj
 925 * to NULL prevents an entry from being created.  class->dev_kobj must
 926 * be set (or cleared) before any devices are registered to the class
 927 * otherwise device_create_sys_dev_entry() and
 928 * device_remove_sys_dev_entry() will disagree about the presence of
 929 * the link.
 930 */
 931static struct kobject *device_to_dev_kobj(struct device *dev)
 932{
 933    struct kobject *kobj;
 934
 935    if (dev->class)
 936        kobj = dev->class->dev_kobj;
 937    else
 938        kobj = sysfs_dev_char_kobj;
 939
 940    return kobj;
 941}

从该函数的注释可以看出，该函数的作用是返回device对应的/sys/dev/下的一个目录。该函数的返回值是一个kobject，一个kobject在sysfs系统中对应一个目录。如果dev->class不为NULL，该函数返回dev->class->dev_kobj；如果dev->class为NULL，该函数返回sysfs_dev_char_kobj，它对应/sys/dev/char目录。

回到device_create_sys_dev_entry函数：

950行，调用format_dev_t宏，该宏定义在include/linux/kdev_t.h文件中，其内容如下：

16#define format_dev_t(buffer, dev)                   \
17    ({                              \
18        sprintf(buffer, "%u:%u", MAJOR(dev), MINOR(dev));   \
19        buffer;                         \
20    })

可以看到，该宏的作用是把“主设备号：次设备号”字符串写入buffer变量中。

951行，调用sysfs_create_link(kobj, &dev->kobj, devt_str)创建一个链接。该函数定义在fs/sysfs/symlink.c文件中，其内容如下：

117/**
118 *  sysfs_create_link - create symlink between two objects.
119 *  @kobj:  object whose directory we're creating the link in.
120 *  @target:    object we're pointing to.
121 *  @name:      name of the symlink.
122 */
123int sysfs_create_link(struct kobject *kobj, struct kobject *target,
124              const char *name)
125{
126    return sysfs_do_create_link(kobj, target, name, 1);
127}

由注释可以看出，第一个参数是我们将要创建的链接所在的目录，第二个参数是链接指向的源，第三个参数是链接的名字。

回到device_create_sys_dev_entry函数，这里我们举个例子，如果dev的设备号major=20, minor=1，设备名为dev->init_name,那么，

如果dev->class为NULL，则产生的链接文件为/sys/dev/char/20:1，链接指向的文件为/sys/devices/dev->init_name。

如果dev->class不为NULL，则产生的链接文件将位于dev->class->dev_kobj所对应的目录下，链接指向的文件为/sys/devices/virual/dev->init_name。

回到device_add函数：

1072行，如果device主设备号不为0，还会调用devtmpfs_create_node(dev)函数在/dev目录下自动生成一个设备节点。在Linux早期，/dev下的设备节点是通过mknod命令手动添加的，现在通过devtmpfs文件系统，就可以在调用device_register函数注册设备时自动向/dev目录下添加设备节点，该节点的名字就是dev->init_name。

1075行，调用device_add_class_symlinks(dev)函数，该函数定义在drivers/base/core.c文件中，创建dev->class相关的链接。这个函数我们不仔细分析了。

1078行，调用device_add_attrs(dev)函数，该函数定义在drivers/base/core.c文件中，该函数创建与dev->class相关的一些属性文件，这个函数我们不仔细分析了。

1081行，调用bus_add_device(dev)函数，该函数定义在drivers/base/bus.c文件中，该函数在/sys/bus/devices目录下创建一个链接文件，指向/sys/devices/dev->init_name。

1097行，调用bus_probe_device(dev)函数为新添加的device探测匹配的device_driver。该函数定义在drivers/base/bus.c文件中，其内容如下：

 523/**
 524 * bus_probe_device - probe drivers for a new device
 525 * @dev: device to probe
 526 *
 527 * - Automatically probe for a driver if the bus allows it.
 528 */
 529void bus_probe_device(struct device *dev)
 530{
 531    struct bus_type *bus = dev->bus;
 532    struct subsys_interface *sif;
 533    int ret;
 534
 535    if (!bus)
 536        return;
 537
 538    if (bus->p->drivers_autoprobe) {
 539        ret = device_attach(dev);
 540        WARN_ON(ret < 0);
 541    }
 542
 543    mutex_lock(&bus->p->mutex);
 544    list_for_each_entry(sif, &bus->p->interfaces, node)
 545        if (sif->add_dev)
 546            sif->add_dev(dev, sif);
 547    mutex_unlock(&bus->p->mutex);
 548}

531行，取得device所依附的bus。

538-539行，如果bus->p->drivers_autoprobe值为1，调用device_attach(dev)函数，该函数定义在drivers/base/dd.c文件中，其内容如下：

398/**
399 * device_attach - try to attach device to a driver.
400 * @dev: device.
401 *
402 * Walk the list of drivers that the bus has and call
403 * driver_probe_device() for each pair. If a compatible
404 * pair is found, break out and return.
405 *
406 * Returns 1 if the device was bound to a driver;
407 * 0 if no matching driver was found;
408 * -ENODEV if the device is not registered.
409 *
410 * When called for a USB interface, @dev->parent lock must be held.
411 */
412int device_attach(struct device *dev)
413{
414    int ret = 0;
415
416    device_lock(dev);
417    if (dev->driver) {
418        if (klist_node_attached(&dev->p->knode_driver)) {
419            ret = 1;
420            goto out_unlock;
421        }
422        ret = device_bind_driver(dev);
423        if (ret == 0)
424            ret = 1;
425        else {
426            dev->driver = NULL;
427            ret = 0;
428        }
429    } else {
430        ret = bus_for_each_drv(dev->bus, NULL, dev, __device_attach);
431        pm_request_idle(dev);
432    }
433out_unlock:
434    device_unlock(dev);
435    return ret;
436}

417-429行，如果dev->driver不为空，表明device已经与相应的device_driver进行了绑定，这种情况下只需要调用device_bind_driver函数在sysfs文件系统中建立device与其device_driver的相互关联。

430行，调用bus_for_each_drv函数，该函数定义在drivers/base/bus.c文件中，其内容如下：

 417/**
 418 * bus_for_each_drv - driver iterator
 419 * @bus: bus we're dealing with.
 420 * @start: driver to start iterating on.
 421 * @data: data to pass to the callback.
 422 * @fn: function to call for each driver.
 423 *
 424 * This is nearly identical to the device iterator above.
 425 * We iterate over each driver that belongs to @bus, and call
 426 * @fn for each. If @fn returns anything but 0, we break out
 427 * and return it. If @start is not NULL, we use it as the head
 428 * of the list.
 429 *
 430 * NOTE: we don't return the driver that returns a non-zero
 431 * value, nor do we leave the reference count incremented for that
 432 * driver. If the caller needs to know that info, it must set it
 433 * in the callback. It must also be sure to increment the refcount
 434 * so it doesn't disappear before returning to the caller.
 435 */
 436int bus_for_each_drv(struct bus_type *bus, struct device_driver *start,
 437             void *data, int (*fn)(struct device_driver *, void *))
 438{
 439    struct klist_iter i;
 440    struct device_driver *drv;
 441    int error = 0;
 442
 443    if (!bus)
 444        return -EINVAL;
 445
 446    klist_iter_init_node(&bus->p->klist_drivers, &i,
 447                 start ? &start->p->knode_bus : NULL);
 448    while ((drv = next_driver(&i)) && !error)
 449        error = fn(drv, data);
 450    klist_iter_exit(&i);
 451    return error;
 452}

448-449行，这个while循环依次遍历bus->p->klist_drivers中的所有device_driver，对于每个device_driver，调用fn(drv, data)函数，看该device_driver是否支持该device。如果匹配成功，fn函数返回非0值。参数传递过来的fn函数是__device_attach，该函数定义在drivers/base/dd.c文件中，其内容如下：

388static int __device_attach(struct device_driver *drv, void *data)
389{
390    struct device *dev = data;
391
392    if (!driver_match_device(drv, dev))
393        return 0;
394
395    return driver_probe_device(drv, dev);
396}

392行，调用driver_match_device(drv, dev)函数，检查device_driver是否支持该device，该函数定义在drivers/base/base.h文件中，其内容如下：

116static inline int driver_match_device(struct device_driver *drv,
117                      struct device *dev)
118{
119    return drv->bus->match ? drv->bus->match(dev, drv) : 1;
120}

如果定义了drv->bus->match函数，则调用该函数，否则返回1。

回到__device_attach函数：

395行，调用driver_probe_device(drv, dev)函数，该函数定义在drivers/base/dd.c文件中，其内容如下：

360/**
361 * driver_probe_device - attempt to bind device & driver together
362 * @drv: driver to bind a device to
363 * @dev: device to try to bind to the driver
364 *
365 * This function returns -ENODEV if the device is not registered,
366 * 1 if the device is bound successfully and 0 otherwise.
367 *
368 * This function must be called with @dev lock held.  When called for a
369 * USB interface, @dev->parent lock must be held as well.
370 */
371int driver_probe_device(struct device_driver *drv, struct device *dev)
372{
373    int ret = 0;
374
375    if (!device_is_registered(dev))
376        return -ENODEV;
377
378    pr_debug("bus: '%s': %s: matched device %s with driver %s\n",
379         drv->bus->name, __func__, dev_name(dev), drv->name);
380
381    pm_runtime_barrier(dev);
382    ret = really_probe(dev, drv);
383    pm_request_idle(dev);
384
385    return ret;
386}

375行，调用device_is_registered函数判断device是否已经在sysfs系统中注册过，如果还没有注册过，则返回ENODEV，退出。该函数定义在include/linux/device.h文件中，其内容如下：

 787static inline int device_is_registered(struct device *dev)
 788{
 789    return dev->kobj.state_in_sysfs;
 790}

382行，调用really_probe(dev, drv)函数，该函数定义在drivers/base/dd.c文件中，其内容如下：

265static int really_probe(struct device *dev, struct device_driver *drv)
266{
267    int ret = 0;
268
269    atomic_inc(&probe_count);
270    pr_debug("bus: '%s': %s: probing driver %s with device %s\n",
271         drv->bus->name, __func__, drv->name, dev_name(dev));
272    WARN_ON(!list_empty(&dev->devres_head));
273
274    dev->driver = drv;
275
276    /* If using pinctrl, bind pins now before probing */
277    ret = pinctrl_bind_pins(dev);
278    if (ret)
279        goto probe_failed;
280
281    if (driver_sysfs_add(dev)) {
282        printk(KERN_ERR "%s: driver_sysfs_add(%s) failed\n",
283            __func__, dev_name(dev));
284        goto probe_failed;
285    }
286
287    if (dev->bus->probe) {
288        ret = dev->bus->probe(dev);
289        if (ret)
290            goto probe_failed;
291    } else if (drv->probe) {
292        ret = drv->probe(dev);
293        if (ret)
294            goto probe_failed;
295    }
296
297    driver_bound(dev);
298    ret = 1;
299    pr_debug("bus: '%s': %s: bound device %s to driver %s\n",
300         drv->bus->name, __func__, dev_name(dev), drv->name);
301    goto done;
302
303probe_failed:
304    devres_release_all(dev);
305    driver_sysfs_remove(dev);
306    dev->driver = NULL;
307    dev_set_drvdata(dev, NULL);
308
309    if (ret == -EPROBE_DEFER) {
310        /* Driver requested deferred probing */
311        dev_info(dev, "Driver %s requests probe deferral\n", drv->name);
312        driver_deferred_probe_add(dev);
313    } else if (ret != -ENODEV && ret != -ENXIO) {
314        /* driver matched but the probe failed */
315        printk(KERN_WARNING
316               "%s: probe of %s failed with error %d\n",
317               drv->name, dev_name(dev), ret);
318    } else {
319        pr_debug("%s: probe of %s rejects match %d\n",
320               drv->name, dev_name(dev), ret);
321    }
322    /*
323     * Ignore errors returned by ->probe so that the next driver can try
324     * its luck.
325     */
326    ret = 0;
327done:
328    atomic_dec(&probe_count);
329    wake_up(&probe_waitqueue);
330    return ret;
331}

287-295行，如果定义了dev->bus->probe函数，则调用该函数；如果没有定义dev->bus->probe函数，但是定义了drv->probe函数，则调用drv->probe函数。这里，我们一般写Linux驱动程序时都要实现的probe函数就会被调用了。

297行，调用driver_bound(dev)函数，该函数定义在drivers/base/dd.c文件中，其内容如下：

182static void driver_bound(struct device *dev)
183{
184    if (klist_node_attached(&dev->p->knode_driver)) {
185        printk(KERN_WARNING "%s: device %s already bound\n",
186            __func__, kobject_name(&dev->kobj));
187        return;
188    }
189
190    pr_debug("driver: '%s': %s: bound to device '%s'\n", dev_name(dev),
191         __func__, dev->driver->name);
192
193    klist_add_tail(&dev->p->knode_driver, &dev->driver->p->klist_devices);
194
195    /*
196     * Make sure the device is no longer in one of the deferred lists and
197     * kick off retrying all pending devices
198     */
199    driver_deferred_probe_del(dev);
200    driver_deferred_probe_trigger();
201
202    if (dev->bus)
203        blocking_notifier_call_chain(&dev->bus->p->bus_notifier,
204                         BUS_NOTIFY_BOUND_DRIVER, dev);
205}

193行，调用klist_add_tail函数将device加入到device_driver的driver->p->klist_devices链表中。

至此，我们一步一步回退
driver_bound->really_probe ->driver_probe_device->__device_attach->device_attach->bus_probe_device->device_add->device_register

这样，device_register函数我们就分析完了，也就清楚了device的注册过程。