底层软件 | Linux设备驱动模型和sysfs文件系统-技术圈


          
           
            
             原文：https://blog.csdn.net/i37193429/article/details/136491517

Linux设备驱动模型和sysfs文件系统

Linux内核在2.6版本中引入设备驱动模型，简化了驱动程序的编写。Linux设备驱动模型包含设备(device)、总线(bus)、类(class)和驱动(driver)，它们之间相互关联。其中设备(device)和驱动(driver)通过总线(bus)绑定在一起。

Linux内核中，分别用bus_type、device_driver和device结构来描述总线、驱动和设备，结构体定义详见linux/device.h。设备和对应的驱动必须依附于同一种总线，因此device_driver和device结构中都包含struct bus_type指针。

Linux sysfs是一个虚拟的文件系统，它把连接在系统上的设备和总线组织成为一个分级的文件，可以由用户空间存取，向用户空间导出内核数据结构以及它们的属性。
sysfs展示出设备驱动模型中各个组件的层次关系，某个系统上的sysfs顶层目录展示如下：

/sys$ ll
total 0
drwxr-xr-x   2 root root 0 Aug 20 15:27 block/
drwxr-xr-x  29 root root 0 Aug 20 15:27 bus/
drwxr-xr-x  61 root root 0 Aug 20 15:27 class/
drwxr-xr-x   4 root root 0 Aug 20 15:27 dev/
drwxr-xr-x  14 root root 0 Aug 20 15:27 devices/
drwxr-xr-x   4 root root 0 Aug 20 15:27 firmware/
drwxr-xr-x   8 root root 0 Aug 20 15:27 fs/
drwxr-xr-x   2 root root 0 Sep  2 17:08 hypervisor/
drwxr-xr-x   8 root root 0 Aug 20 15:27 kernel/
drwxr-xr-x 147 root root 0 Aug 20 15:27 module/
drwxr-xr-x   2 root root 0 Aug 20 15:27 power/

重要子目录介绍：

block: 包含所有的块设备，如ram，sda等
bus: 包含系统中所有的总线类型，如pci，usb，i2c等
class: 包含系统中的设备类型，如input，pci_bus，mmc_host等
dev: 包含两个子目录：char和block，分别存放字符设备和块设备的主次设备号(major:minor)，指向/sys/devices目录下的设备
devices:包含系统所有的设备

sysfs中显示的每一个对象都对应一个kobject结构（完整定义位于linux/kobject.h，结构内部包含一个parent指针），而另一个相联系的结构为kset。kset是嵌入相同类型结构的kobject对象的集合。内核用kobject、kset和parent之间的关系将各个对象连接起来组成一个分层的结构体系，从而与模型化的子系统相匹配。（有机会详细介绍）

sysfs中能清晰地看出device、driver和bus的相互联系，以某系统上pci总线上的igb驱动为例。/sys/bus/pci/下存在devices和drivers两个目录，分别包含了依附于pci总线上的设备和驱动。进入igb驱动目录，可以发现存在指向设备的链接。

/sys/bus/pci/drivers/igb$ ll
total 0
...   0 Sep  2 17:08 0000:07:00.0 -> ../../../../devices/pci0000:00/0000:00:1c.4/0000:07:00.0/
...   0 Sep  2 17:08 0000:07:00.1 -> ../../../../devices/pci0000:00/0000:00:1c.4/0000:07:00.1/
...

对应地，在/sys/devices/目录下，可以看到设备存在一个指向igb的driver项：

/sys/devices/pci0000:00/0000:00:1c.4/0000:07:00.0$ ll
total 0
...
lrwxrwxrwx  1 root root       0 Aug 20 15:27 driver -> ../../../../bus/pci/drivers/igb/
...

同样地，/sys/bus/pci/devices目录下可以找到指向同样设备的一个链接：

/sys/bus/pci/devices$ ll
total 0
...
...   0 Aug 20 15:27 0000:07:00.0 -> ../../../devices/pci0000:00/0000:00:1c.4/0000:07:00.0/
...   0 Aug 20 15:27 0000:07:00.1 -> ../../../devices/pci0000:00/0000:00:1c.4/0000:07:00.1/
...

对于早期的Linux内核（2.6版本以前）来说，通常在驱动代码中xxx_driver注册过程中调用probe()函数来对设备进行初始化。
引入Linux设备驱动模型下，设备和驱动可以分开注册，依赖总线完成相互绑定。系统每注册一个设备的时候，会寻找与之匹配的驱动；相反，系统每注册一个驱动的时候，会寻找与之匹配的设备。这个过程中，设备和驱动的匹配工作由总线完成。

下文中将会用关键的内核源码（基于linux 5.2.14 Kernel）说明驱动和设备间匹配机制的实现，分析的过程中以platform总线为例。
platform总线是一种虚拟的总线，与之相对应的是PCI、I2C、SPI等实体总线。引入虚拟platform总线是为了解决某些设备无法直接依附在现有实体总线上的问题，例如SoC系统中集成的独立外设控制器，挂接在SoC内存空间的外设等等。

platform总线的注册

platform总线作为Linux的基础总线，在内核启动阶段便完成了注册，注册的入口函数为platform_bus_init()。内核启动阶段调用该函数的路径为：

start_kernel()                  --> arch_call_rest_init()[last step in start_kernel] 
    --> rest_init()             --> kernel_init() 
    --> kernel_init_freeable()  --> do_basic_setup() 
    --> driver_init()           --> platform_bus_init()

Linux内核中定义了platform_bus_type结构体来描述platform总线，同时也定义了设备platform_bus，用于管理所有挂载在platform总线下的设备，定义如下：

struct bus_type platform_bus_type = {
    .name               = 'platform',
    .dev_groups         = platform_dev_groups,
    .match              = platform_match,
    .uevent             = platform_uevent,
    .dma_configure      = platform_dma_configure,
    .pm                 = &platform_dev_pm_ops,
};

struct device platform_bus = {
    .init_name  = 'platform',
};

platform_bus_init()对platform总线的注册主要分为两步：

device_register(&platform_bus)
bus_register(&platform_bus_type)。

int __init platform_bus_init(void)
{
    int error;

    /* Clear up early_platform_device_list, then only remain head_list */
    early_platform_cleanup();

    /* register platform_bus device (platform_bus is also regarded as a device) */
    error = device_register(&platform_bus);
    if (error) {
        put_device(&platform_bus);
        return error;
    }
    /* Main process to register platform_bus */
    error =  bus_register(&platform_bus_type);
    if (error)
        device_unregister(&platform_bus);
    of_platform_register_reconfig_notifier();
    return error;
}

device_register(&platform_bus)

/*****  drivers/base/core.c  *****/
int device_register(struct device *dev)
{
    device_initialize(dev);     // init device structure
    return device_add(dev);     // add device to device hierarchy
}

device_initialize()：对struct device中基本成员进行初始化，包括kobject、struct device_private、struct mutex等。
device_add(dev)：将platform总线也作为一个设备platform_bus注册到驱动模型中，重要的函数包括device_create_file()、device_add_class_symlinks()、bus_add_device()、bus_probe_device()等，下文中对设备注册的介绍一节，将对这个函数做更详细的介绍。device_add(&platform_bus)主要功能是完成/sys/devices/platform目录的建立。

bus_register(&platform_bus_type)

/*****  drivers/base/bus.c  *****/
int bus_register(struct bus_type *bus)
{
    struct subsys_private *priv;
    struct lock_class_key *key = &bus->lock_key;

    priv = kzalloc(sizeof(struct subsys_private), GFP_KERNEL);

    priv->bus = bus;
    bus->p = priv;

    BLOCKING_INIT_NOTIFIER_HEAD(&priv->bus_notifier);

    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;

    /* Register kset (subsys) */
    retval = kset_register(&priv->subsys);

    retval = bus_create_file(bus, &bus_attr_uevent);

    /* Setup 'devices' and 'drivers' subfolder under 'platform' */
    priv->devices_kset = kset_create_and_add('devices', NULL,
            &priv->subsys.kobj);

    priv->drivers_kset = kset_create_and_add('drivers', NULL,
            &priv->subsys.kobj);

    INIT_LIST_HEAD(&priv->interfaces);
    __mutex_init(&priv->mutex, 'subsys mutex', key);
    klist_init(&priv->klist_devices, klist_devices_get, klist_devices_put);
    klist_init(&priv->klist_drivers, NULL, NULL);

    /* bus_create_file(bus, &bus_attr_drivers_probe);     BUS_ATTR_WO(drivers_probe)
     * bus_create_file(bus, &bus_attr_drivers_autoprobe); BUS_ATTR_RW(drivers_autoprobe)
     * Add two attribute files for current bus /sys/bus/platform 
     */
    retval = add_probe_files(bus);

    retval = bus_add_groups(bus, bus->bus_groups);

    return 0;
}

bus_register(&platform_bus_type)将总线platform注册到Linux的总线系统中，主要完成了subsystem的注册，对struct subsys_private结构进行了初始化，具体包括：

platform_bus_type->p->drivers_autoprobe = 1
对struct kset类型成员subsys进行初始化，作为子系统中kobject对象的parent。kset本身也包含kobject对象，在sysfs中也表现为一个目录，即/sys/bus/platform。
建立struct kset类型的drivers_kset和devices_kset，作为总线下挂载的所有驱动和设备的集合，sysfs中表现为/sys/bus/platform/drivers和/sys/bus/platform/devices。
初始化链表klist_drivers和klist_devices，将总线下的驱动和设备分别链接在一起。
增加probe文件，对应/sys/bus/platform目录的文件drivers_autoprobe和drivers_probe。

注册完成后platform_bus_type结构重要的成员列举如下：

struct bus_type platform_bus_type = {
    .name           = 'platform',
    .dev_groups     = platform_dev_groups,
    .match          = platform_match,
    .uevent         = platform_uevent,
    .dma_configure  = platform_dma_configure,
    .pm             = &platform_dev_pm_ops,
    .p  (struct subsys_private) = {
        .bus        = &platform_bus_type,
        .subsys (struct kset) = {
            .kobj = {
                .name   = “platform”
                .kref->refcount->refs       = 1,    // kset_init()
                INIT_LIST_HEAD(.entry),
                .state_in_sysfs             = 0,
                .state_add_uevent_sent      = 1,    // kset_register()
                .state_remove_uevent_sent   = 0,
                .state_initialized          = 1,
                .kset   = bus_kset,                 // attached to /sys/bus/
                .ktype= bus_ktype,

                .parent = bus_kset->kobj,
                .sd (kernfs_node) = {               // create_dir, kobject_add_internal
                    .parent = bus_kset->kobj->sd,
                    .dir.root = bus_kset->kobj->sd->dir.root,
                    .ns = NULL,
                    .priv = .kobj
                }
            }
            INIT_LIST_HEAD(&k->list);
            spin_lock_init(&k->list_lock);
        }
        /* key point for driver to autoprobe device, set in bus_register() */
        . drivers_autoprobe = 1
        klist_init(&priv->klist_devices, klist_devices_get, klist_devices_put);
        klist_init(&priv->klist_drivers, NULL, NULL);
        .devices_kset = kset_create_and_add('devices', NULL, &.p->subsys.kobj);
        /* .drivers_kset = kset_create_and_add('drivers', NULL, &.p->subsys.kobj) */
        .drivers_kset = {
            .kobj = {
                .name = “drivers”,
                .parent = &.subsys.kobj,
                .ktype = &kset_ktype,
                .kset = NULL,

                .kref->refcount->refs       = 1,    // kset_init
                INIT_LIST_HEAD(.entry),
                .state_in_sysfs             = 0,
                .state_add_uevent_sent      = 1,    // kset_register
                .state_remove_uevent_sent   = 0,
                .state_initialized          = 1,

                .sd = {                             // create_dir: /sys/bus/platform/drivers
                    /* kobject_add_internal */
                    .parent = &.subsys.kobj.sd,
                    .dir.root = = &.subsys.kobj.sd->dir.root
                        .ns = NULL,
                    .priv = .kobj
                }
            }
            INIT_LIST_HEAD(.list);
            spin_lock_init(.list_lock);
            .uevent_ops = NULL,
        }
    }
};

platform驱动的注册

Linux内核中对依赖于platform总线的驱动定义了platform_driver结构体，内部封装了前述的struct device_driver。

struct platform_driver {
    int (*probe)(struct platform_device *);
    int (*remove)(struct platform_device *);
    void (*shutdown)(struct platform_device *);
    int (*suspend)(struct platform_device *, pm_message_t state);
    int (*resume)(struct platform_device *);
    struct device_driver driver;
    const struct platform_device_id *id_table;
    bool prevent_deferred_probe;
};

为了更好地说明platform驱动的注册过程，以驱动globalfifo_driver为实例，globalfifo_driver结构成员定义如下：

static struct platform_driver globalfifo_driver = {
    .driver = {
        .name   = 'globalfifo_platform',
        .owner  = THIS_MODULE,
    },
    .probe  = globalfifo_probe,
    .remove = globalfifo_remove,
};

globalfifo_driver注册的入口函数为platform_driver_register(&globalfifo_driver)，具体实现为__platform_driver_register(&globalfifo_driver, THIS_MODULE)。
该函数会对struct device_driver的bus、probe、remove等回调函数进行初始化，紧接着调用driver_register(&globalfifo_driver->driver)。

/*****  drivers/base/platform.c  *****/
/**
 * __platform_driver_register - register a driver for platform-level devices
 * @drv: platform driver structure
 * @owner: owning module/driver
 */
int __platform_driver_register(struct platform_driver *drv,
        struct module *owner)
{
    drv->driver.owner       = owner;
    drv->driver.bus         = &platform_bus_type;
    drv->driver.probe       = platform_drv_probe;
    drv->driver.remove      = platform_drv_remove;
    drv->driver.shutdown    = platform_drv_shutdown;

    return driver_register(&drv->driver);
}

driver_register(&(globalfifo_driver.driver))

/*****  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;

    if (!drv->bus->p) {
        pr_err('Driver '%s' was unable to register with bus_type '%s' because the bus was not initialized.\n',
                drv->name, drv->bus->name);
        return -EINVAL;
    }

    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);

    ret = bus_add_driver(drv);

    ret = driver_add_groups(drv, drv->groups);

    kobject_uevent(&drv->p->kobj, KOBJ_ADD);

    return ret;
}

driver_register(&(globalfifo_driver.driver))主要的工作包括：

确认驱动依附的总线platform_bus已经被注册并初始化（必要条件）。
对probe、remove、shutdown等回调函数初始化进行判断，保证总线和驱动上相应的函数只能存在一个。
driver_find()查找总线上是否已存在当前驱动的同名驱动。
bus_add_driver(&(globalfifo_driver.driver))，将驱动注册到总线上，下文详述。
发起KOBJ_ADD类型uevent，指示驱动已经添加完成，TODO。

bus_add_driver(&(globalfifo_driver.driver))

/*****  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);

    priv = kzalloc(sizeof(*priv), GFP_KERNEL);

    klist_init(&priv->klist_devices, NULL, NULL);
    priv->driver = drv;
    drv->p = priv;
    priv->kobj.kset = bus->p->drivers_kset;
    error = kobject_init_and_add(&priv->kobj, &driver_ktype, NULL,
            '%s', drv->name);

    klist_add_tail(&priv->knode_bus, &bus->p->klist_drivers);

    /* Entrance to match device: try to bind driver to devices */
    if (drv->bus->p->drivers_autoprobe) {
        error = driver_attach(drv);
    }
    module_add_driver(drv->owner, drv);

    error = driver_create_file(drv, &driver_attr_uevent);
    error = driver_add_groups(drv, bus->drv_groups);

    if (!drv->suppress_bind_attrs) {
        error = add_bind_files(drv);
    }
    return 0;
}

bus_add_driver(&(globalfifo_driver.driver))的主要工作包括：

为struct device_driver中结构struct driver_private动态分配空间，并完成后者kobject对象初始化。对应地，在/sys/bus/platform/drivers下建立目录globalfifo_platform。
初始化klist_devices链表，用来维护驱动相关联的设备。对应sysfs中在每个驱动目录下关联的设备。
klist_add_tail()将当前驱动加入到总线对应的klist_drivers链表中。
如果总线使能drivers_autoprobe，将调用driver_attach()尝试匹配设备。下文中将详述此过程。
module_add_driver(drv->owner, drv)通过sysfs_create_link()，在globalfifo_platform目录下新建module项指向/sys/module/globalfifo_platform。同时，也在/sys/module/globalfifo_platform/目录下新建driver目录，建立bus->name:drv->name 链接到/sys/bus/platform/drivers/globalfifo_platform。
uevent设置。

初始化后globalfifo_driver结构主要的成员列举如下：

static struct platform_driver globalfifo_driver = {
    .driver = {
        .name       = 'globalfifo_platform',
        .owner      = THIS_MODULE,
        .bus        = &platform_bus_type,
        .probe      = platform_drv_probe
        .remove     = platform_drv_remove,
        .shutdown   = platform_drv_shutdown,
        .p (struct driver_private)  = {
            .driver = & globalfifo_driver.driver,
            klist_init(&.klist_devices, NULL, NULL);
            klist_add_tail(&priv->knode_bus, &bus->p->klist_drivers);

            .kobj = {
                .kset = platform_bus_type->p->drivers_kset,
                .ktype = driver_ktype,

                .kref->refcount->refs       = 1,    // kset_init
                INIT_LIST_HEAD(.entry),
                .state_in_sysfs             = 1,
                .state_add_uevent_sent      = 0,
                .state_remove_uevent_sent   = 0,
                .state_initialized          = 1,
                .name       = 'globalfifo_platform',
                .parent     = platform_bus_type->p->drivers_kset->kobj,
            }
        }
    },
    .probe  = globalfifo_probe,
    .remove = globalfifo_remove,
};

driver_attach(&(globalfifo_driver.driver))

/*****  drivers/base/dd.c  *****/
/**
 * driver_attach - try to bind driver to devices.
 * @drv: driver.
 */
int driver_attach(struct device_driver *drv)
{
    return bus_for_each_dev(drv->bus, NULL, drv, __driver_attach);
}

/*****  drivers/base/bus.c  *****/
/**
 * 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.
 */
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 || !bus->p)
        return -EINVAL;

    klist_iter_init_node(&bus->p->klist_devices, &i,
            (start ? &start->p->knode_bus : NULL));
    while (!error && (dev = next_device(&i)))
        error = fn(dev, data);
    klist_iter_exit(&i);
    return error;
}

driver_attach()函数找到驱动依附的总线信息，遍历总线上链表klist_devices得到当前总线上存在的设备，然后调用__driver_attach(dev, drv)函数，尝试将驱动和设备绑定。
__driver_attach(dev, drv)函数包含两个主要的部分：

driver_match_device(drv, dev) : 尝试将驱动和设备匹配，返回值指示是否能匹配。
device_driver_attach(drv, dev): 将驱动和设备绑定。

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

    /*
     * 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.
     */
    ret = driver_match_device(drv, dev);
    if (ret == 0) {
        /* no match */
        return 0;
    } else if (ret == -EPROBE_DEFER) {
        dev_dbg(dev, 'Device match requests probe deferral\n');
        driver_deferred_probe_add(dev);
    } else if (ret < 0) {
        dev_dbg(dev, 'Bus failed to match device: %d', ret);
        return ret;
    }   /* ret > 0 means positive match */

    ... ...

    device_driver_attach(drv, dev);
    return 0;
}

driver_match_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_match_device(drv, dev)回调drv->bus->match()函数，对于platform_bus为platform_match()。
platform_match()函数会依次尝试如下几种方式：

driver_override有效时，尝试将驱动名字和driver_override匹配
基于设备树风格的匹配
基于ACPI风格的匹配
匹配ID表
匹配platform_device设备名和驱动的名字

/**
 * platform_match - bind platform device to platform driver.
 * @dev: device.
 * @drv: driver.
 */
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);

    /* When driver_override is set, only bind to the matching driver */
    if (pdev->driver_override)
        return !strcmp(pdev->driver_override, drv->name);

    /* Attempt an OF style match first */
    if (of_driver_match_device(dev, drv))
        return 1;

    /* Then try ACPI style match */
    if (acpi_driver_match_device(dev, drv))
        return 1;

    /* Then try to match against the id table */
    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);
}

device_driver_attach(drv, dev)

/**
 * device_driver_attach - attach a specific driver to a specific device
 * @drv: Driver to attach
 * @dev: Device to attach it to
 */
int device_driver_attach(struct device_driver *drv, struct device *dev)
{
    int ret = 0;

    __device_driver_lock(dev, dev->parent);

    /*
     * If device has been removed or someone has already successfully
     * bound a driver before us just skip the driver probe call.
     */
    if (!dev->p->dead && !dev->driver)
        ret = driver_probe_device(drv, dev);

    __device_driver_unlock(dev, dev->parent);

    return ret;
}

在驱动和设备匹配成功之后，便将驱动和设备进行绑定。调用driver_probe_device(drv, dev)完成此工作，进一步调用really_probe(dev, drv)。

/**
 * 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
 */
int driver_probe_device(struct device_driver *drv, struct device *dev)
{
    int ret = 0;

    if (!device_is_registered(dev))
        return -ENODEV;

    ... ...
    if (initcall_debug)
        ret = really_probe_debug(dev, drv);
    else
        ret = really_probe(dev, drv);
    ... ...
    return ret;
}

really_probe(dev, drv)

/*****  drivers/base/dd.c  *****/
static int really_probe(struct device *dev, struct device_driver *drv)
{
    dev->driver = drv;
    ... ...
    driver_sysfs_add(dev);
    ... ...
    /* Routine to probe device */
    if (dev->bus->probe) {
        ret = dev->bus->probe(dev);
        if (ret)
            goto probe_failed;
    } else if (drv->probe) {
        ret = drv->probe(dev);
        if (ret)
            goto probe_failed;
    }
    ... ...
    driver_bound(dev);
}

really_probe(dev, drv)主要完成的工作包括：

将设备struct device中driver指针指向globalfifo_driver->driver。
driver_sysfs_add(dev)完成sysfs中设备和驱动的链接，包括在驱动目录下建立到设备的链接，和在设备目录下建立到驱动的链接。
设备probe函数的调用：优先使用platform_device->bus->probe函数，其次使用platform_driver->probe函数。对于globalfifo_driver，会回调globalfifo_probe()，完成设备的初始化。
driver_bound(dev)将设备添加到驱动维护的设备链表中，并发起KOBJ_BIND事件。

platform设备的注册

最后，对设备的注册过程进行简要梳理。
和驱动类似，Linux内核中对依赖于platform总线的设备也定义了特有的结构：platform_device，内部封装了struct device结构。

struct platform_device {
    const char  *name;
    int     id;
    bool        id_auto;
    struct device   dev;
    u32     num_resources;
    struct resource *resource;

    const struct platform_device_id *id_entry;
    char *driver_override; /* Driver name to force a match */

    /* MFD cell pointer */
    struct mfd_cell *mfd_cell;

    /* arch specific additions */
    struct pdev_archdata    archdata;
};

与globalfifo_driver相对应，同样定义globalfifo_device结构体，成员定义如下：

static struct platform_device globalfifo_device = {
    .name       = 'globalfifo_platform',
    .id         = -1,
};

对设备globalfifo_device进行注册的入口函数为platform_device_register(&globalfifo_device)。

int platform_device_register(struct platform_device *pdev)
{
    device_initialize(&pdev->dev);
    arch_setup_pdev_archdata(pdev);
    return platform_device_add(pdev);
}

其中device_initialize(&pdev->dev)在第一节platform_bus注册中也提到过，主要对struct device中基本成员进行初始化，包括kobject、struct device_private、struct mutex等。着重介绍platform_device_add(pdev)。

platform_device_add(&globalfifo_device)

int platform_device_add(struct platform_device *pdev)
{
    int i, ret;

    if (!pdev->dev.parent)
        pdev->dev.parent = &platform_bus;

    pdev->dev.bus = &platform_bus_type;

    switch (pdev->id) {
        default:
            dev_set_name(&pdev->dev, '%s.%d', pdev->name,  pdev->id);
            break;
        case PLATFORM_DEVID_NONE:
            dev_set_name(&pdev->dev, '%s', pdev->name);
            break;
        case PLATFORM_DEVID_AUTO:
            /*
             * Automatically allocated device ID. We mark it as such so
             * that we remember it must be freed, and we append a suffix
             * to avoid namespace collision with explicit IDs.
             */
            ret = ida_simple_get(&platform_devid_ida, 0, 0, GFP_KERNEL);
            pdev->id = ret;
            pdev->id_auto = true;
            dev_set_name(&pdev->dev, '%s.%d.auto', pdev->name, pdev->id);
            break;
    }
    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);

        p = r->parent;
        if (!p) {
            if (resource_type(r) == IORESOURCE_MEM)
                p = &iomem_resource;
            else if (resource_type(r) == IORESOURCE_IO)
                p = &ioport_resource;
        }

        if (p) {
            ret = insert_resource(p, r);
        }
    }

    ret = device_add(&pdev->dev);
    if (ret == 0)
        return ret;
    ... ...
}

platform_device_add(&globalfifo_device)主要工作如下：

对globalfifo_device.dev.parent和globalfifo_device->dev.bus初始化，分别指向platform_bus和platform_bus_type。
globalfifo_device.dev.kobj->name初始化为globalfifo_device.name ('globalfifo_platform')。
调用device_add(&globalfifo_device.dev)添加设备。

device_add(&globalfifo_device.dev)

int device_add(struct device *dev)
{
    struct device *parent;
    struct kobject *kobj;
    int error = -EINVAL;

    /* This will incr the ref_count */
    dev = get_device(dev);

    /*  Init dev->p->device = dev  */
    if (!dev->p)
        error = device_private_init(dev);

    /* if init_name exists, use it to initialize dev.kobj->name */
    if (dev->init_name) {
        dev_set_name(dev, '%s', dev->init_name);
        dev->init_name = NULL;
    }

    /* subsystems can specify simple device enumeration */
    if (!dev_name(dev) && dev->bus && dev->bus->dev_name)
        dev_set_name(dev, '%s%u', dev->bus->dev_name, dev->id);

    /* Return ERROR if dev name is not specified */
    if (!dev_name(dev)) {
        error = -EINVAL;
        goto name_error;
    }

    ... ...
    parent = get_device(dev->parent);
    /* get_device_parent(dev, parent) --> platform_bus.kobj */
    kobj = get_device_parent(dev, parent);
    if (kobj)
        dev->kobj.parent = kobj;

    ... ...
    /* first, register with generic layer. */
    /* we require the name to be set before, and pass NULL */
    error = kobject_add(&dev->kobj, dev->kobj.parent, NULL);

    /* notify platform of device entry */
    error = device_platform_notify(dev, KOBJ_ADD);

    error = device_create_file(dev, &dev_attr_uevent);

    error = device_add_class_symlinks(dev);
    error = device_add_attrs(dev);

    /* Main Entrance to add device into existing bus */
    error = bus_add_device(dev);

    error = dpm_sysfs_add(dev);
    device_pm_add(dev);

    /* Create related node in devfs */
    if (MAJOR(dev->devt)) {
        error = device_create_file(dev, &dev_attr_dev);

        error = device_create_sys_dev_entry(dev);

        devtmpfs_create_node(dev);
    }
    ... ...

    kobject_uevent(&dev->kobj, KOBJ_ADD);

    /* Try to find driver to bind this device */
    bus_probe_device(dev);

    ... ...

}

主要工作如下：

globalfifo_device.dev.kobj.parent初始化为&platform_bus.kobj。
kobject_add()函数初始化globalfifo_device.dev.kobj对象，在sysfs中建立相关的目录，例如/sys/devices/platform/globalfifo_platform。
bus_add_device(&globalfifo_device.dev)：将globalfifo_device注册到总线系统里，并建立sysfs的相关目录：总线系统中建立到设备的链接，同时也在设备目录下建立到总线的subsystem链接。
bus_probe_device(dev)：尝试在总线上寻找可以绑定的驱动。下文详细介绍。

globalfifo_devices初步初始化后主要成员列举如下：

static struct platform_device globalfifo_device = {
    .name  = 'globalfifo_platform',
    .id    = -1,
    .dev   = {
        .parent   = &platform_bus,
        .bus     = &platform_bus_type,
        .p = {
            .device = & globalfifo_device.dev,

            INIT_LIST_HEAD(.klist_children->k_list),
            spin_lock_init(.klist_children->k_lock),
            .klist_children->get    = klist_children_get,
            .klist_children->put    = klist_children_put,

            INIT_LIST_HEAD(&.deferred_probe)
        },

        .kobj       = {
            .name = 'globalfifo_platform',
            .kref->refcount->refs       = 1,

            INIT_LIST_HEAD(.entry),
            .state_in_sysfs         = 0,
            .state_add_uevent_sent      = 0,
            .state_remove_uevent_sent   = 0,
            .state_initialized      = 1,
            .kset = devices_kset,
            .ktype = device_ktype,
            .name = 'globalfifo_platform',

            .parent = & platform_bus.kobj,
            .sd = {                 //create_dir: /sys/devices/platform/globalfifo_platform
                .parent = platform_bus.kobj.sd,
                .dir.root = platform_bus.kobj.sd->dir.root,
                .ns = NULL,
                .priv = .kobj
            }
        },
        INIT_LIST_HEAD(.dma_pools),
        Mutex_init(.mutex),
        spin_lock_init(.devres_lock),
        INIT_LIST_HEAD(&dev->devres_head),
        device_pm_init(.),
        .numa_node  = -1,
        INIT_LIST_HEAD(&dev->msi_list),
        INIT_LIST_HEAD(&dev->links.consumers);
        INIT_LIST_HEAD(&dev->links.suppliers);
        dev->links.status = DL_DEV_NO_DRIVER;
    },
};

bus_probe_device(&globalfifo_device.dev)

/*****  drivers/base/bus.c  *****/
/**
 * bus_probe_device - probe drivers for a new device
 * @dev: device to probe
 *
 * - Automatically probe for a driver if the bus allows it.
 */
void bus_probe_device(struct device *dev)
{
    struct bus_type *bus = dev->bus;

    if (!bus)
        return;

    if (bus->p->drivers_autoprobe)
        device_initial_probe(dev);
    ... ... 
}

/*****  drivers/base/dd.c  *****/
void device_initial_probe(struct device *dev)
{
    __device_attach(dev, true);
}

static int __device_attach(struct device *dev, bool allow_async)
{
    ... ...
    ret = bus_for_each_drv(dev->bus, NULL, &data,
            __device_attach_driver);
    ... ...
}

static int __device_attach_driver(struct device_driver *drv, void *_data)
{
    struct device_attach_data *data = _data;
    struct device *dev = data->dev;
    bool async_allowed;
    int ret;

    ret = driver_match_device(drv, dev);
    if (ret == 0) {
        /* no match */
        return 0;
    } else if (ret == -EPROBE_DEFER) {
        dev_dbg(dev, 'Device match requests probe deferral\n');
        driver_deferred_probe_add(dev);
    } else if (ret < 0) {
        dev_dbg(dev, 'Bus failed to match device: %d', ret);
        return ret;
    } /* ret > 0 means positive match */

    async_allowed = driver_allows_async_probing(drv);

    if (async_allowed)
        data->have_async = true;

    if (data->check_async && async_allowed != data->want_async)
        return 0;

    return driver_probe_device(drv, dev);
}

bus_probe_device(&globalfifo_device.dev)的执行函数路线分析如下所示，经过层层调用，最终又调用到driver_match_device()和driver_probe_device()函数，查找总线上能和当前设备匹配的驱动，并将驱动和设备绑定在了一起。

    struct device *dev = &globalfifo_device.dev;
    struct device_attach_data *data = {
        .dev = dev,
        .check_async = allow_async,
        .want_async = false,
    };
    struct device_driver *drv;
---------------------------------------------------
        bus_probe_device(dev)
                |
                V
        device_initial_probe(dev)
                |
                V
        __device_attach(dev, true) 
                |
                V
        bus_for_each_drv(dev->bus, NULL, &data, __device_attach_driver)
                |
                V
        __device_attach_driver(drv, data)
                |
                V
        driver_match_device(drv, dev) / driver_probe_device(drv, dev)

总结

综上述分析，可以看到驱动注册的过程中，会尝试寻找总线上可以与之匹配的设备；同样地，设备注册的过程中，也会尝试寻找总线上可以与之绑定的驱动。整个过程中，总线、设备、驱动的关键注册函数分别为：

总线注册：bus_register()
驱动注册：platform_driver_register() --> driver_register() --> bus_add_driver()
设备注册：platform_device_add() --> device_add() --> bus_add_device() / bus_probe_device()

从sysfs的角度，可以清楚地看到platform_device、platform_driver、platform_bus之间的联系：

/sys/bus/platform/drivers/globalfifo_platform$ ll
total 0
     bind
     globalfifo_platform -> ../../../../devices/platform/globalfifo_platform/
     module -> ../../../../module/globalfifo_platform/
     uevent
     unbind

/sys/bus/platform/devices$ ll
total 0
... ... 
     globalfifo_platform -> ../../../devices/platform/globalfifo_platform/


/sys/devices/platform/globalfifo_platform$ ll
total 0
     driver -> ../../../bus/platform/drivers/globalfifo_platform/
     modalias
     power/
     subsystem -> ../../../bus/platform/
     uevent

/sys/module/globalfifo_platform/drivers$ ll
total 0
     platform:globalfifo_platform -> ../../../bus/platform/drivers/globalfifo_platform/

参考资料

[1] Linux设备驱动开发详解（基于最新的Linux4.0内核），宋宝华编著，2016年
[2] 知识整理–linux设备驱动模型：https://blog.csdn.net/TongxinV/article/details/54853122^[1]
[3] linux设备驱动模型：https://blog.csdn.net/qq_40732350/article/details/82992904^[2]

原文链接

Linux设备驱动模型简述（源码剖析）^[3]

参考资料

[1]

2] 知识整理–linux设备驱动模型：[https://blog.csdn.net/TongxinV/article/details/54853122: https://blog.csdn.net/TongxinV/article/details/54853122

[2]

3] linux设备驱动模型：[https://blog.csdn.net/qq_40732350/article/details/82992904: https://blog.csdn.net/qq_40732350/article/details/82992904

[3]

Linux设备驱动模型简述（源码剖析）: https://hughesxu.github.io/posts/Linux_device_and_driver_model/

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