platform_device_系列函式，實際上是註冊了一個叫platform的虛擬匯流排。使用約定是如果一個不屬於任何匯流排的裝置，例如藍芽，串列埠等裝置，都需要掛在這個虛擬總線上。

river/base/platform.c

//platform裝置宣告
struct device platform_bus = {
    .bus_id        = "platform",
};
EXPORT_SYMBOL_GPL(platform_bus);
//platform匯流排裝置宣告
struct bus_type platform_bus_type = {
    .name        = "platform",
    .dev_attrs    = platform_dev_attrs,
    .match        = platform_match,
    .uevent        = platform_uevent,
    .suspend    = platform_suspend,
    .suspend_late    = platform_suspend_late,
    .resume_early    = platform_resume_early,
    .resume        = platform_resume,
};
EXPORT_SYMBOL_GPL(platform_bus_type);
int __init platform_bus_init(void)
{
    int error;
    error = device_register(&platform_bus);//註冊了"platform"的裝置
    if (error)
        return error;
    error = bus_register(&platform_bus_type);//註冊了叫"platform"的匯流排
    if (error)
        device_unregister(&platform_bus);
    return error;
}
//這裡在platform總線上掛裝置
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;//父裝置設定為platform_bus
    pdev->dev.bus = &platform_bus_type;//設定掛在platform總線上
    if (pdev->id != -1)
        snprintf(pdev->dev.bus_id, BUS_ID_SIZE, "%s.%d", pdev->name,pdev->id);
    else
        strlcpy(pdev->dev.bus_id, pdev->name, BUS_ID_SIZE);

    for (i = 0; i < pdev->num_resources; i++) {
        struct resource *p, *r = &pdev->resource[i];
        if (r->name == NULL)
            r->name = pdev->dev.bus_id;
        p = r->parent;
        if (!p) {
            if (r->flags & IORESOURCE_MEM)
                p = &iomem_resource;
            else if (r->flags & IORESOURCE_IO)
                p = &ioport_resource;
        }

        if (p && insert_resource(p, r)) {
            printk(KERN_ERR"%s: failed to claim resource %d\n",\
                   pdev->dev.bus_id, i);
            ret = -EBUSY;
            goto failed;
        }
    }

    pr_debug("Registering platform device '%s'. Parent at %s\n",\
         pdev->dev.bus_id, pdev->dev.parent->bus_id);
    ret = device_add(&pdev->dev);
    if (ret == 0)
        return ret;

failed:
    while (--i >= 0)
        if (pdev->resource[i].flags & (IORESOURCE_MEM|IORESOURCE_IO))
            release_resource(&pdev->resource[i]);
    return ret;
}
EXPORT_SYMBOL_GPL(platform_device_add);

//常用的platform_device_register，內部呼叫了platform_device_add，將裝置掛在了platform總線上
/**
* 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);
 

要用註冊一個platform驅動的步驟：
1，註冊裝置platform_device_register
2，註冊驅動platform_driver_register
註冊時候的兩個名字必須一樣，才能match上，才能work,例如：

struct platform_device pxa3xx_device_nand = {
    .name        = "pxa3xx-nand",
    .id        = -1,
    .dev        = {
        .dma_mask = &pxa3xx_nand_dma_mask,
        .coherent_dma_mask = DMA_BIT_MASK(32),
    },
    .resource    = pxa3xx_resource_nand,
    .num_resources    = ARRAY_SIZE(pxa3xx_resource_nand),
};

static struct platform_driver pxa3xx_nand_driver = {
    .driver = {
        .name    = "pxa3xx-nand",
    },
    .probe        = pxa3xx_nand_probe,
    .remove        = pxa3xx_nand_remove,
#ifdef CONFIG_PM
    .suspend    = pxa3xx_nand_suspend,
    .resume        = pxa3xx_nand_resume,
#endif
};
 

struct device {
    struct klist        klist_children;
    struct klist_node    knode_parent;    /* node in sibling list */
    struct klist_node    knode_driver;
    struct klist_node    knode_bus;
    struct device        *parent;

    struct kobject kobj;
    char    bus_id[BUS_ID_SIZE];    /* position on parent bus */
    struct device_type    *type;
    unsigned        is_registered:1;
    unsigned        uevent_suppress:1;

    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;

    spinlock_t        devres_lock;
    struct list_head    devres_head;

    /* class_device migration path */
    struct list_head    node;
    struct class        *class;
    dev_t            devt;    /* dev_t, creates the sysfs "dev" */
    struct attribute_group    **groups;    /* optional groups */

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

傳引數都是通過platform_data傳，所以定義為void *
    void        *platform_data;  

static struct pxa3xx_nand_platform_data XXX_nand_info = {
    .parts            = android_256m_v75_partitions,
    .nr_parts        = ARRAY_SIZE(android_256m_v75_partitions),
};

static void __init XXX_init_nand(void)
{
    pxa3xx_device_nand.dev.platform_data = &XXX_nand_info;
    platform_device_register(&pxa3xx_device_nand);
}

static int __init pxa3xx_nand_probe(struct platform_device *pdev)
{
    struct pxa3xx_nand_platform_data *pdata;
    struct nand_chip *this;
    struct pxa3xx_nand_info *info;
    struct resource *res;
    struct clk *clk = NULL, *smc_clk = NULL;
    int status = -1;
    struct mtd_partition *parts;
    unsigned int data_buf_len;
#ifdef CONFIG_MTD_NAND_PXA3xx_DMA
    unsigned int buf_len;
#endif
    int i, ret = 0;
#ifdef CONFIG_MTD_PARTITIONS
    int err;
#endif

    pdata = pdev->dev.platform_data;
....
....
....
}

void __init pxa_set_ohci_info(struct pxaohci_platform_data *info)
{
    pxa_register_device(&pxa27x_device_ohci, info);
}

void __init pxa_register_device(struct platform_device *dev, void *data)
{
    int ret;

    dev->dev.platform_data = data;

    ret = platform_device_register(dev);
    if (ret)
        dev_err(&dev->dev, "unable to register device: %d\n", ret);
}

platform_device_register向系統註冊裝置

platform_driver_register向系統註冊驅動，過程中在系統尋找註冊的裝置（根據.name)，找到後執行.probe進行初始化。

首先看device和platform_device區別
由struct platform_device {
const char * name; //NOTE:此處裝置的命名應和相應驅動程式命名一致
u32 id;            //以實現driver binding
struct device dev;
u32 num_resources;
struct resource * resource;
};
可知：platform_device由device派生而來

核心中關於Platform devices的註釋
Platform devices
~~~~~~~~~~~~~~~~
Platform devices are devices that typically appear as autonomous
entities in the system. This includes legacy port-based devices and
host bridges to peripheral buses, and most controllers integrated
into system-on-chip platforms. What they usually have in common
is direct addressing from a CPU bus. Rarely, a platform_device will
be connected through a segment of some other kind of bus; but its
registers will still be directly addressable.

Platform devices are given a name, used in driver binding, and a
list of resources such as addresses and IRQs.
個人理解：Platform devices是相對獨立的裝置，擁有各自獨自的資源（addresses and IRQs）

一個Platform devices例項：

static struct platform_device *smdk2410_devices[] __initdata = {
    &s3c_device_usb, //片上的各個裝置
    &s3c_device_lcd, //下面以s3c_device_lcd為例
    &s3c_device_wdt,
    &s3c_device_i2c,
    &s3c_device_iis,
};

/* LCD Controller */
static struct resource s3c_lcd_resource[] = { //LCD的兩個資源
    [0] = {
    .start = S3C2410_PA_LCD,
    .end = S3C2410_PA_LCD + S3C2410_SZ_LCD,
    .flags = IORESOURCE_MEM,
    },
    [1] = {
    .start = IRQ_LCD,
    .end = IRQ_LCD,
    .flags = IORESOURCE_IRQ,
    }

};

struct platform_device s3c_device_lcd = {//s3c_device_lcd裝置
    .name = "s3c2410-lcd",
    .id = -1,
    .num_resources = ARRAY_SIZE(s3c_lcd_resource),
    .resource = s3c_lcd_resource,
    .dev = { //device例項
        .dma_mask = &s3c_device_lcd_dmamask,
        .coherent_dma_mask = 0xffffffffUL
    }
};

#define S3C2410_LCDREG(x) ((x) + S3C2410_VA_LCD)

/* LCD control registers */
#define S3C2410_LCDCON1 S3C2410_LCDREG(0x00)
#define S3C2410_LCDCON2 S3C2410_LCDREG(0x04)
#define S3C2410_LCDCON3 S3C2410_LCDREG(0x08)
#define S3C2410_LCDCON4 S3C2410_LCDREG(0x0C)
#define S3C2410_LCDCON5 S3C2410_LCDREG(0x10)

#define S3C2410_LCDCON1_CLKVAL(x) ((x) << 8)
#define S3C2410_LCDCON1_MMODE (1<<7)
#define S3C2410_LCDCON1_DSCAN4 (0<<5)
#define S3C2410_LCDCON1_STN4 (1<<5)
#define S3C2410_LCDCON1_STN8 (2<<5)
#define S3C2410_LCDCON1_TFT (3<<5)
－－－－－－－－－－－－－－－－－－－－－－－－－－
#define S3C2410_ADDR(x) (0xF0000000 + (x))

/* LCD controller */
#define S3C2410_VA_LCD S3C2410_ADDR(0x00600000)
#define S3C2410_PA_LCD (0x4D000000)
#define S3C2410_SZ_LCD SZ_1M

device_register()－－－－－－－－－－－－－－－－－－－－－－－－

/**
* 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.
*/

int device_register(struct device *dev)
{
    device_initialize(dev); //初始化裝置結構
    return device_add(dev); //新增裝置到裝置層
}

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); //初始化裝置結構
    return platform_device_add(pdev); //新增一個片上的裝置到裝置層
}

platform_device_add()－－－－－－－－－－－－－－－－－－－－－－
/**
* 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;
    pdev->dev.bus = &platform_bus_type;

    /*＋＋＋＋＋＋＋＋＋＋＋＋＋＋
    The platform_device.dev.bus_id is the canonical name for the devices.
    It's built from two components:

    * platform_device.name ... which is also used to for driver matching.
    * platform_device.id ... the device instance number, or else "-1"
    to indicate there's only one.

    These are concatenated, so name/id "serial"/0 indicates bus_id "serial.0", and
    "serial/3" indicates bus_id "serial.3"; both would use the platform_driver
    named "serial". While "my_rtc"/-1 would be bus_id "my_rtc" (no instance id)
    and use the platform_driver called "my_rtc".
    ＋＋＋＋＋＋＋＋＋＋＋＋＋＋*/

    if (pdev->id != -1)
    snprintf(pdev->dev.bus_id, BUS_ID_SIZE, "%s.%u", pdev->name, pdev->id);
    else //"-1" indicate there's only one
    strlcpy(pdev->dev.bus_id, pdev->name, BUS_ID_SIZE);
    for (i = 0; i < pdev->num_resources; i++) { //遍歷裝置資源個數，如LCD的兩個資源：控制器和IRQ
        struct resource *p, *r = &pdev->resource[i];
        if (r->name == NULL)
        r->name = pdev->dev.bus_id;
        p = r->parent;
        if (!p) { //resources分為兩種IORESOURCE_MEM和IORESOURCE_IO
            //CPU對外設IO埠實體地址的編址方式有兩種：I/O對映方式和記憶體對映方式
            if (r->flags & IORESOURCE_MEM)
            p = &iomem_resource;
            else if (r->flags & IORESOURCE_IO)
            p = &ioport_resource;
        }

        if (p && insert_resource(p, r)) {
            printk(KERN_ERR
            "%s: failed to claim resource %d/n",
            pdev->dev.bus_id, i);
            ret = -EBUSY;
            goto failed;
        }
    }
    pr_debug("Registering platform device '%s'. Parent at %s/n",
    pdev->dev.bus_id, pdev->dev.parent->bus_id);
    ret = device_add(&pdev->dev);
    if (ret == 0)
    return ret;
    failed:
    while (--i >= 0)
    if (pdev->resource[i].flags & (IORESOURCE_MEM|IORESOURCE_IO))
    release_resource(&pdev->resource[i]);
    return ret;
}

device_initialize()－－－－－－－－－－－－－－－－－－
/** </drivers/base/core.c>
* device_initialize - init device structure.
* @dev: device.
*
* This prepares the device for use by other layers,
* including adding it to the device hierarchy.
* It is the first half of device_register(), if called by
* that, though it can also be called separately, so one
* may use @dev's fields (e.g. the refcount).
*/

void device_initialize(struct device *dev)
{
kobj_set_kset_s(dev, devices_subsys);
kobject_init(&dev->kobj);
klist_init(&dev->klist_children, klist_children_get,
klist_children_put);
INIT_LIST_HEAD(&dev->dma_pools);
INIT_LIST_HEAD(&dev->node);
init_MUTEX(&dev->sem);
spin_lock_init(&dev->devres_lock);
INIT_LIST_HEAD(&dev->devres_head);
device_init_wakeup(dev, 0);
set_dev_node(dev, -1);
}
// device_add(struct device *dev)－－－－－－－－－－－－－
/**
* 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 it 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.
*/
//結構體resource－－－－－－－－－－－－－－－－－－－－－－
/* < /include/linux/ioport.h>
* Resources are tree-like, allowing
* nesting etc..
*/
struct resource {
    resource_size_t start;
    resource_size_t end;
    const char *name;
    unsigned long flags;
    struct resource *parent, *sibling, *child;
};
//－－－－－－－－－－－－－－－－－－－－－－－－－－－

原文地址:http://blog.chinaunix.net/u1/58968/showart_467998.html ,

在8250.c(driver/serial/8250.c)的初始化函式serial8250_init（）中，給出了一個很簡單的例子

static struct platform_device *serial8250_isa_devs;

......

//create a platform_device 

serial8250_isa_devs = platform_device_alloc("serial8250",PLAT8250_DEV_LEGACY);              

  platform_device_add(serial8250_isa_devs);   //add the platform_device to system

platform_driver_register(&serial8250_isa_driver);//then register the platform_driver        

還有另外一個比較類似的比較，就是driver_register和platform_driver_register的比較
    platform_driver_register(&xx_driver) 會向系統註冊xx_driver這個驅動程式，這個函式會根據 xx_driver中的.name內容，搜尋系統註冊的device中有沒有這個platform_device,如果有，就會執行 platform_driver(也就是xx_driver的型別)中的.probe函式。

      對只需要初始化執行一次的函式都加上__init屬性，__init 巨集告訴編譯器如果這個模組被編譯到核心則把這個函式放到（.init.text）段，module_exit的引數解除安裝時同__init類似，如果驅動被編譯進核心，則__exit巨集會忽略清理函式，因為編譯進核心的模組不需要做清理工作，顯然__init和__exit對動態載入的模組是無效的，只支援完全編譯進核心。