platform总线和普通总线，驱动模型间的一些关系和理解

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http://blog.csdn.net/sailor_8318/archive/2010/01/29/5267698.aspx

【摘要】本文以Linux 2.6.25 内核为例，分析了基于platform总线的驱动模型。首先介绍了Platform总线的基本概念，接着介绍了platform device和platform driver的定义和加载过程，分析了其与基类device 和driver的派生关系及在此过程中面向对象的设计思想。最后以ARM S3C2440中I2C控制器为例介绍了基于platform总线的驱动开发流程。

【关键字】platform_bus, platform_device, resource , platform_driver, file_operations

目录

1    何谓platform bus？    2
2    device和platform_device    3
3    device_register和platform_device_register    5
4    device_driver和platform driver    8
5    driver_register 和platform_driver_register    10
6    bus、device及driver三者之间的关系    17
7    哪些适用于plarform驱动？    18
8    基于platform总线的驱动开发流程    18
8.1    初始化platform_bus    19
8.2    定义platform_device    22
8.3    注册platform_device    22
8.4    定义platform_driver    28
8.5    注册platform_driver    29
8.6    操作设备    32

1    何谓platform bus？
Linux系统中许多部分对设备是如何链接的并不感兴趣，但是他们需要知道哪些类型的设备是可以使用的。设备模型提供了一种机制来对设备进行分类，在更高的功能层面上描述这些设备，并使得这些设备对用户空间可见。因此从2.6内核开始引入了设备模型。

总线是处理器和一个或多个设备之间的通道，在设备模型中, 所有的设备都通过总线相连。总线可以相互插入。设备模型展示了总线和它们所控制的设备之间的实际连接。

Platform总线是2.6 kernel中最近引入的一种虚拟总线，主要用来管理CPU的片上资源，具有更好的移植性，因此在2.6 kernel中，很多驱动都用platform改写了。

platform_bus_type的定义如下:

http://lxr.linux.no/#linux+v2.6.25/drivers/base/platform.c#L609 609struct bus_type platform_bus_type = { 610        .name           = "platform", 611        .dev_attrs      = platform_dev_attrs, 612        .match          = platform_match, 613        .uevent         = platform_uevent, 614        .suspend        = platform_suspend, 615        .suspend_late   = platform_suspend_late, 616        .resume_early   = platform_resume_early, 617        .resume         = platform_resume, 618}; 619EXPORT_SYMBOL_GPL(platform_bus_type);http://lxr.linux.no/#linux+v2.6.25/include/linux/device.h#L55  55struct bus_type {  56        const char              *name;  57        struct bus_attribute    *bus_attrs;  58        struct device_attribute *dev_attrs;  59        struct driver_attribute *drv_attrs;  60  61        int (*match)(struct device *dev, struct device_driver *drv);  62        int (*uevent)(struct device *dev, struct kobj_uevent_env *env);  63        int (*probe)(struct device *dev);  64        int (*remove)(struct device *dev);  65        void (*shutdown)(struct device *dev);  66  67        int (*suspend)(struct device *dev, pm_message_t state);  68        int (*suspend_late)(struct device *dev, pm_message_t state);  69        int (*resume_early)(struct device *dev);  70        int (*resume)(struct device *dev);  71  72        struct bus_type_private *p;  73};

总线名称是"platform"，其只是bus_type的一种，定义了总线的属性，同时platform_bus_type还有相关操作方法，如挂起、中止、匹配及hotplug事件等。

总线bus是联系driver和device的中间枢纽。Device通过所属的bus找到driver，由match操作方法进行匹配。

Bus、driver及devices的连接关系

2 device和platform_device
Plarform device会有一个名字用于driver binding（在注册driver的时候会查找driver的目标设备的bus位置，这个过程称为driver binding），另外IRQ以及地址空间等资源也要给出。

platform_device结构体用来描述设备的名称、资源信息等。该结构被定义在http://lxr.linux.no/#linux+v2.6.25/include/linux/platform_device.h#L16中，定义原型如下：

 16struct platform_device {  17        const char      * name; //定义平台设备的名称，此处设备的命名应和相应驱动程序命名一致  18        int             id;  19        struct device   dev;  20        u32             num_resources;  21        struct resource * resource;  //定义平台设备的资源  22};

在这个结构里封装了struct device及struct resource。可知：platform_device由device派生而来，是一种特殊的device。

下面来看一下platform_device结构体中最重要的一个成员struct resource * resource。struct resource被定义在

http://lxr.linux.no/#linux+v2.6.25/include/linux/ioport.h#L18中，定义原型如下：  14/*  15 * Resources are tree-like, allowing  16 * nesting etc..  17 */  18struct resource {  19        resource_size_t start;  //定义资源的起始地址  20        resource_size_t end;  //定义资源的结束地址  21        const char *name; //定义资源的名称  22        unsigned long flags; 定义资源的类型，比如MEM，IO，IRQ，DMA类型  23        struct resource *parent, *sibling, *child;  24};

这个结构表示设备所拥有的资源，即I/O端口、I/O映射内存、中断及DMA等。这里的地址指的是物理地址。

另外还需要注意platform_device中的device结构，它详细描述了设备的情况，其为所有设备的基类，定义如下：

http://lxr.linux.no/#linux+v2.6.25/include/linux/device.h#L422422struct device { 423        struct klist            klist_children; 424        struct klist_node       knode_parent;   /* node in sibling list */ 425        struct klist_node       knode_driver; 426        struct klist_node       knode_bus; 427        struct device           *parent; 428 429        struct kobject kobj; 430        char    bus_id[BUS_ID_SIZE];    /* position on parent bus */ 431        struct device_type      *type; 432        unsigned                is_registered:1; 433        unsigned                uevent_suppress:1; 434 435        struct semaphore        sem;    /* semaphore to synchronize calls to 436                                         * its driver. 437                                         */ 438 439        struct bus_type *bus;           /* type of bus device is on */ 440        struct device_driver *driver;   /* which driver has allocated this 441                                           device */ 442        void            *driver_data;   /* data private to the driver */ 443        void            *platform_data; /* Platform specific data, device 444                                           core doesn't touch it */ 445        struct dev_pm_info      power; 446 447#ifdef CONFIG_NUMA 448        int             numa_node;      /* NUMA node this device is close to */ 449#endif 450        u64             *dma_mask;      /* dma mask (if dma'able device) */ 451        u64             coherent_dma_mask;/* Like dma_mask, but for 452                                             alloc_coherent mappings as 453                                             not all hardware supports 454                                             64 bit addresses for consistent 455                                             allocations such descriptors. */ 456 457        struct device_dma_parameters *dma_parms; 458 459        struct list_head        dma_pools;      /* dma pools (if dma'ble) */ 460 461        struct dma_coherent_mem *dma_mem; /* internal for coherent mem 462                                             override */ 463        /* arch specific additions */ 464        struct dev_archdata     archdata; 465 466        spinlock_t              devres_lock; 467        struct list_head        devres_head; 468 469        /* class_device migration path */ 470        struct list_head        node; 471        struct class            *class; 472        dev_t                   devt;   /* dev_t, creates the sysfs "dev" */ 473        struct attribute_group  **groups;       /* optional groups */ 474 475        void    (*release)(struct device *dev); 476}; 477

3 device_register和platform_device_register

http://lxr.linux.no/#linux+v2.6.25/drivers/base/core.c#L881 870/** 871 * device_register - register a device with the system. 872 * @dev: pointer to the device structure 873 * 874 * This happens in two clean steps - initialize the device 875 * and add it to the system. The two steps can be called 876 * separately, but this is the easiest and most common. 877 * I.e. you should only call the two helpers separately if 878 * have a clearly defined need to use and refcount the device 879 * before it is added to the hierarchy. 880 */ 881int device_register(struct device *dev) 882{ 883        device_initialize(dev); 884        return device_add(dev); 885}初始化一个设备，然后加入到系统中。http://lxr.linux.no/#linux+v2.6.25/drivers/base/platform.c#L325316/** 317 * platform_device_register - add a platform-level device 318 * @pdev: platform device we're adding 319 */ 320int platform_device_register(struct platform_device *pdev) 321{ 322        device_initialize(&pdev->dev); 323        return platform_device_add(pdev); 324} 325EXPORT_SYMBOL_GPL(platform_device_register);

我们看到注册一个platform device分为了两部分，初始化这个platform_device，然后将此platform_device添加到platform总线中。输入参数platform_device可以是静态的全局设备。

另外一种机制就是动态申请platform_device_alloc一个platform_device设备，然后通过platform_device_add_resources及platform_device_add_data等添加相关资源和属性。

无论哪一种platform_device，最终都将通过platform_device_add这册到platform总线上。

229/** 230 * platform_device_add - add a platform device to device hierarchy 231 * @pdev: platform device we're adding 232 * 233 * This is part 2 of platform_device_register(), though may be called 234 * separately _iff_ pdev was allocated by platform_device_alloc(). 235 */ 236int platform_device_add(struct platform_device *pdev) 237{ 238        int i, ret = 0; 239 240        if (!pdev) 241                return -EINVAL; 242初始化设备的parent为platform_bus，初始化驱备的总线为platform_bus_type。 243        if (!pdev->dev.parent) 244                pdev->dev.parent = &platform_bus; 245 246        pdev->dev.bus = &platform_bus_type; 247/*＋＋＋＋＋＋＋＋＋＋＋＋＋＋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_drivernamed "serial". While "my_rtc"/-1 would be bus_id "my_rtc" (no instance id)and use the platform_driver called "my_rtc".＋＋＋＋＋＋＋＋＋＋＋＋＋＋*/ 248        if (pdev->id != -1) 249                snprintf(pdev->dev.bus_id, BUS_ID_SIZE, "%s.%d", pdev->name, 250                         pdev->id); 251        else 252                strlcpy(pdev->dev.bus_id, pdev->name, BUS_ID_SIZE); 253设置设备struct device 的bus_id成员，留心这个地方，在以后还需要用到这个的。 254        for (i = 0; i < pdev->num_resources; i++) { 255                struct resource *p, *r = &pdev->resource[i]; 256 257                if (r->name == NULL) 258                        r->name = pdev->dev.bus_id; 259 260                p = r->parent; 261                if (!p) { 262                        if (r->flags & IORESOURCE_MEM) 263                                p = &iomem_resource; 264                        else if (r->flags & IORESOURCE_IO) 265                                p = &ioport_resource; 266                }//resources分为两种IORESOURCE_MEM和IORESOURCE_IO //CPU对外设IO端口物理地址的编址方式有两种：I/O映射方式和内存映射方式 267 268                if (p && insert_resource(p, r)) { 269                        printk(KERN_ERR 270                               "%s: failed to claim resource %d/n", 271                               pdev->dev.bus_id, i); 272                        ret = -EBUSY; 273                        goto failed; 274                } 275        } 276 277        pr_debug("Registering platform device '%s'. Parent at %s/n", 278                 pdev->dev.bus_id, pdev->dev.parent->bus_id); 279 280        ret = device_add(&pdev->dev); 281        if (ret == 0) 282                return ret; 283 284 failed: 285        while (--i >= 0) 286                if (pdev->resource[i].flags & (IORESOURCE_MEM|IORESOURCE_IO)) 287                        release_resource(&pdev->resource[i]); 288        return ret; 289} 290EXPORT_SYMBOL_GPL(platform_device_add);

由platform_device_register和platform_device_add的实现可知，device_register()和platform_device_register()都会首先初始化设备
区别在于第二步：其实platform_device_add()包括device_add()，不过要先注册resources，然后将设备挂接到特定的platform总线。

4 device_driver和platform driver
Platform device是一种device自己是不会做事情的，要有人为它做事情，那就是platform driver。platform driver遵循linux系统的driver model。对于device的discovery/enumerate都不是driver自己完成的而是由由系统的driver注册机制完成。driver编写人员只要将注册必须的数据结构初始化并调用注册driver的kernel API就可以了。

接下来来看platform_driver结构体的原型定义，在http://lxr.linux.no/#linux+v2.6.25/include/linux/platform_device.h#L48中，代码如下：

48 struct platform_driver {  49        int (*probe)(struct platform_device *);  50        int (*remove)(struct platform_device *);  51        void (*shutdown)(struct platform_device *);  52        int (*suspend)(struct platform_device *, pm_message_t state);  53        int (*suspend_late)(struct platform_device *, pm_message_t state);  54        int (*resume_early)(struct platform_device *);  55        int (*resume)(struct platform_device *);  56        struct device_driver driver;  57};

可见，它包含了设备操作的几个功能函数，同时包含了一个device_driver结构，说明device_driver是platform_driver的基类。驱动程序中需要初始化这个变量。下面看一下这个变量的定义，位于http://lxr.linux.no/#linux+v2.6.25/include/linux/device.h#L121中：

121struct device_driver { 122        const char              *name; 123        struct bus_type         *bus; 124 125        struct module           *owner; 126        const char              *mod_name;      /* used for built-in modules */ 127 128        int (*probe) (struct device *dev); 129        int (*remove) (struct device *dev); 130        void (*shutdown) (struct device *dev); 131        int (*suspend) (struct device *dev, pm_message_t state); 132        int (*resume) (struct device *dev); 133        struct attribute_group **groups; 134 135        struct driver_private *p; 136};

device_driver提供了一些操作接口，但其并没有实现，相当于一些虚函数，由派生类platform_driver进行重载，无论何种类型的driver都是基于device_driver派生而来的，具体的各种操作都是基于统一的基类接口的，这样就实现了面向对象的设计。

需要注意这两个变量：name和owner。其作用主要是为了和相关的platform_device关联起来，owner的作用是说明模块的所有者，驱动程序中一般初始化为THIS_MODULE。

device_driver结构中也有一个name变量。platform_driver从字面上来看就知道是设备驱动。设备驱动是为谁服务的呢？当然是设备了。内核正是通过这个一致性来为驱动程序找到资源，即 platform_device中的resource。

5 driver_register 和platform_driver_register

内核提供的platform_driver结构体的注册函数为platform_driver_register（），其原型定义在http://lxr.linux.no/#linux+v2.6.25/drivers/base/platform.c#L458文件中，具体实现代码如下：

439/** 440 * platform_driver_register 441 * @drv: platform driver structure 442 */ 443int platform_driver_register(struct platform_driver *drv) 444{ 445        drv->driver.bus = &platform_bus_type;   /*设置成platform_bus_type这个很重要，因为driver和device是通过bus联系在一起的，具体在本例中是通过  platform_bus_type中注册的回调例程和属性来是实现的， driver与device的匹配就是通过 platform_bus_type注册的回调例程platform_match ()来完成的。*/ 446        if (drv->probe) 447                drv->driver.probe = platform_drv_probe; //在really_probe函数中，回调了platform_drv_probe函数448        if (drv->remove) 449                drv->driver.remove = platform_drv_remove; 450        if (drv->shutdown) 451                drv->driver.shutdown = platform_drv_shutdown; 452        if (drv->suspend) 453                drv->driver.suspend = platform_drv_suspend; 454        if (drv->resume) 455                drv->driver.resume = platform_drv_resume; 456        return driver_register(&drv->driver); 457} 458EXPORT_SYMBOL_GPL(platform_driver_register);

不要被上面的platform_drv_XXX吓倒了，它们其实很简单，就是将struct device转换为struct platform_device和struct platform_driver，然后调用platform_driver中的相应接口函数。那为什么不直接调用platform_drv_XXX等接口呢？这就是Linux内核中面向对象的设计思想。

device_driver提供了一些操作接口，但其并没有实现，相当于一些虚函数，由派生类platform_driver进行重载，无论何种类型的driver都是基于device_driver派生而来的，device_driver中具体的各种操作都是基于统一的基类接口的，这样就实现了面向对象的设计。

在文件http://lxr.linux.no/#linux+v2.6.25/drivers/base/driver.c#L234中，实现了driver_register()函数。

209/** 210 * driver_register - register driver with bus 211 * @drv: driver to register 212 * 213 * We pass off most of the work to the bus_add_driver() call, 214 * since most of the things we have to do deal with the bus 215 * structures. 216 */ 217int driver_register(struct device_driver *drv) 218{ 219        int ret; 220//如果总线的方法和设备自己的方法同时存在，将打印告警信息，对于platform bus，其没有probe等接口 221        if ((drv->bus->probe && drv->probe) || 222            (drv->bus->remove && drv->remove) || 223            (drv->bus->shutdown && drv->shutdown)) 224                printk(KERN_WARNING "Driver '%s' needs updating - please use " 225                        "bus_type methods/n", drv->name); 226        ret = bus_add_driver(drv); 227        if (ret) 228                return ret; 229        ret = driver_add_groups(drv, drv->groups); 230        if (ret) 231                bus_remove_driver(drv); 232        return ret; 233} 234EXPORT_SYMBOL_GPL(driver_register);

226 其主要将驱动挂接到总线上，通过总线来驱动设备。

644/** 645 * bus_add_driver - Add a driver to the bus. 646 * @drv: driver. 647 */ 648int bus_add_driver(struct device_driver *drv) 649{ 650        struct bus_type *bus; 651        struct driver_private *priv; 652        int error = 0; 653 654        bus = bus_get(drv->bus); 655        if (!bus) 656                return -EINVAL; 657 658        pr_debug("bus: '%s': add driver %s/n", bus->name, drv->name); 659 660        priv = kzalloc(sizeof(*priv), GFP_KERNEL); 661        if (!priv) { 662                error = -ENOMEM; 663                goto out_put_bus; 664        } 665        klist_init(&priv->klist_devices, NULL, NULL); 666        priv->driver = drv; 667        drv->p = priv; 668        priv->kobj.kset = bus->p->drivers_kset; 669        error = kobject_init_and_add(&priv->kobj, &driver_ktype, NULL, 670                                     "%s", drv->name); 671        if (error) 672                goto out_unregister; 673 674        if (drv->bus->p->drivers_autoprobe) { 675                error = driver_attach(drv); 676                if (error) 677                        goto out_unregister; 678        } 679        klist_add_tail(&priv->knode_bus, &bus->p->klist_drivers); 680        module_add_driver(drv->owner, drv); 681 682        error = driver_create_file(drv, &driver_attr_uevent); 683        if (error) { 684                printk(KERN_ERR "%s: uevent attr (%s) failed/n", 685                        __FUNCTION__, drv->name); 686        } 687        error = driver_add_attrs(bus, drv); 688        if (error) { 689                /* How the hell do we get out of this pickle? Give up */ 690                printk(KERN_ERR "%s: driver_add_attrs(%s) failed/n", 691                        __FUNCTION__, drv->name); 692        } 693        error = add_bind_files(drv); 694        if (error) { 695                /* Ditto */ 696                printk(KERN_ERR "%s: add_bind_files(%s) failed/n", 697                        __FUNCTION__, drv->name); 698        } 699 700        kobject_uevent(&priv->kobj, KOBJ_ADD); 701        return error; 702out_unregister: 703        kobject_put(&priv->kobj); 704out_put_bus: 705        bus_put(bus); 706        return error; 707}

如果总线上的driver是自动probe的话，则将该总线上的driver和device绑定起来。

http://lxr.linux.no/#linux+v2.6.25/drivers/base/dd.c#L285272/** 273 * driver_attach - try to bind driver to devices. 274 * @drv: driver. 275 * 276 * Walk the list of devices that the bus has on it and try to 277 * match the driver with each one.  If driver_probe_device() 278 * returns 0 and the @dev->driver is set, we've found a 279 * compatible pair. 280 */ 281int driver_attach(struct device_driver *drv) 282{ 283        return bus_for_each_dev(drv->bus, NULL, drv, __driver_attach); 284} 285EXPORT_SYMBOL_GPL(driver_attach);

扫描该总线上的每一个设备，将当前driver和总线上的设备进行match，如果匹配成功，则将设备和driver绑定起来。

246static int __driver_attach(struct device *dev, void *data) 247{ 248        struct device_driver *drv = data; 249 250        /* 251         * Lock device and try to bind to it. We drop the error 252         * here and always return 0, because we need to keep trying 253         * to bind to devices and some drivers will return an error 254         * simply if it didn't support the device. 255         * 256         * driver_probe_device() will spit a warning if there 257         * is an error. 258         */ 259 260        if (dev->parent)        /* Needed for USB */ 261                down(&dev->parent->sem); 262        down(&dev->sem); 263        if (!dev->driver) 264                driver_probe_device(drv, dev); 265        up(&dev->sem); 266        if (dev->parent) 267                up(&dev->parent->sem); 268 269        return 0; 270}

263，如果该设备尚没有匹配的driver，则尝试匹配。

http://lxr.linux.no/#linux+v2.6.25/drivers/base/dd.c#L187170/** 171 * driver_probe_device - attempt to bind device & driver together 172 * @drv: driver to bind a device to 173 * @dev: device to try to bind to the driver 174 * 175 * First, we call the bus's match function, if one present, which should 176 * compare the device IDs the driver supports with the device IDs of the 177 * device. Note we don't do this ourselves because we don't know the 178 * format of the ID structures, nor what is to be considered a match and 179 * what is not. 180 * 181 * This function returns 1 if a match is found, -ENODEV if the device is 182 * not registered, and 0 otherwise. 183 * 184 * This function must be called with @dev->sem held.  When called for a 185 * USB interface, @dev->parent->sem must be held as well. 186 */ 187int driver_probe_device(struct device_driver *drv, struct device *dev) 188{ 189        int ret = 0; 190 191        if (!device_is_registered(dev)) 192                return -ENODEV; 193        if (drv->bus->match && !drv->bus->match(dev, drv)) 194                goto done; 195 196        pr_debug("bus: '%s': %s: matched device %s with driver %s/n", 197                 drv->bus->name, __FUNCTION__, dev->bus_id, drv->name); 198 199        ret = really_probe(dev, drv); 200 201done: 202        return ret; 203}

193，如果该总线上的设备需要进行匹配，则验证是否匹配。对于platform总线，其匹配过程如下：

http://lxr.linux.no/#linux+v2.6.25/drivers/base/platform.c#L555542/** 543 * platform_match - bind platform device to platform driver. 544 * @dev: device. 545 * @drv: driver. 546 * 547 * Platform device IDs are assumed to be encoded like this: 548 * "<name><instance>", where <name> is a short description of the type of 549 * device, like "pci" or "floppy", and <instance> is the enumerated 550 * instance of the device, like '0' or '42'.  Driver IDs are simply 551 * "<name>".  So, extract the <name> from the platform_device structure, 552 * and compare it against the name of the driver. Return whether they match 553 * or not. 554 */ 555static int platform_match(struct device *dev, struct device_driver *drv) 556{ 557        struct platform_device *pdev; 558 559        pdev = container_of(dev, struct platform_device, dev); 560        return (strncmp(pdev->name, drv->name, BUS_ID_SIZE) == 0); 561}

560，简单的进行字符串匹配，这也是我们强调platform_device和platform_driver中的name属性需要一致的原因。

匹配成功后，则调用probe接口。

http://lxr.linux.no/#linux+v2.6.25/drivers/base/dd.c#L101  98static atomic_t probe_count = ATOMIC_INIT(0);  99static DECLARE_WAIT_QUEUE_HEAD(probe_waitqueue); 100 101static int really_probe(struct device *dev, struct device_driver *drv) 102{ 103        int ret = 0; 104 105        atomic_inc(&probe_count); 106        pr_debug("bus: '%s': %s: probing driver %s with device %s/n", 107                 drv->bus->name, __FUNCTION__, drv->name, dev->bus_id); 108        WARN_ON(!list_empty(&dev->devres_head)); 109 110        dev->driver = drv; 111        if (driver_sysfs_add(dev)) { 112                printk(KERN_ERR "%s: driver_sysfs_add(%s) failed/n", 113                        __FUNCTION__, dev->bus_id); 114                goto probe_failed; 115        } 116 117        if (dev->bus->probe) { 118                ret = dev->bus->probe(dev); 119                if (ret) 120                        goto probe_failed; 121        } else if (drv->probe) { 122                ret = drv->probe(dev); 123                if (ret) 124                        goto probe_failed; 125        } 126 127        driver_bound(dev); 128        ret = 1; 129        pr_debug("bus: '%s': %s: bound device %s to driver %s/n", 130                 drv->bus->name, __FUNCTION__, dev->bus_id, drv->name); 131        goto done; 132 133probe_failed: 134        devres_release_all(dev); 135        driver_sysfs_remove(dev); 136        dev->driver = NULL; 137 138        if (ret != -ENODEV && ret != -ENXIO) { 139                /* driver matched but the probe failed */ 140                printk(KERN_WARNING 141                       "%s: probe of %s failed with error %d/n", 142                       drv->name, dev->bus_id, ret); 143        } 144        /* 145         * Ignore errors returned by ->probe so that the next driver can try 146         * its luck. 147         */ 148        ret = 0; 149done: 150        atomic_dec(&probe_count); 151        wake_up(&probe_waitqueue); 152        return ret; 153} 154

如果bus和driver同时具备probe方法，则优先调用总线的probe函数。否则调用device_driver的probe函数，此probe函数是经过各种类型的driver重载的函数，这就实现了利用基类的统一方法来实现不同的功能。对于platform_driver来说，其就是：

http://lxr.linux.no/#linux+v2.6.25/drivers/base/platform.c#L394394static int platform_drv_probe(struct device *_dev) 395{ 396        struct platform_driver *drv = to_platform_driver(_dev->driver); 397        struct platform_device *dev = to_platform_device(_dev); 398 399        return drv->probe(dev); 400}

然后调用特定platform_driver所定义的操作方法，这个是在定义某个platform_driver时静态指定的操作接口。

至此，platform_driver成功挂接到platform bus上了，并与特定的设备实现了绑定，并对设备进行了probe处理。

6    bus、device及driver三者之间的关系
在数据结构设计上，总线、设备及驱动三者相互关联。

platform device包含device，根据device可以获得相应的bus及driver。

设备添加到总线上后形成一个双向循环链表，根据总线可以获得其上挂接的所有device，进而获得了 platform device。根据device也可以获得驱动该总线上所有设备的相关driver。

platform driver包含driver，根据driver可以获得相应的bus，进而获得bus上所有的device，进一步获得platform device，根据name对driver与platform device进行匹配，匹配成功后将device与相应的driver关联起来，即实现了platform device和platform driver的关联。

匹配成功后调用driver的probe进而调用platform driver的probe，在probe里实现驱动特定的功能。

7    哪些适用于plarform驱动？
platform机制将设备本身的资源注册进内核，由内核统一管理，在驱动程序中使用这些资源时通过platform device提供的标准接口进行申请并使用。这样提高了驱动和资源管理的独立性，这样拥有更好的可移植性。platform机制的本身使用并不复杂，由两部分组成：platform_device和platfrom_driver。Platform driver通过platform bus获取platform_device。

通常情况下只要和内核本身运行依赖性不大的外围设备，相对独立的，拥有各自独立的资源(地址总线和IRQs)，都可以用 platform_driver来管理，而timer，irq等小系统之内的设备则最好不用platfrom_driver机制。

platform_device最大的特定是CPU直接寻址设备的寄存器空间，即使对于其他总线设备，设备本身的寄存器无法通过CPU总线访问，但总线的controller仍然需要通过platform bus来管理。

总之，platfrom_driver的根本目的是为了统一管理系统的外设资源，为驱动程序提供统一的接口来访问系统资源，将驱动和资源分离，提高程序的可移植性。

8    基于platform总线的驱动开发流程
基于Platform总线的驱动开发流程如下：
•    定义初始化platform bus
•    定义各种platform devices
•    注册各种platform devices
•    定义相关platform driver
•    注册相关platform driver
•    操作相关设备

图 platform机制开发驱动流程

以S3C24xx平台为例，来简单讲述下platform驱动的实现流程。
8.1    初始化platform_bus
Platform总线的初始化是在platform_bus_init()完成的，代码如下:

http://lxr.linux.no/#linux+v2.6.25/drivers/base/platform.c#L621  26struct device platform_bus = {  27        .bus_id         = "platform",  28};  29EXPORT_SYMBOL_GPL(platform_bus);621int __init platform_bus_init(void) 622{ 623        int error; 624 625        error = device_register(&platform_bus); 626        if (error) 627                return error; 628        error =  bus_register(&platform_bus_type); 629        if (error) 630                device_unregister(&platform_bus); 631        return error; 632}

该函数创建了一个名为 “platform”的设备，后续platform的设备都会以此为parent。在sysfs中表示为：所有platform类型的设备都会添加在 platform_bus所代表的目录下，即 /sys/devices/platform下面。
-sh-3.1# ls /sys/devices/platform/
Fixed MDIO bus.0     fsl-i2c.0            serial8250
fsl-ehci.0           fsl-i2c.1            serial8250.0
fsl-gianfar.0        mpc83xx_spi.0        uevent
fsl-gianfar.1        mpc83xx_wdt.0
fsl-gianfar_mdio.-5 power

-sh-3.1# ls /sys/
block/    class/    firmware/ kernel/   power/
bus/      devices/ fs/       module/
-sh-3.1# ls /sys/bus/
i2c/         of_platform/ pci_express/ scsi/        usb/
mdio_bus/    pci/         platform/    spi/
-sh-3.1# ls /sys/bus/i2c/
devices/           drivers_autoprobe uevent
drivers/           drivers_probe

-sh-3.1# ls /sys/bus/platform/devices/
Fixed MDIO bus.0/    fsl-gianfar_mdio.-5/ mpc83xx_wdt.0/
fsl-ehci.0/          fsl-i2c.0/           serial8250/
fsl-gianfar.0/       fsl-i2c.1/           serial8250.0/
fsl-gianfar.1/       mpc83xx_spi.0/
-sh-3.1# ls /sys/bus/platform/drivers
drivers/           drivers_autoprobe drivers_probe
-sh-3.1# ls /sys/bus/platform/drivers/
fsl-ehci/         fsl-gianfar_mdio/ mpc83xx_spi/      serial8250/
fsl-gianfar/      fsl-i2c/          mpc83xx_wdt/

platform_bus必须在系统注册任何platform driver和platform device之前初始化，那么这是如何实现的呢？

http://lxr.linux.no/#linux+v2.6.25/drivers/base/init.c

  14/**  15 * driver_init - initialize driver model.  16 *  17 * Call the driver model init functions to initialize their  18 * subsystems. Called early from init/main.c.  19 */  20void __init driver_init(void)  21{  22        /* These are the core pieces */  23        devices_init();  24        buses_init();  25        classes_init();  26        firmware_init();  27        hypervisor_init();  28  29        /* These are also core pieces, but must come after the  30         * core core pieces.  31         */  32        platform_bus_init();  33        system_bus_init();  34        cpu_dev_init();  35        memory_dev_init();  36}

init/main.c
start_kernel 》 rest_init 》 kernel_init 》 do_basic_setup》driver_init 》platform_bus_init

http://lxr.linux.no/#linux+v2.6.25/drivers/base/init.c#L32

724/* 725 * Ok, the machine is now initialized. None of the devices 726 * have been touched yet, but the CPU subsystem is up and 727 * running, and memory and process management works. 728 * 729 * Now we can finally start doing some real work.. 730 */ 731static void __init do_basic_setup(void) 732{ 733        /* drivers will send hotplug events */ 734        init_workqueues(); 735        usermodehelper_init(); 736        driver_init(); 737        init_irq_proc(); 738        do_initcalls(); 739}

platform driver和platform device的初始化是在do_initcalls中进行的。

8.2 定义platform_device
http://lxr.linux.no/#linux+v2.6.25/arch/arm/plat-s3c24xx/devs.c#L276中定义了系统的资源，是一个高度可移植的文件，大部分板级资源都在这里集中定义。

274/* I2C */ 275 276static struct resource s3c_i2c_resource[] = { 277        [0] = { 278                .start = S3C24XX_PA_IIC, 279                .end   = S3C24XX_PA_IIC + S3C24XX_SZ_IIC - 1, 280                .flags = IORESOURCE_MEM, 281        }, 282        [1] = { 283                .start = IRQ_IIC, 284                .end   = IRQ_IIC, 285                .flags = IORESOURCE_IRQ, 286        } 287 288}; 289 290struct platform_device s3c_device_i2c = { 291        .name             = "s3c2410-i2c", 292        .id               = -1, 293        .num_resources    = ARRAY_SIZE(s3c_i2c_resource), 294        .resource         = s3c_i2c_resource, 295}; 296 297EXPORT_SYMBOL(s3c_device_i2c);

设备名称为s3c2410-i2c，“-1”只有一个i2c设备，两个资源s3c_i2c_resource，分别为i2c控制器的寄存器空间和中断信息。

8.3 注册platform_device

定义了platform_device后，需要添加到系统中，就可以调用函数platform_add_devices。
http://lxr.linux.no/#linux+v2.6.25/arch/arm/mach-s3c2440/mach-smdk2440.c

smdk2440_devices将系统资源组织起来，统一注册进内核。

151static struct platform_device *smdk2440_devices[] __initdata = { 152        &s3c_device_usb, 153        &s3c_device_lcd, 154        &s3c_device_wdt, 155        &s3c_device_i2c, 156        &s3c_device_iis, 157};166static void __init smdk2440_machine_init(void) 167{ 168        s3c24xx_fb_set_platdata(&smdk2440_fb_info); 169 170        platform_add_devices(smdk2440_devices, ARRAY_SIZE(smdk2440_devices)); 171        smdk_machine_init(); 172} 173 174MACHINE_START(S3C2440, "SMDK2440") 175        /* Maintainer: Ben Dooks <ben@fluff.org> */ 176        .phys_io        = S3C2410_PA_UART, 177        .io_pg_offst    = (((u32)S3C24XX_VA_UART) >> 18) & 0xfffc, 178        .boot_params    = S3C2410_SDRAM_PA + 0x100, 179 180        .init_irq       = s3c24xx_init_irq, 181        .map_io         = smdk2440_map_io, 182        .init_machine   = smdk2440_machine_init, 183        .timer          = &s3c24xx_timer, 184MACHINE_END

170 platform_add_devices(smdk2440_devices, ARRAY_SIZE(smdk2440_devices));
将系统所有资源注册进系统，在此之前platform bus需要初始化成功，否则无法将platform devices挂接到platform bus上。为了保证platform drive初始化时，相关platform资源已经注册进系统，smdk2440_machine_init需要很早执行，而其作为平台初始化init_machine 时，将优先于系统所有驱动的初始化。

其调用顺序如下：
start_kernel》setup_arch》init_machine》arch_initcall(customize_machine)
http://lxr.linux.no/#linux+v2.6.25/arch/arm/kernel/setup.c#L788

786arch_initcall(customize_machine); 787 788void __init setup_arch(char **cmdline_p) 789{ 790        struct tag *tags = (struct tag *)&init_tags; 791        struct machine_desc *mdesc; 792        char *from = default_command_line; 793 794        setup_processor(); 795        mdesc = setup_machine(machine_arch_type);//根据machine id获得移植时定义的machine desc结构 796        machine_name = mdesc->name; 797 798        if (mdesc->soft_reboot) 799                reboot_setup("s"); 800 801        if (__atags_pointer) 802                tags = phys_to_virt(__atags_pointer); 803        else if (mdesc->boot_params) 804                tags = phys_to_virt(mdesc->boot_params); 805 806        /* 807         * If we have the old style parameters, convert them to 808         * a tag list. 809         */ 810        if (tags->hdr.tag != ATAG_CORE) 811                convert_to_tag_list(tags); 812        if (tags->hdr.tag != ATAG_CORE) 813                tags = (struct tag *)&init_tags; 814 815        if (mdesc->fixup) 816                mdesc->fixup(mdesc, tags, &from, &meminfo); 817 818        if (tags->hdr.tag == ATAG_CORE) { 819                if (meminfo.nr_banks != 0) 820                        squash_mem_tags(tags); 821                save_atags(tags); 822                parse_tags(tags); 823        } 824 825        init_mm.start_code = (unsigned long) &_text; 826        init_mm.end_code   = (unsigned long) &_etext; 827        init_mm.end_data   = (unsigned long) &_edata; 828        init_mm.brk        = (unsigned long) &_end; 829 830        memcpy(boot_command_line, from, COMMAND_LINE_SIZE); 831        boot_command_line[COMMAND_LINE_SIZE-1] = '/0'; 832        parse_cmdline(cmdline_p, from); 833        paging_init(&meminfo, mdesc); 834        request_standard_resources(&meminfo, mdesc); 835 836#ifdef CONFIG_SMP 837        smp_init_cpus(); 838#endif 839 840        cpu_init(); 841 842        /* 843         * Set up various architecture-specific pointers 844         */ 845        init_arch_irq = mdesc->init_irq; 846        system_timer = mdesc->timer; 847        init_machine = mdesc->init_machine;//对init_machine指针赋值 848 849#ifdef CONFIG_VT 850#if defined(CONFIG_VGA_CONSOLE) 851        conswitchp = &vga_con; 852#elif defined(CONFIG_DUMMY_CONSOLE) 853        conswitchp = &dummy_con; 854#endif 855#endif 856}777static void (*init_machine)(void) __initdata; 778 779static int __init customize_machine(void) 780{ 781        /* customizes platform devices, or adds new ones */ 782        if (init_machine) 783                init_machine(); 784        return 0; 785} 786arch_initcall(customize_machine);

arch_initcall将customize_machine放在特定的段中，系统将在某个地方运行所有的arch_initcall修饰的函数。

http://lxr.linux.no/#linux+v2.6.25/include/linux/init.h#L182

152#ifndef MODULE  //非可加载模块，即编译链接进内核的代码 153 154#ifndef __ASSEMBLY__ 155 156/* initcalls are now grouped by functionality into separate  157 * subsections. Ordering inside the subsections is determined 158 * by link order.  159 * For backwards compatibility, initcall() puts the call in  160 * the device init subsection. 161 * 162 * The `id' arg to __define_initcall() is needed so that multiple initcalls 163 * can point at the same handler without causing duplicate-symbol build errors. 164 */ 165 166#define __define_initcall(level,fn,id) / 167        static initcall_t __initcall_##fn##id __used / 168        __attribute__((__section__(".initcall" level ".init"))) = fn 169 170/* 171 * A "pure" initcall has no dependencies on anything else, and purely 172 * initializes variables that couldn't be statically initialized. 173 * 174 * This only exists for built-in code, not for modules. 175 */ 176#define pure_initcall(fn)               __define_initcall("0",fn,0) 177 178#define core_initcall(fn)               __define_initcall("1",fn,1) 179#define core_initcall_sync(fn)          __define_initcall("1s",fn,1s) 180#define postcore_initcall(fn)           __define_initcall("2",fn,2) 181#define postcore_initcall_sync(fn)      __define_initcall("2s",fn,2s) 182#define arch_initcall(fn)               __define_initcall("3",fn,3) 183#define arch_initcall_sync(fn)          __define_initcall("3s",fn,3s) 184#define subsys_initcall(fn)             __define_initcall("4",fn,4) 185#define subsys_initcall_sync(fn)        __define_initcall("4s",fn,4s) 186#define fs_initcall(fn)                 __define_initcall("5",fn,5) 187#define fs_initcall_sync(fn)            __define_initcall("5s",fn,5s) 188#define rootfs_initcall(fn)             __define_initcall("rootfs",fn,rootfs) 189#define device_initcall(fn)             __define_initcall("6",fn,6) 190#define device_initcall_sync(fn)        __define_initcall("6s",fn,6s) 191#define late_initcall(fn)               __define_initcall("7",fn,7) 192#define late_initcall_sync(fn)          __define_initcall("7s",fn,7s) 193 194#define __initcall(fn) device_initcall(fn) 195 196#define __exitcall(fn) / 197        static exitcall_t __exitcall_##fn __exit_call = fn 198。。。。。。。。。 239#endif /* __ASSEMBLY__ */ 240 241/** 242 * module_init() - driver initialization entry point 243 * @x: function to be run at kernel boot time or module insertion 244 *  245 * module_init() will either be called during do_initcalls() (if 246 * builtin) or at module insertion time (if a module).  There can only 247 * be one per module. 248 */ 249#define module_init(x)  __initcall(x); 250 251/** 252 * module_exit() - driver exit entry point 253 * @x: function to be run when driver is removed 254 *  255 * module_exit() will wrap the driver clean-up code 256 * with cleanup_module() when used with rmmod when 257 * the driver is a module.  If the driver is statically 258 * compiled into the kernel, module_exit() has no effect. 259 * There can only be one per module. 260 */ 261#define module_exit(x)  __exitcall(x); 262 263#else /* MODULE */

各种xx_core_initcall被定义到了不同的分级的段中
所以arch_initcall == __initcall_fn3 它将被链接器放于section .initcall3.init. 中

module_init()==__initcall(fn)==device_initcall(fn)== __initcall_fn6

各个段的优先级由链接脚本定义
http://lxr.linux.no/#linux+v2.6.25/include/asm-generic/vmlinux.lds.h#L328

#define INITCALLS       /   *(.initcall0.init)      /   *(.initcall0s.init)      /   *(.initcall1.init)      /   *(.initcall1s.init)      /   *(.initcall2.init)      /   *(.initcall2s.init)      /   *(.initcall3.init)      /   *(.initcall3s.init)      /   *(.initcall4.init)      /   *(.initcall4s.init)      /   *(.initcall5.init)      /   *(.initcall5s.init)      / *(.initcallrootfs.init)      /   *(.initcall6.init)      /   *(.initcall6s.init)      /   *(.initcall7.init)      /   *(.initcall7s.init)这个__initcall_start是在文件arch/xxx/kernel/vmlinux.lds.S定义的:__initcall_start = .;   INITCALLS  __initcall_end = .;http://lxr.linux.no/#linux+v2.6.25/init/main.c#L664664static void __init do_initcalls(void) 665{ 666        initcall_t *call; 667        int count = preempt_count(); 668 669        for (call = __initcall_start; call < __initcall_end; call++) {.。。。。 682 683                result = (*call)(); 684。。。 }                720        /* Make sure there is no pending stuff from the initcall sequence */ 721        flush_scheduled_work(); 722}

因此__initcall_fnx，数字越小，越先被调用，故arch_initcall优先于module_init所修饰的函数。

arch_initcall修饰的函数的调用顺序如下：
start_kernel 》 rest_init（在setup_arch之后）》 kernel_init 》 do_basic_setup》do_initcalls（在driver_init()之后），因为platform_bus_init在此之前已经初始化完毕了，便可将设备挂接到总线上了。

8.4 定义platform_driver
Platform bus和设备都定义好了后，需要定义一个platform driver用来驱动此设备。

对于设备来说：

290struct platform_device s3c_device_i2c = { 291        .name             = "s3c2410-i2c", 292        .id               = -1, 293        .num_resources    = ARRAY_SIZE(s3c_i2c_resource), 294        .resource         = s3c_i2c_resource, 295}; 296 297EXPORT_SYMBOL(s3c_device_i2c);

根据platform总线上device和driver的匹配规则可知，I2C 的platform driver的名字是s3c2410-i2c。

http://lxr.linux.no/#linux+v2.6.25/drivers/i2c/busses/i2c-s3c2410.c#L1

903/* device driver for platform bus bits */ 904 905static struct platform_driver s3c2410_i2c_driver = { 906        .probe          = s3c24xx_i2c_probe, 907        .remove         = s3c24xx_i2c_remove, 908        .resume         = s3c24xx_i2c_resume, 909        .driver         = { 910                .owner  = THIS_MODULE, 911                .name   = "s3c2410-i2c", 912        }, 913};

8.5 注册platform_driver
http://lxr.linux.no/#linux+v2.6.25/drivers/i2c/busses/i2c-s3c2410.c#L1

925static int __init i2c_adap_s3c_init(void) 926{ 927        int ret; 928 929        ret = platform_driver_register(&s3c2410_i2c_driver); 930        if (ret == 0) { 931                ret = platform_driver_register(&s3c2440_i2c_driver); 932                if (ret) 933                        platform_driver_unregister(&s3c2410_i2c_driver); 934        } 935 936        return ret; 937} 938945module_init(i2c_adap_s3c_init); 946module_exit(i2c_adap_s3c_exit);

在i2c_adap_s3c_init中注册s3c2410_i2c_driver，那么i2c_adap_s3c_init何时执行的呢？module_init(i2c_adap_s3c_init)表明其存放在initcall段，调用顺序如下：
init/main.c
start_kernel 》 rest_init 》 kernel_init 》 do_basic_setup》do_initcalls，因为platform_bus_init在此之前已经初始化完毕了，且设备已经注册到内核中了，驱动将和内核绑定，并最终调用s3c24xx_i2c_probe。

748/* s3c24xx_i2c_probe 749 * 750 * called by the bus driver when a suitable device is found 751*/ 752 753static int s3c24xx_i2c_probe(struct platform_device *pdev) 754{ 755        struct s3c24xx_i2c *i2c = &s3c24xx_i2c; 756        struct resource *res; 757        int ret; 758 759        /* find the clock and enable it */ 760 761        i2c->dev = &pdev->dev; 762        i2c->clk = clk_get(&pdev->dev, "i2c"); 763        if (IS_ERR(i2c->clk)) { 764                dev_err(&pdev->dev, "cannot get clock/n"); 765                ret = -ENOENT; 766                goto err_noclk; 767        } 768 769        dev_dbg(&pdev->dev, "clock source %p/n", i2c->clk); 770 771        clk_enable(i2c->clk); 772 773        /* map the registers */ 774 775        res = platform_get_resource(pdev, IORESOURCE_MEM, 0); 776        if (res == NULL) { 777                dev_err(&pdev->dev, "cannot find IO resource/n"); 778                ret = -ENOENT; 779                goto err_clk; 780        } 781 782        i2c->ioarea = request_mem_region(res->start, (res->end-res->start)+1, 783                                         pdev->name); 784 785        if (i2c->ioarea == NULL) { 786                dev_err(&pdev->dev, "cannot request IO/n"); 787                ret = -ENXIO; 788                goto err_clk; 789        } 790 791        i2c->regs = ioremap(res->start, (res->end-res->start)+1); 792 793        if (i2c->regs == NULL) { 794                dev_err(&pdev->dev, "cannot map IO/n"); 795                ret = -ENXIO; 796                goto err_ioarea; 797        } 798 799        dev_dbg(&pdev->dev, "registers %p (%p, %p)/n", i2c->regs, i2c->ioarea, res); 800 801        /* setup info block for the i2c core */ 802 803        i2c->adap.algo_data = i2c; 804        i2c->adap.dev.parent = &pdev->dev; 805 806        /* initialise the i2c controller */ 807 808        ret = s3c24xx_i2c_init(i2c); 809        if (ret != 0) 810                goto err_iomap; 811 812        /* find the IRQ for this unit (note, this relies on the init call to 813         * ensure no current IRQs pending  814         */ 815 816        res = platform_get_resource(pdev, IORESOURCE_IRQ, 0); 817        if (res == NULL) { 818                dev_err(&pdev->dev, "cannot find IRQ/n"); 819                ret = -ENOENT; 820                goto err_iomap; 821        } 822 823        ret = request_irq(res->start, s3c24xx_i2c_irq, IRQF_DISABLED, 824                          pdev->name, i2c); 825 826        if (ret != 0) { 827                dev_err(&pdev->dev, "cannot claim IRQ/n"); 828                goto err_iomap; 829        } 830 831        i2c->irq = res; 832                 833        dev_dbg(&pdev->dev, "irq resource %p (%lu)/n", res, 834                (unsigned long)res->start); 835 836        ret = i2c_add_adapter(&i2c->adap); 837        if (ret < 0) { 838                dev_err(&pdev->dev, "failed to add bus to i2c core/n"); 839                goto err_irq; 840        } 841 842        platform_set_drvdata(pdev, i2c); 843 844        dev_info(&pdev->dev, "%s: S3C I2C adapter/n", i2c->adap.dev.bus_id); 845        return 0; 846 847 err_irq: 848        free_irq(i2c->irq->start, i2c); 849 850 err_iomap: 851        iounmap(i2c->regs); 852 853 err_ioarea: 854        release_resource(i2c->ioarea); 855        kfree(i2c->ioarea); 856 857 err_clk: 858        clk_disable(i2c->clk); 859        clk_put(i2c->clk); 860 861 err_noclk: 862        return ret; 863}

当进入probe函数后，需要获取设备的资源信息，常用获取资源的函数主要是：
struct resource * platform_get_resource(struct platform_device *dev, unsigned int type, unsigned int num);
根据参数type所指定类型，例如IORESOURCE_MEM，来获取指定的资源。
struct int platform_get_irq(struct platform_device *dev, unsigned int num);
获取资源中的中断号。
struct resource * platform_get_resource_byname(struct platform_device *dev, unsigned int type, char *name);
根据参数name所指定的名称，来获取指定的资源。
int platform_get_irq_byname(struct platform_device *dev, char *name);
根据参数name所指定的名称，来获取资源中的中断号。

此probe函数获取物理IO空间，通过request_mem_region和ioremap等操作物理地址转换成内核中的虚拟地址，初始化I2C控制器，通过platform_get_irq或platform_get_resource得到设备的中断号以后，就可以调用request_irq函数来向系统注册中断，并将此I2C控制器添加到系统中。

8.6 操作设备
进行了platform_device_register 和platform_driver_register后，驱动的相应信息就出现在sys目录的相应文件夹下，然后，我们该如何调用设备呢？？怎么对设备进行打开读写等操作呢？？？

Platform总线只是为了方便管理挂接在CPU总线上的设备，与用户空间的交互，如读写还是需要利用file_operations。当然如果此platform设备无需和用户空间交互，则无需file_operations实例。

对于I2C总线来说，其file_operations如下：
http://lxr.linux.no/#linux+v2.6.25/drivers/i2c/i2c-core.c#L461

478static const struct file_operations i2cdev_fops = { 479        .owner          = THIS_MODULE, 480        .llseek         = no_llseek, 481        .read           = i2cdev_read, 482        .write          = i2cdev_write, 483        .ioctl          = i2cdev_ioctl, 484        .open           = i2cdev_open, 485        .release        = i2cdev_release, 486};

其和platform bus的区别在于，platform bus提供机制访问I2C 控制器本身的资源，而I2C总线提供访问I2C 控制器上挂接的I2C设备的机制。

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