【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#L422
422struct 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#L325
316/**
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_driver
named "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#L285
272/**
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#L187
170/**
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#L555
542/**
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 * "", where is a short description of the type of
549 * device, like "pci" or "floppy", and is the enumerated
550 * instance of the device, like ''0'' or ''42''. Driver IDs are simply
551 * "". So, extract the 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#L394
394static 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#L664
664static 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}
938
945module_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。
原文地址：http://blog.csdn.net/sailor_8318/archive/2010/01/29/5267698.aspx