【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: #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);
#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
#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#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： #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
#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。
#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注册到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，初始化设备的总线为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
#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
#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 #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#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);
//将驱动挂接到总线上，通过总线来驱动设备。 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);
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。
#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);
//如果该设备尚没有匹配的driver，则尝试匹配。 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}
#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： #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 * "<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。 #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}
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： #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
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: #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？
#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
#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 #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。 #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) #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。
#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 #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 = .;
#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。
#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 #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： #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。
