【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
struct bus_type platform_bus_type = {
.name = "platform",
.dev_attrs = platform_dev_attrs,
.match = platform_match,
.uevent = platform_uevent,
.suspend = platform_suspend,
.suspend_late = platform_suspend_late,
.resume_early = platform_resume_early,
.resume = platform_resume,
};
EXPORT_SYMBOL_GPL(platform_bus_type);
#linux+v2.6.25/include/linux/device.h#L55
struct bus_type {
const char *name;
struct bus_attribute *bus_attrs;
struct device_attribute *dev_attrs;
struct driver_attribute *drv_attrs;
int (*match)(struct device *dev, struct device_driver *drv);
int (*uevent)(struct device *dev, struct kobj_uevent_env *env);
int (*probe)(struct device *dev);
int (*remove)(struct device *dev);
void (*shutdown)(struct device *dev);
int (*suspend)(struct device *dev, pm_message_t state);
int (*suspend_late)(struct device *dev, pm_message_t state);
int (*resume_early)(struct device *dev);
int (*resume)(struct device *dev);
struct bus_type_private *p;
};
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：
struct platform_device {
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
int id;
struct device dev;
u32 num_resources;
struct resource *resource; //定义平台设备的资源
};
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：
/*
* Resources are tree-like, allowing
* nesting etc..
*/
struct resource {
resource_size_t start; //定义资源的起始地址
resource_size_t end; //定义资源的结束地址
const char *name; //定义资源的名称
unsigned long flags; 定义资源的类型，比如MEM，IO，IRQ，DMA类型
struct resource *parent, *sibling, *child;
};
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
struct device {
struct klist klist_children;
struct klist_node knode_parent; /* node in sibling list */
struct klist_node knode_driver;
struct klist_node knode_bus;
struct device *parent;
struct kobject kobj;
char bus_id[BUS_ID_SIZE]; /* position on parent bus */
struct device_type *type;
unsigned is_registered:1;
unsigned uevent_suppress:1;
struct semaphore sem; /* semaphore to synchronize calls to
* its driver.
*/
struct bus_type *bus; /* type of bus device is on */
struct device_driver *driver; /* which driver has allocated this
device */
void *driver_data; /* data private to the driver */
void *platform_data; /* Platform specific data, device
core doesn't touch it */
struct dev_pm_info power;
#ifdef CONFIG_NUMA
int numa_node; /* NUMA node this device is close to */
#endif
u64 *dma_mask; /* dma mask (if dma'able device) */
u64 coherent_dma_mask;/* Like dma_mask, but for
alloc_coherent mappings as
not all hardware supports
64 bit addresses for consistent
allocations such descriptors. */
struct device_dma_parameters *dma_parms;
struct list_head dma_pools; /* dma pools (if dma'ble) */
struct dma_coherent_mem *dma_mem; /* internal for coherent mem override */
/* arch specific additions */
struct dev_archdata archdata;
spinlock_t devres_lock;
struct list_head devres_head;
/* class_device migration path */
struct list_head node;
struct class *class;
dev_t devt; /* dev_t, creates the sysfs "dev" */
struct attribute_group **groups; /* optional groups */
void (*release)(struct device *dev);
};
3 device_register和platform_device_register
#linux+v2.6.25/drivers/base/core.c#L881
/**
* device_register - register a device with the system.
* @dev: pointer to the device structure
*
* This happens in two clean steps - initialize the device
* and add it to the system. The two steps can be called
* separately, but this is the easiest and most common.
* I.e. you should only call the two helpers separately if
* have a clearly defined need to use and refcount the device
* before it is added to the hierarchy.
*/
1int device_register(struct device *dev)
{
device_initialize(dev);
return device_add(dev);
}
初始化一个设备，然后加入到系统中。
#linux+v2.6.25/drivers/base/platform.c#L325
/**
* platform_device_register - add a platform-level device
* @pdev: platform device we're adding
*/
int platform_device_register(struct platform_device *pdev)
{
device_initialize(&pdev->dev);
return platform_device_add(pdev);
}
EXPORT_SYMBOL_GPL(platform_device_register);
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。
/**
* platform_device_add - add a platform device to device hierarchy
* @pdev: platform device we're adding
*
* This is part 2 of platform_device_register(), though may be called
* separately _iff_ pdev was allocated by platform_device_alloc().
*/
int platform_device_add(struct platform_device *pdev)
{
int i, ret = 0;
if (!pdev)
return -EINVAL;
//初始化设备的parent为platform_bus，初始化设备的总线为platform_bus_type。
if (!pdev->dev.parent)
pdev->dev.parent = &platform_bus;
pdev->dev.bus = &platform_bus_type;
/*＋＋＋＋＋＋＋＋＋＋＋＋＋＋
The platform_device.dev.bus_id is the canonical name for the devices.
It's built from two components:
* platform_device.name ... which is also used to for driver matching.
* platform_device.id ... the device instance number, or else "-1"
to indicate there's only one.
These are concatenated, so name/id "serial"/0 indicates bus_id "serial.0", and
"serial/3" indicates bus_id "serial.3"; both would use the platform_driver
named "serial". While "my_rtc"/-1 would be bus_id "my_rtc" (no instance id)
and use the platform_driver called "my_rtc".
＋＋＋＋＋＋＋＋＋＋＋＋＋＋*/
if (pdev->id != -1)
snprintf(pdev->dev.bus_id, BUS_ID_SIZE, "%s.%d", pdev->name,
pdev->id);
else
strlcpy(pdev->dev.bus_id, pdev->name, BUS_ID_SIZE);
//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。
for (i = 0; i < pdev->num_resources; i++) {
struct resource *p, *r = &pdev->resource[i];
if (r->name == NULL)
r->name = pdev->dev.bus_id;
p = r->parent;
if (!p) {
if (r->flags & IORESOURCE_MEM)
p = &iomem_resource;
else if (r->flags & IORESOURCE_IO)
p = &ioport_resource;
}
//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
if (p && insert_resource(p, r)) {
printk(KERN_ERR
"%s: failed to claim resource %d/n",
pdev->dev.bus_id, i);
ret = -EBUSY;
goto failed;
}
}
pr_debug("Registering platform device '%s'. Parent at %s/n",
pdev->dev.bus_id, pdev->dev.parent->bus_id);
ret = device_add(&pdev->dev);
if (ret == 0)
return ret;
failed:
while (--i >= 0)
if (pdev->resource[i].flags & (IORESOURCE_MEM|IORESOURCE_IO))
release_resource(&pdev->resource[i]);
return ret;
}
EXPORT_SYMBOL_GPL(platform_device_add);
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：
struct platform_driver {
int (*probe)(struct platform_device *);
int (*remove)(struct platform_device *);
void (*shutdown)(struct platform_device *);
int (*suspend)(struct platform_device *, pm_message_t state);
int (*suspend_late)(struct platform_device *, pm_message_t state);
int (*resume_early)(struct platform_device *);
int (*resume)(struct platform_device *);
struct device_driver driver;
};
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：
struct device_driver {
const char *name;
struct bus_type *bus;
struct module *owner;
const char *mod_name; /* used for built-in modules */
int (*probe) (struct device *dev);
int (*remove) (struct device *dev);
void (*shutdown) (struct device *dev);
int (*suspend) (struct device *dev, pm_message_t state);
int (*resume) (struct device *dev);
struct attribute_group **groups;
struct driver_private *p;
};
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：
/**
* platform_driver_register
* @drv: platform driver structure
*/
int platform_driver_register(struct platform_driver *drv)
{
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。*/
if (drv->probe)
drv->driver.probe = platform_drv_probe;
//在really_probe函数中，回调了platform_drv_probe函数
if (drv->remove)
drv->driver.remove = platform_drv_remove;
if (drv->shutdown)
drv->driver.shutdown = platform_drv_shutdown;
if (drv->suspend)
drv->driver.suspend = platform_drv_suspend;
if (drv->resume)
drv->driver.resume = platform_drv_resume;
return driver_register(&drv->driver);
}
EXPORT_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。
/**
* driver_register - register driver with bus
* @drv: driver to register
*
* We pass off most of the work to the bus_add_driver() call,
* since most of the things we have to do deal with the bus
* structures.
*/
int driver_register(struct device_driver *drv)
{
int ret;
//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
if ((drv->bus->probe && drv->probe) ||
(drv->bus->remove && drv->remove) ||
(drv->bus->shutdown && drv->shutdown))
printk(KERN_WARNING "Driver '%s' needs updating - please use "
"bus_type methods/n", drv->name);
//将驱动挂接到总线上，通过总线来驱动设备。
ret = bus_add_driver(drv);
if (ret)
return ret;
ret = driver_add_groups(drv, drv->groups);
if (ret)
bus_remove_driver(drv);
return ret;
}
EXPORT_SYMBOL_GPL(driver_register);
/** * bus_add_driver - Add a driver to the bus. * @drv: driver. */int bus_add_driver(struct device_driver *drv){ struct bus_type *bus; struct driver_private *priv; int error = 0; bus = bus_get(drv->bus); if (!bus) return -EINVAL; pr_debug("bus: '%s': add driver %s/n", bus->name, drv->name); priv = kzalloc(sizeof(*priv), GFP_KERNEL); if (!priv) { error = -ENOMEM; goto out_put_bus; } klist_init(&priv->klist_devices, NULL, NULL); priv->driver = drv; drv->p = priv; priv->kobj.kset = bus->p->drivers_kset; error = kobject_init_and_add(&priv->kobj, &driver_ktype, NULL, "%s", drv->name); if (error) goto out_unregister; if (drv->bus->p->drivers_autoprobe) { error = driver_attach(drv); if (error) goto out_unregister; } klist_add_tail(&priv->knode_bus, &bus->p->klist_drivers); module_add_driver(drv->owner, drv); error = driver_create_file(drv, &driver_attr_uevent); if (error) { printk(KERN_ERR "%s: uevent attr (%s) failed/n", __FUNCTION__, drv->name); } error = driver_add_attrs(bus, drv); if (error) { /* How the hell do we get out of this pickle? Give up */ printk(KERN_ERR "%s: driver_add_attrs(%s) failed/n", __FUNCTION__, drv->name); } error = add_bind_files(drv); if (error) { /* Ditto */ printk(KERN_ERR "%s: add_bind_files(%s) failed/n", __FUNCTION__, drv->name); } kobject_uevent(&priv->kobj, KOBJ_ADD); return error;out_unregister: kobject_put(&priv->kobj);out_put_bus: bus_put(bus); return error;}
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
/**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。
* driver_attach - try to bind driver to devices.
* @drv: driver.
*
* Walk the list of devices that the bus has on it and try to
* match the driver with each one. If driver_probe_device()
* returns 0 and the @dev->driver is set, we've found a
* compatible pair.
*/
int driver_attach(struct device_driver *drv)
{
return bus_for_each_dev(drv->bus, NULL, drv, __driver_attach);
}
EXPORT_SYMBOL_GPL(driver_attach);
static int __driver_attach(struct device *dev, void *data)#linux+v2.6.25/drivers/base/dd.c#L187
{
struct device_driver *drv = data;
/*
* Lock device and try to bind to it. We drop the error
* here and always return 0, because we need to keep trying
* to bind to devices and some drivers will return an error
* simply if it didn't support the device.
*
* driver_probe_device() will spit a warning if there
* is an error.
*/
if (dev->parent) /* Needed for USB */
down(&dev->parent->sem);
down(&dev->sem);
//如果该设备尚没有匹配的driver，则尝试匹配。
if (!dev->driver)
driver_probe_device(drv, dev);
up(&dev->sem);
if (dev->parent)
up(&dev->parent->sem);
return 0;
}
/**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：
* driver_probe_device - attempt to bind device & driver together
* @drv: driver to bind a device to
* @dev: device to try to bind to the driver
*
* First, we call the bus's match function, if one present, which should
* compare the device IDs the driver supports with the device IDs of the
* device. Note we don't do this ourselves because we don't know the
* format of the ID structures, nor what is to be considered a match and
* what is not.
*
* This function returns 1 if a match is found, -ENODEV if the device is
* not registered, and 0 otherwise.
*
* This function must be called with @dev->sem held. When called for a
* USB interface, @dev->parent->sem must be held as well.
*/
int driver_probe_device(struct device_driver *drv, struct device *dev)
{
int ret = 0;
if (!device_is_registered(dev))
return -ENODEV;
if (drv->bus->match && !drv->bus->match(dev, drv))
goto done;
pr_debug("bus: '%s': %s: matched device %s with driver %s/n",
drv->bus->name, __FUNCTION__, dev->bus_id, drv->name);
ret = really_probe(dev, drv);
done:
return ret;
}
#linux+v2.6.25/drivers/base/platform.c#L555
/**
* platform_match - bind platform device to platform driver.
* @dev: device.
* @drv: driver.
*
* Platform device IDs are assumed to be encoded like this:
* "<name><instance>", where <name> is a short description of the type of
* device, like "pci" or "floppy", and <instance> is the enumerated
* instance of the device, like '0' or '42'. Driver IDs are simply
* "<name>". So, extract the <name> from the platform_device structure,
* and compare it against the name of the driver. Return whether they match
* or not.
*/
static int platform_match(struct device *dev, struct device_driver *drv)
{
struct platform_device *pdev;
pdev = container_of(dev, struct platform_device, dev);
return (strncmp(pdev->name, drv->name, BUS_ID_SIZE) == 0);
}
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
static atomic_t probe_count = ATOMIC_INIT(0);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：
static DECLARE_WAIT_QUEUE_HEAD(probe_waitqueue);
static int really_probe(struct device *dev, struct device_driver *drv)
{
int ret = 0;
atomic_inc(&probe_count);
pr_debug("bus: '%s': %s: probing driver %s with device %s/n",
drv->bus->name, __FUNCTION__, drv->name, dev->bus_id);
WARN_ON(!list_empty(&dev->devres_head));
dev->driver = drv;
if (driver_sysfs_add(dev)) {
printk(KERN_ERR "%s: driver_sysfs_add(%s) failed/n",
__FUNCTION__, dev->bus_id);
goto probe_failed;
}
if (dev->bus->probe) {
ret = dev->bus->probe(dev);
if (ret)
goto probe_failed;
} else if (drv->probe) {
ret = drv->probe(dev);
if (ret)
goto probe_failed;
}
driver_bound(dev);
ret = 1;
pr_debug("bus: '%s': %s: bound device %s to driver %s/n",
drv->bus->name, __FUNCTION__, dev->bus_id, drv->name);
goto done;
probe_failed:
devres_release_all(dev);
driver_sysfs_remove(dev);
dev->driver = NULL;
if (ret != -ENODEV && ret != -ENXIO) {
/* driver matched but the probe failed */
printk(KERN_WARNING
"%s: probe of %s failed with error %d/n",
drv->name, dev->bus_id, ret);
}
/*
* Ignore errors returned by ->probe so that the next driver can try
* its luck.
*/
ret = 0;
done:
atomic_dec(&probe_count);
wake_up(&probe_waitqueue);
return ret;
}
#linux+v2.6.25/drivers/base/platform.c#L394
static int platform_drv_probe(struct device *_dev)
{
struct platform_driver *drv = to_platform_driver(_dev->driver);
struct platform_device *dev = to_platform_device(_dev);
return drv->probe(dev);
}
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
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
struct device platform_bus = {
.bus_id = "platform",
};
EXPORT_SYMBOL_GPL(platform_bus);
int __init platform_bus_init(void)
{
int error;
error = device_register(&platform_bus);
if (error)
return error;
error = bus_register(&platform_bus_type);
if (error)
device_unregister(&platform_bus);
return error;
}
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/<span style="font-family: Arial;"> </span>
Fixed MDIO bus.0 fsl-i2c.0 serial8250fsl-ehci.0 fsl-i2c.1 serial8250.0fsl-gianfar.0 mpc83xx_spi.0 ueventfsl-gianfar.1 mpc83xx_wdt.0fsl-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/ <span style="font-family: Arial;"> </span>
-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
/**
* driver_init - initialize driver model.
*
* Call the driver model init functions to initialize their
* subsystems. Called early from init/main.c.
*/
void __init driver_init(void)
{
/* These are the core pieces */
devices_init();
buses_init();
classes_init();
firmware_init();
hypervisor_init();
/* These are also core pieces, but must come after the
* core core pieces.
*/
platform_bus_init();
system_bus_init();
cpu_dev_init();
memory_dev_init();
}
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
/*
* Ok, the machine is now initialized. None of the devices
* have been touched yet, but the CPU subsystem is up and
* running, and memory and process management works.
*
* Now we can finally start doing some real work..
*/
static void __init do_basic_setup(void)
{
/* drivers will send hotplug events */
init_workqueues();
usermodehelper_init();
driver_init();
init_irq_proc();
do_initcalls();
}
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。
/* I2C */
static struct resource s3c_i2c_resource[] = {
[0] = {
.start = S3C24XX_PA_IIC,
.end = S3C24XX_PA_IIC + S3C24XX_SZ_IIC - 1,
.flags = IORESOURCE_MEM,
},
[1] = {
.start = IRQ_IIC,
.end = IRQ_IIC,
.flags = IORESOURCE_IRQ,
}
};
struct platform_device s3c_device_i2c = {
.name = "s3c2410-i2c",
.id = -1,
.num_resources = ARRAY_SIZE(s3c_i2c_resource),
.resource = s3c_i2c_resource,
};
EXPORT_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。
static struct platform_device *smdk2440_devices[] __initdata = {
&s3c_device_usb,
&s3c_device_lcd,
&s3c_device_wdt,
&s3c_device_i2c,
&s3c_device_iis,
};
static void __init smdk2440_machine_init(void)
{
s3c24xx_fb_set_platdata(&smdk2440_fb_info);
platform_add_devices(smdk2440_devices, ARRAY_SIZE(smdk2440_devices));
smdk_machine_init();
}
MACHINE_START(S3C2440, "SMDK2440")
/* Maintainer: Ben Dooks <ben@fluff.org> */
.phys_io = S3C2410_PA_UART,
.io_pg_offst = (((u32)S3C24XX_VA_UART) >> 18) & 0xfffc,
.boot_params = S3C2410_SDRAM_PA + 0x100,
.init_irq = s3c24xx_init_irq,
.map_io = smdk2440_map_io,
.init_machine = smdk2440_machine_init,
.timer = &s3c24xx_timer,
MACHINE_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
arch_initcall(customize_machine);
void __init setup_arch(char **cmdline_p)
{
struct tag *tags = (struct tag *)&init_tags;
struct machine_desc *mdesc;
char *from = default_command_line;
setup_processor();
mdesc = setup_machine(machine_arch_type);
//根据machine id获得移植时定义的machine desc结构
machine_name = mdesc->name;
if (mdesc->soft_reboot)
reboot_setup("s");
if (__atags_pointer)
tags = phys_to_virt(__atags_pointer);
else if (mdesc->boot_params)
tags = phys_to_virt(mdesc->boot_params);
/*
* If we have the old style parameters, convert them to
* a tag list.
*/
if (tags->hdr.tag != ATAG_CORE)
convert_to_tag_list(tags);
if (tags->hdr.tag != ATAG_CORE)
tags = (struct tag *)&init_tags;
if (mdesc->fixup)
mdesc->fixup(mdesc, tags, &from, &meminfo);
if (tags->hdr.tag == ATAG_CORE) {
if (meminfo.nr_banks != 0)
squash_mem_tags(tags);
save_atags(tags);
parse_tags(tags);
}
init_mm.start_code = (unsigned long) &_text;
init_mm.end_code = (unsigned long) &_etext;
init_mm.end_data = (unsigned long) &_edata;
init_mm.brk = (unsigned long) &_end;
memcpy(boot_command_line, from, COMMAND_LINE_SIZE);
boot_command_line[COMMAND_LINE_SIZE-1] = '/0';
parse_cmdline(cmdline_p, from);
paging_init(&meminfo, mdesc);
request_standard_resources(&meminfo, mdesc);
#ifdef CONFIG_SMP
smp_init_cpus();
#endif
cpu_init();
/*
* Set up various architecture-specific pointers
*/
init_arch_irq = mdesc->init_irq;
system_timer = mdesc->timer;
init_machine = mdesc->init_machine;
//对init_machine指针赋值
#ifdef CONFIG_VT
#if defined(CONFIG_VGA_CONSOLE)
conswitchp = &vga_con;
#elif defined(CONFIG_DUMMY_CONSOLE)
conswitchp = &dummy_con;
#endif
#endif
}
static void (*init_machine)(void) __initdata;
static int __init customize_machine(void)
{
/* customizes platform devices, or adds new ones */
if (init_machine)
init_machine();
return 0;
}
arch_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
#ifndef __ASSEMBLY__
/* initcalls are now grouped by functionality into separate
* subsections. Ordering inside the subsections is determined
* by link order.
* For backwards compatibility, initcall() puts the call in
* the device init subsection.
*
* The `id' arg to __define_initcall() is needed so that multiple initcalls
* can point at the same handler without causing duplicate-symbol build errors.
*/
#define __define_initcall(level,fn,id) /
static initcall_t __initcall_##fn##id __used /
__attribute__((__section__(".initcall" level ".init"))) = fn
/*
* A "pure" initcall has no dependencies on anything else, and purely
* initializes variables that couldn't be statically initialized.
*
* This only exists for built-in code, not for modules.
*/
#define pure_initcall(fn) __define_initcall("0",fn,0)
#define core_initcall(fn) __define_initcall("1",fn,1)
#define core_initcall_sync(fn) __define_initcall("1s",fn,1s)
#define postcore_initcall(fn) __define_initcall("2",fn,2)
#define postcore_initcall_sync(fn) __define_initcall("2s",fn,2s)
#define arch_initcall(fn) __define_initcall("3",fn,3)
#define arch_initcall_sync(fn) __define_initcall("3s",fn,3s)
#define subsys_initcall(fn) __define_initcall("4",fn,4)
#define subsys_initcall_sync(fn) __define_initcall("4s",fn,4s)
#define fs_initcall(fn) __define_initcall("5",fn,5)
#define fs_initcall_sync(fn) __define_initcall("5s",fn,5s)
#define rootfs_initcall(fn) __define_initcall("rootfs",fn,rootfs)
#define device_initcall(fn) __define_initcall("6",fn,6)
#define device_initcall_sync(fn) __define_initcall("6s",fn,6s)
#define late_initcall(fn) __define_initcall("7",fn,7)
#define late_initcall_sync(fn) __define_initcall("7s",fn,7s)
#define __initcall(fn) device_initcall(fn)
#define __exitcall(fn) /
static exitcall_t __exitcall_##fn __exit_call = fn
#endif /* __ASSEMBLY__ */
/**
* module_init() - driver initialization entry point
* @x: function to be run at kernel boot time or module insertion
*
* module_init() will either be called during do_initcalls() (if
* builtin) or at module insertion time (if a module). There can only
* be one per module.
*/
#define module_init(x) __initcall(x);
/**
* module_exit() - driver exit entry point
* @x: function to be run when driver is removed
*
* module_exit() will wrap the driver clean-up code
* with cleanup_module() when used with rmmod when
* the driver is a module. If the driver is statically
* compiled into the kernel, module_exit() has no effect.
* There can only be one per module.
*/
#define module_exit(x) __exitcall(x);
#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 /这个__initcall_start是在文件arch/xxx/kernel/vmlinux.lds.S定义的:
*(.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)
__initcall_start = .;#linux+v2.6.25/init/main.c#L664
INITCALLS
__initcall_end = .;
static void __init do_initcalls(void)
{
initcall_t *call;
int count = preempt_count();
for (call = __initcall_start; call < __initcall_end; call++) {
......
result = (*call)();......
}
/* Make sure there is no pending stuff from the initcall sequence */
flush_scheduled_work();
}
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：
struct platform_device s3c_device_i2c = {
.name = "s3c2410-i2c",
.id = -1,
.num_resources = ARRAY_SIZE(s3c_i2c_resource),
.resource = s3c_i2c_resource,
};
EXPORT_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
/* device driver for platform bus bits */8.5 注册platform_driver
static struct platform_driver s3c2410_i2c_driver = {
.probe = s3c24xx_i2c_probe,
.remove = s3c24xx_i2c_remove,
.resume = s3c24xx_i2c_resume,
.driver = {
.owner = THIS_MODULE,
.name = "s3c2410-i2c",
},
};
#linux+v2.6.25/drivers/i2c/busses/i2c-s3c2410.c#L1
static int __init i2c_adap_s3c_init(void)
{
int ret;
ret = platform_driver_register(&s3c2410_i2c_driver);
if (ret == 0) {
ret = platform_driver_register(&s3c2440_i2c_driver);
if (ret)
platform_driver_unregister(&s3c2410_i2c_driver);
}
return ret;
}
<span style="font-family: Arial;">module_init(i2c_adap_s3c_init);</span>
<span style="font-family: Arial;">module_exit(i2c_adap_s3c_exit);</span>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。
/* s3c24xx_i2c_probe
*
* called by the bus driver when a suitable device is found
*/
static int s3c24xx_i2c_probe(struct platform_device *pdev)
{
struct s3c24xx_i2c *i2c = &s3c24xx_i2c;
struct resource *res;
int ret;
/* find the clock and enable it */
i2c->dev = &pdev->dev;
i2c->clk = clk_get(&pdev->dev, "i2c");
if (IS_ERR(i2c->clk)) {
dev_err(&pdev->dev, "cannot get clock/n");
ret = -ENOENT;
goto err_noclk;
}
dev_dbg(&pdev->dev, "clock source %p/n", i2c->clk);
clk_enable(i2c->clk);
/* map the registers */
res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
if (res == NULL) {
dev_err(&pdev->dev, "cannot find IO resource/n");
ret = -ENOENT;
goto err_clk;
}
i2c->ioarea = request_mem_region(res->start, (res->end-res->start)+1,
pdev->name);
if (i2c->ioarea == NULL) {
dev_err(&pdev->dev, "cannot request IO/n");
ret = -ENXIO;
goto err_clk;
}
i2c->regs = ioremap(res->start, (res->end-res->start)+1);
if (i2c->regs == NULL) {
dev_err(&pdev->dev, "cannot map IO/n");
ret = -ENXIO;
goto err_ioarea;
}
dev_dbg(&pdev->dev, "registers %p (%p, %p)/n", i2c->regs, i2c->ioarea, res);
/* setup info block for the i2c core */
i2c->adap.algo_data = i2c;
i2c->adap.dev.parent = &pdev->dev;
/* initialise the i2c controller */
ret = s3c24xx_i2c_init(i2c);
if (ret != 0)
goto err_iomap;
/* find the IRQ for this unit (note, this relies on the init call to
* ensure no current IRQs pending
*/
res = platform_get_resource(pdev, IORESOURCE_IRQ, 0);
if (res == NULL) {
dev_err(&pdev->dev, "cannot find IRQ/n");
ret = -ENOENT;
goto err_iomap;
}
ret = request_irq(res->start, s3c24xx_i2c_irq, IRQF_DISABLED,
pdev->name, i2c);
if (ret != 0) {
dev_err(&pdev->dev, "cannot claim IRQ/n");
goto err_iomap;
}
i2c->irq = res;
dev_dbg(&pdev->dev, "irq resource %p (%lu)/n", res,
(unsigned long)res->start);
ret = i2c_add_adapter(&i2c->adap);
if (ret < 0) {
dev_err(&pdev->dev, "failed to add bus to i2c core/n");
goto err_irq;
}
platform_set_drvdata(pdev, i2c);
dev_info(&pdev->dev, "%s: S3C I2C adapter/n", i2c->adap.dev.bus_id);
return 0;
err_irq:
free_irq(i2c->irq->start, i2c);
err_iomap:
iounmap(i2c->regs);
err_ioarea:
release_resource(i2c->ioarea);
kfree(i2c->ioarea);
err_clk:
clk_disable(i2c->clk);
clk_put(i2c->clk);
err_noclk:
return ret;
}
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
static const struct file_operations i2cdev_fops = {
.owner = THIS_MODULE,
.llseek = no_llseek,
.read = i2cdev_read,
.write = i2cdev_write,
.ioctl = i2cdev_ioctl,
.open = i2cdev_open,
.release = i2cdev_release,
};
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。