接下来来看platform_driver结构体的原型定义，在include/linux/platform_device.h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;
};
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driver/base/platform.c文件中，具体实现代码如下：
int platform_driver_register(struct platform_driver *drv)
{
drv->driver.bus = &platform_bus_type;
if (drv->probe)
drv->driver.probe = platform_drv_probe;
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);
}
总结，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，拥有各自独自的资源(地址总线和IRQs)，都可以用platform_driver实现。如：LCD，网卡、USB、UART等，都可以用platfrom_driver写，而timer，irq等小系统之内的设备则最好不用platfrom_driver机制。
/*******************************************************************************/
device和driver之间是如何通过注册的名字进行连接的：
下面是转载的一份网友用dm9000为例写的联系：
1.3 驱动注册
下面是DM9000网卡的驱动加载代码：
static int __init
dm9000_init(void)
{
printk(KERN_INFO "%s Ethernet Driver\n", CARDNAME);
return platform_driver_register(&dm9000_driver); /* search board and register */
}
module_init(dm9000_init);
很简单的代码，直接调用platform_driver_register注册驱动，这里dm9000_driver的定义为：
static struct platform_driver dm9000_driver = {
.driver = {
.name = "dm9000",
.owner = THIS_MODULE,
},
.probe = dm9000_probe,
.remove = dm9000_drv_remove,
.suspend = dm9000_drv_suspend,
.resume = dm9000_drv_resume,
};
1.3.1 platform_driver_register
这个函数定义为：
/**
* platform_driver_register
* @drv: platform driver structure
*/
int platform_driver_register(struct platform_driver *drv)
{
drv->driver.bus = &platform_bus_type;
if (drv->probe)
drv->driver.probe = platform_drv_probe;
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);
}
注意由于DM9000的platform_driver中指定了probe，remove，suspend，resume这四个函数，因此device_driver结构体中的这几个函数指针将进行初始化设置。最后再调用driver_register注册driver成员，有点奇怪，怎么就抛弃了platform_driver呢？
1.3.2 driver_register
这个函数定义为：
/**
* 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)
{
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);
}
klist_init(&drv->klist_devices, NULL, NULL);
return bus_add_driver(drv);
}
当函数执行到这里的时候，drv->bus指向的是platform_bus_type这一全局变量。
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,
};
1.3.3 bus_add_driver
这个函数定义为：
/**
* bus_add_driver - Add a driver to the bus.
* @drv: driver.
*
*/
int bus_add_driver(struct device_driver *drv)
{
struct bus_type * bus = get_bus(drv->bus);
int error = 0;
if (!bus)
return -EINVAL;
pr_debug("bus %s: add driver %s\n", bus->name, drv->name);
error = kobject_set_name(&drv->kobj, "%s", drv->name);
if (error)
goto out_put_bus;
drv->kobj.kset = &bus->drivers;
if ((error = kobject_register(&drv->kobj)))
goto out_put_bus;
if (drv->bus->drivers_autoprobe) {
error = driver_attach(drv);
if (error)
goto out_unregister;
}
klist_add_tail(&drv->knode_bus, &bus->klist_drivers);
module_add_driver(drv->owner, drv);
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);
}
return error;
out_unregister:
kobject_unregister(&drv->kobj);
out_put_bus:
put_bus(bus);
return error;
}
当函数执行到此的时候，drv->bus将指向platform_bus_type这一全局变量，而这一全局变量的drivers_autoprobe成员在bus_register这一全局初始化函数中设置为1。因此这里将调用driver_attach函数，注意此时传递进去的参数drv指向的是dm9000_driver的driver成员。
1.3.4 driver_attach
这一函数定义为：
/**
* 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);
}
很简单，转向bus_for_each_dev。
1.3.5 bus_for_each_dev
这一函数定义为：
/**
* bus_for_each_dev - device iterator.
* @bus: bus type.
* @start: device to start iterating from.
* @data: data for the callback.
* @fn: function to be called for each device.
*
* Iterate over @bus's list of devices, and call @fn for each,
* passing it @data. If @start is not NULL, we use that device to
* begin iterating from.
*
* We check the return of @fn each time. If it returns anything
* other than 0, we break out and return that value.
*
* NOTE: The device that returns a non-zero value is not retained
* in any way, nor is its refcount incremented. If the caller needs
* to retain this data, it should do, and increment the reference
* count in the supplied callback.
*/
int bus_for_each_dev(struct bus_type * bus, struct device * start,
void * data, int (*fn)(struct device *, void *))
{
struct klist_iter i;
struct device * dev;
int error = 0;
if (!bus)
return -EINVAL;
klist_iter_init_node(&bus->klist_devices, &i,
(start ? &start->knode_bus : NULL));
while ((dev = next_device(&i)) && !error)
error = fn(dev, data);
klist_iter_exit(&i);
return error;
}
简单枚举此总线上注册的device，然后为其调用__driver_attach函数，试图将一个device和传递进来的driver相匹配。
1.3.6 __driver_attach
这一函数定义为：
static int __driver_attach(struct device * dev, void * data)
{
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);
if (!dev->driver)
driver_probe_device(drv, dev);
up(&dev->sem);
if (dev->parent)
up(&dev->parent->sem);
return 0;
}
很简单，转而调用driver_probe_device进行驱动的匹配。
1.3.7 driver_probe_device
这个函数定义为：
/**
* 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("%s: Matched Device %s with Driver %s\n",
drv->bus->name, dev->bus_id, drv->name);
ret = really_probe(dev, drv);
done:
return ret;
}
此时的drv->bus指向platform_bus_type这一全局变量，而它的match函数为platform_match，且让我们看看它是如何确定device和driver是否匹配的。
/**
* platform_match - bind platform device to platform driver.
* @dev: device.
* @drv: driver.
*
* Platform device IDs are assumed to be encoded like this:
* "", where is a short description of the
* type of device, like "pci" or "floppy", and is the
* enumerated instance of the device, like '0' or '42'.
* Driver IDs are simply "".
* So, extract the 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 = container_of(dev, struct platform_device, dev);
return (strncmp(pdev->name, drv->name, BUS_ID_SIZE) == 0);
}
也就是说，它通过比较pdev->name和drv->name是否匹配来决定。
对于DM9000的驱动来说，这里的pdev指向dm9000_bfin_device，看看它的初始值：
static struct platform_device dm9000_bfin_device = {
.name = "dm9000",
.id = -1,
.num_resources = ARRAY_SIZE(dm9000_bfin_resources),
.resource = dm9000_bfin_resources,
};
再看drv，其指向dm9000_driver这一变量中的driver成员。
static struct platform_driver dm9000_driver = {
.driver = {
.name = "dm9000",
.owner = THIS_MODULE,
},
.probe = dm9000_probe,
.remove = dm9000_drv_remove,
.suspend = dm9000_drv_suspend,
.resume = dm9000_drv_resume,
};
在进行了正确的名称匹配之后，将调用really_probe进行硬件检测。
1.3.8 really_probe
这一函数定义为：
static int really_probe(struct device *dev, struct device_driver *drv)
{
int ret = 0;
atomic_inc(&probe_count);
pr_debug("%s: Probing driver %s with device %s\n",
drv->bus->name, 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("%s: Bound Device %s to Driver %s\n",
drv->bus->name, 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;
}
此时的drv->bus指向platform_bus_type这一全局变量，其probe回调函数没有指定，而drv->probe函数则指向dm9000_probe。因此转向dm9000_probe执行，并将dm9000_bfin_device做为参数传递进去。
1.4 结论
platform device和driver分别向platform_bus_type这一中介注册，并通过名称进行相互间的匹配。很是有点婚姻中介的味道，还有点对暗号的神秘，呵呵！
2 参考资料
从DM9000驱动看platform device与driver的关系(2009-6-8)
uclinux内核驱动的初始化顺序(2009-6-7)
)
