这种注册设备的方法是最经常看到的，据说是以前老版本的内核推荐的方法，个人感觉虽然这种方法使用起来方便，但是
却屏蔽了很多细节，对于学系内核来说并不是很直观。
使用此种方法注册的驱动程序，主次设备号都不能大于255
下面看一下几个函数的调用关系
register_chrdev
|---- __register_chrdev
|---- __register_chrdev_region
|---- cdev_alloc
|---- cdev_add
可以看出register_chardev函数封装了，分配设备号的__register_chrdev_region函数和注册设备的cdev_alloc函数和cdev_add函数，就是将上边的过程封装了起来，
使用设备类自动注册设备文件
自动注册设备节点需要mdev的支持，（PC平台叫做udev）mdev会以守护进程的形式运行，在后台监听sysfs的uevent事件，自动在/dev，目录下创建相应的设备节点。
为了满足这样的需求，根文件系统首先需要相应的支持。
1、必须存在sysfs文件系统 mount -t sysfs sysfs /sys
2、/dev目录需要挂载为tmpfs mount -t tmpfs mdev /dev
3、在设备开机时启动mdev, 在/etc/init.d/rcS中添加echo /bin/mdev > /proc/sys/kernel/hotplug && mdev -s
其次代码中也要添加创建设备节点的相应代码。
/* This
is a
#define to keep the compiler from merging different
* instances
of the __key variable */
#define class_create(owner, name)
/**
* class_destroy - destroys a struct
class structure
* @cls: pointer
to the struct
class that is to be destroyed
*
* Note,
the pointer
to be destroyed must have been created
with a call
*
to class_create().
*/
void class_destroy(struct
class *cls)
{
if ((cls == NULL) || (IS_ERR(cls)))
return;
class_unregister(cls);
}
class_create 是一个宏，参数有两个，owner指所属模块，一般为THIS_MODULE，name为设备类名，
此宏调用了 __class_create(owner, name, &__key); 函数，调用关系如下
__class_create
| ---- __class_register
| ---- kset_init
| ---- kobject_set_name
| ---- kset_register
| ---- kobject_add_internal
| ---- kobject_uevent v
| ---- add_class_attrs
| ---- class_create_file
/**
* device_create - creates a device
and registers
it with sysfs
* @
class: pointer
to the struct
class that this device should be registered
to
* @parent: pointer
to the parent struct device
of this new device,
if any
* @devt:
the dev_t
for the char device
to be added
* @drvdata:
the data
to be added
to the device
for callbacks
* @fmt:
string for the device's
name
*
* This function can be used
by char device classes. A struct device
* will be created
in sysfs, registered
to the specified
class.
*
* A
"dev" file will be created, showing
the dev_t
for the device,
if
*
the dev_t
is not 0,
0.
* If a pointer
to a parent struct device
is passed
in,
the newly created
* struct device will be a child
of that device
in sysfs.
* The pointer
to the struct device will be returned
from the call.
* Any further sysfs files
that might be required can be created using this
* pointer.
*
* Returns &struct device pointer
on success,
or ERR_PTR()
on error.
*
* Note:
the struct
class passed
to this function must have previously
* been created
with a call
to class_create().
*/
struct device *device_create(struct
class *
class, struct device *parent,
dev_t devt, void *drvdata, const char *fmt, ...)
/**
* device_destroy - removes a device
that was created
with device_create()
* @cla: pointer
to the struct
class that this device was registered
with
* @devt:
the dev_t
of the device
that was previously registered
*
* This call unregisters
and cleans up a device
that was created
with a
* call
to device_create().
*/
void device_destroy(struct
class *
class, dev_t devt)
