linux device_create_file属性 怎么调用,device_create_file创建多级目录
开始写Linux设备驱动程序的时候，很多时候都是利用mknod命令手动创建设备节点(包括ldd3中不少例子也是这样)，实际上现在Linux内核为我们提供了一组函数，可以用来在模块加载的时候自动在/dev目录下创建相应设备节点，并在卸载模块时删除该节点。
内核中定义了struct class结构体，顾名思义，一个struct class结构体类型变量对应一个类，内核同时提供了class_create(…)函数，可以用它来创建一个类，这个类存放于sysfs下面，一旦创建好了这个类，再调用device_create(…)函数来在/dev目录下创建相应的设备节点。这样，加载模块的时候，用户空间中的udev会自动响应device_create(…)函数，去/sysfs下寻找对应的类从而创建设备节点。
此外，利用device_create_file函数可以在/sys/class/下创建对应的属性文件，从而通过对该文件的读写实现特定的数据操作。
一、class_create
官方说明：
/* This is a
#define to keep the compiler from merging different
* instances of the __key variable */
#define
class_create(owner, name) \
({ \
static struct lock_class_key __key; \
__class_create(owner, name, &__key); \
})
/**
* class_create - create a struct class
structure
* @owner: pointer to the module that is to
"own" this struct class
* @name: pointer to a string for the name of
this class.
* @key: the lock_class_key for this class;
used by mutex lock debugging
*
* This is used to create a struct class
pointer that can then be used
* in calls to device_create().
*
* Returns &struct class pointer on
success, or ERR_PTR() on error.
*
* Note, the pointer created here is to be
destroyed when finished by
* making a call to class_destroy().
*/
struct class *__class_create(struct
module *owner, const char *name,
struct lock_class_key *key)
关键的一句是：
* This is used to create a struct class
pointer that can then be used
* in calls to device_create().
-->这个函数用来创建一个struct class的结构体指针，这个指针可用作device_create()函数的参数。
也就是说，这个函数主要是在调用device_create()前使用，创建一个struct class类型的变量，并返回其指针。
二、device_create
官方说明：
/**
* 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, ...)
首先解释一下"sysfs"：sysfs是linux2.6所提供的一种虚拟档案系统；在设备模型中，sysfs文件系统用来表示设备的结构，将设备的层次结构形象的反应到用户空间中，从而可以通过修改sysfs中的文件属性来修改设备的属性值；sysfs被挂载到根目录下的"/sys"文件夹下。
三、device_create_file
官方说明：
/**
* device_create_file - create sysfs attribute
file for device.
* @dev: device.
* @attr: device attribute descriptor.
*/
int
device_create_file(struct device *dev,
const struct device_attribute *attr)
使用这个函数时要引用 device_create所返回的device*指针，作用是在/sys/class/下创建一个属性文件，从而通过对这个属性文件进行读写就能完成对应的数据操作。
如：
a.在驱动程序中使用device_create_file创建属性文件
static DEVICE_ATTR(val,
S_IRUGO | S_IWUSR, hello_val_show, hello_val_store);
/*读取寄存器val的值到缓冲区buf中，内部使用*/
static ssize_t
__hello_get_val(struct xxx_dev* dev, char* buf) {
int val = 0;
/*同步访问*/
if(down_interruptible(&(dev->sem))) {
return -ERESTARTSYS;
}
val = dev->val;
up(&(dev->sem));
return snprintf(buf, PAGE_SIZE, "%d/n",
val);
}
/*把缓冲区buf的值写到设备寄存器val中去，内部使用*/
static ssize_t
__hello_set_val(struct xxx_dev* dev, const char* buf, size_t count) {
int val = 0;
/*将字符串转换成数字*/
val = simple_strtol(buf, NULL, 10);
/*同步访问*/
if(down_interruptible(&(dev->sem))) {
return -ERESTARTSYS;
}
dev->val = val;
up(&(dev->sem));
return count;
}
/*读取设备属性val*/
static ssize_t
hello_val_show(struct device* dev, struct device_attribute* attr, char* buf) {
struct xxx_dev* hdev = (struct xxx_dev*)dev_get_drvdata(dev);
return __hello_get_val(hdev, buf);
}
/*写设备属性val*/
static ssize_t
hello_val_store(struct device* dev, struct device_attribute* attr, const char*
buf, size_t count) {
struct xxx_dev* hdev = (struct xxx_dev*)dev_get_drvdata(dev);
return __hello_set_val(hdev, buf, count);
}
/*模块加载方法*/
static int __init
xxx_init(void){
...
/*在/sys/class/xxx/xxx目录下创建属性文件val*/
err = device_create_file(temp, &dev_attr_val);
if(err < 0) {
printk(KERN_ALERT"Failed to create
attribute val.");
goto destroy_device;
}
...
}
b.在用户空间读取属性
...
read(dev->fd,
val, sizeof(*val));
...
write(dev->fd, &val,
sizeof(val));
...
四、使用示例
/*在/sys/class/目录下创建设备类别目录xxx*/
g_vircdev_class = class_create(THIS_MODULE,
VIRCDEV_CLASS_NAME);
if(IS_ERR(g_vircdev_class)) {
err = PTR_ERR(g_vircdev_class);
printk(KERN_ALERT "Failed to
create class.\n");
goto CLASS_CREATE_ERR;
}
/*在/dev/目录和/sys/class/xxx目录下分别创建设备文件xxx*/
dev = device_create(g_vircdev_class, NULL,
devt, NULL, VIRCDEV_DEVICE_NAME);
if(IS_ERR(dev)) {
err = PTR_ERR(dev);
printk(KERN_ALERT "Failed to
create device.\n");
goto DEVICE_CREATE_ERR;
}
/*在/sys/class/xxx/xxx目录下创建属性文件val*/
err = device_create_file(dev, attr);
if(err < 0) {
printk(KERN_ALERT"Failed to create
attribute file.");
goto DEVICE_CREATE_FILE_ERR;
}
参考资料：
sysfs: