设备驱动dev驱动文件的创建(写驱动时再详细看可能会比较清楚)

    xiaoxiao2025-02-02  11

    转自讨论帖:http://bbs.chinaunix.net/thread-3575768-1-1.html

       我们在刚开始写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,实际上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,当然前提条件是用户空间移植了udev。 

        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。     注意,在2.6较早的内核版本中,device_create(…)函数名称不同,是class_device_create(…),所以在新的内核中编译以前的模块程序有时会报错,就是因为函数名称不同,而且里面的参数设置也有一些变化。  struct class和device_create(…) 以及device_create(…)都定义在/include/linux/device.h中,使用的时候一定要包含这个头文件,否则编译器会报错。  在2.6.26.6内核版本中,struct class定义在头文件include/linux/device.h中:  /*        * device classes        */      struct class {        const char        *name;        struct module     *owner;   nbsp;struct kset         subsys;        struct list_head         devices;        struct list_head         interfaces;        struct kset              class_dirs;        struct semaphore sem;    /* locks children, devices, interfaces */        struct class_attribute   *class_attrs;        struct device_attribute      *dev_attrs;    int (*dev_uevent)(struct device *dev, struct kobj_uevent_env *env);    void (*class_release)(struct class *class);        void (*dev_release)(struct device *dev);   int (*suspend)(struct device *dev, pm_message_t state);        int (*resume)(struct device *dev); }; class_create(…)在/drivers/base/class.c中实现:        /**      * 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.      *      * This is used to create a struct class pointer that can then be used      * in calls to device_create().      *      * 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 class *cls;        int retval;        cls = kzalloc(sizeof(*cls), GFP_KERNEL);        if (!cls) {             retval = -ENOMEM;             goto error;        }   cls->name = name;        cls->owner = owner;        cls->class_release = class_create_release;   retval = class_register(cls);        if (retval)             goto error;   return cls; error:        kfree(cls);        return ERR_PTR(retval);      }      第一个参数指定类的所有者是哪个模块,第二个参数指定类名。       在class.c中,还定义了class_destroy(…)函数,用于在模块卸载时删除类。  device_create(…)函数在/drivers/base/core.c中实现:       /**       * 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       * @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.       *       * 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, const char *fmt, ...)      {           va_list vargs;           struct device *dev;      va_start(vargs, fmt);           dev = device_create_vargs(class, parent, devt, NULL, fmt, vargs);           va_end(vargs);           return dev;      } 第一个参数指定所要创建的设备所从属的类,第二个参数是这个设备的父设备,如果没有就指定为NULL,第三个参数是设备号,第四个参数是设备名称,第五个参数是从设备号。  下面以一个简单字符设备驱动来展示如何使用这几个函数       #include <linux/module.h>      #include <linux/kernel.h>      #include <linux/init.h>      #include <linux/fs.h>      #include <linux/cdev.h>      #include <linux/device.h> MODULE_LICENSE ("GPL"); int hello_major = 555;      int hello_minor = 0;      int number_of_devices = 1; struct cdev cdev;      dev_t dev = 0; struct file_operations hello_fops = {        .owner = THIS_MODULE      }; static void char_reg_setup_cdev (void)      {         int error, devno = MKDEV (hello_major, hello_minor);         cdev_init (&cdev, &hello_fops);         cdev.owner = THIS_MODULE;         cdev.ops = &hello_fops;         error = cdev_add (&cdev, devno , 1);         if (error)             printk (KERN_NOTICE "Error %d adding char_reg_setup_cdev", error); } struct class *my_class; static int __init hello_2_init (void)      {         int result;         dev = MKDEV (hello_major, hello_minor);         result = register_chrdev_region (dev, number_of_devices, "hello");         if (result<0) {             printk (KERN_WARNING "hello: can't get major number %d\n", hello_major);             return result;       } char_reg_setup_cdev ();   /* create your own class under /sysfs */       my_class = class_create(THIS_MODULE, "my_class");       if(IS_ERR(my_class))        {            printk("Err: failed in creating class.\n");            return -1;         }    /* register your own device in sysfs, and this will cause udev to create corresponding device node */        device_create( my_class, NULL, MKDEV(hello_major, 0), "hello" "%d", 0 );   printk (KERN_INFO "Registered character driver\n");        return 0;      } static void __exit hello_2_exit (void)      {         dev_t devno = MKDEV (hello_major, hello_minor);         cdev_del (&cdev);    device_destroy(my_class, MKDEV(adc_major, 0));         //delete device node under /dev         class_destroy(my_class);                               //delete class created by us    unregister_chrdev_region (devno, number_of_devices);    printk (KERN_INFO "char driver cleaned up\n");      } module_init (hello_2_init);      module_exit (hello_2_exit); 这样,模块加载后,就能在/dev目录下找到hello0这个设备节点了。
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