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分类: LINUX

2014-05-28 18:13:03

在去年刚开始学Linux驱动的时候就跟着宋宝华的《Linux设备驱动开发详解》写了一个按键驱动,但是最近在看代码的时候发现很多时候人们写代码都是采用Platform bus管理的机制,所以将之前写过的按键驱动修改了一下,采用Platform_device和Platform_driver的机制实现,同时添加了在模块初始化函数中实现设备节点的初始化功能的自动创建。
在自动创建设备节点这个功能的时候比较郁闷,老是出现段错误,调了好久才发现原来里面用到的一个函数device_create函数在Linux 2.6版本中更新了好多次,网上的参考资料里面的这个函数都和我系统里面的不一致,所以出现了错误。不过在调试过程中总结出一个非常重要的结论:如果调试过程中出现了段错误,首先进行定位,看看具体是哪个函数导致段错误,如果是系统函数,那么一般就是你传递进去的参数错误,所以根据系统里面提供的函数进行逐个参数确定无误。
在Linux 2.6.27-14里面device_create原型是:
 

extern struct device *device_create(struct class *cls, struct device *parent,dev_t devt, void *drvdata, const char *fmt, ...)


下面转一篇文章,留意文章中的这个函数看看是否和你系统提供的一致。

------------------------------华丽的分界线-------------------------------

在驱动模块初始化函数中实现设备节点的自动创建

作者:杨硕,讲师。

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 classdevice_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()/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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