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分类: LINUX

2013-04-24 15:09:09

作者:苗德行,讲师。

从linux内核2.6的某个版本之后,devfs不复存在,udev成为devfs的替代。提醒一点,udev是应用层的,不要试图在内核的配置选项里找到它;加入对udev的支持很简单,以作者所写的一个字符设备驱动为例,在驱动初始化的代码里调用class_create为该设备创建一个class,再为每个设备调用device_create创建对应的设备。大致用法如下:

struct class *myclass ;
        class_create(THIS_MODULE, “my_device_driver”);
                device_create(myclass, NULL, MKDEV(major_num, minor_num), NULL, “my_device”);

这样的module被加载时,udev daemon就会自动在/dev下创建my_device设备文件。

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2.6.35内核版本中,struct class定义在头文件include/linux/device.h中
    

点击(此处)折叠或打开

  1. /*
  2.                 * device classes
  3.                 */
  4.                 struct class {
  5.                         const char *name;
  6.                         struct module *owner;
  7.                         struct class_attribute *class_attrs;
  8.                         struct device_attribute *dev_attrs;
  9.                         struct kobject *dev_kobj;
  10.                         int (*dev_uevent)(struct device *dev, struct kobj_uevent_env *env);
  11.                         char *(*devnode)(struct device *dev, mode_t *mode);
  12.                         void (*class_release)(struct class *class);
  13.                         void (*dev_release)(struct device *dev);
  14.                         int (*suspend)(struct device *dev, pm_message_t state);
  15.                         int (*resume)(struct device *dev);
  16.                         const struct kobj_ns_type_operations *ns_type;
  17.                         const void *(*namespace)(struct device *dev);
  18.                         const struct dev_pm_ops *pm;
  19.                         struct class_private *p;
  20.                         };
  21.                 class_create()/drivers/base/class.c中实现:
  22.         /**
  23.                 * class_create - create a struct class structure
  24.                 * @owner: pointer to the module that is to "own" this struct class
  25.                 * @name: pointer to a string for the name of this class.
  26.                 ;*
  27.                 * This is used to create a struct class pointer that can then be used
  28.                 * in calls to device_create().
  29.                 *
  30.                 * Note, the pointer created here is to be destroyed when finished by
  31.                 * making a call to class_destroy().
  32.                 */
  33.                 struct class *__class_create(struct module *owner, const char *name,
  34.                 struct lock_class_key *key)
  35.                 {
  36.                        struct class *cls;
  37.                         int retval;
  38.                         cls = kzalloc(sizeof(*cls), GFP_KERNEL);
  39.                         if (!cls) {
  40.                                 retval = -ENOMEM;
  41.                                goto error;
  42.                                }
  43.                         cls->name = name;
  44.                        cls->owner = owner;
  45.                         cls->class_release = class_create_release;
  46.                         retval = __class_register(cls, key);
  47.                         if (retval)
  48.                                 goto error;
  49.                                 return cls;
  50.                         error:
  51.                                 kfree(cls);
  52.                                 return ERR_PTR(retval);
  53.                 }

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, void *drvdata, const char *fmt, ...)
                {
                        va_list vargs;
                        struct device *dev;
                        va_start(vargs, fmt);
                        dev = device_create_vargs(class, parent, devt, drvdata, 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:

点击(此处)折叠或打开

  1. /*
  2.                  * Copyright (C) 2005 Farsight
  3.                  *
  4.                  * This program is free software; you can redistribute it and/or modify
  5.                  * it under the terms of the GNU General Public License as published by
  6.                  * the Free Software Foundation; either version 2 of the License, or
  7.                  * (at your option) any later version.
  8.                  * This program is distributed in the hope that it will be useful,
  9.                  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  10.                  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  11.                  * GNU General Public License for more details.
  12.                  * You should have received a copy of the GNU General Public License
  13.                  * along with this program; if not, write to the Free Software
  14.                  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
  15.                  *
  16.                  */
  17.                  #include <linux/module.h>
  18.                  #include <linux/kernel.h>
  19.                  #include <linux/init.h>
  20.                  #include <linux/fs.h>
  21.                  #include <linux/cdev.h>
  22.                  #include <asm/uaccess.h>
  23.                  #include <linux/device.h>
  24.                  MODULE_LICENSE ("GPL");
  25.                  int hello_major = 250;
  26.                  int hello_minor = 0;
  27.                  int number_of_devices = 1;
  28.                  struct cdev cdev;
  29.                  dev_t dev = 0;
  30.                  struct file_operations hello_fops = {
  31.                          .owner = THIS_MODULE,
  32.                   };
  33.                  struct class *my_class;
  34.                  static void char_reg_setup_cdev (void)
  35.                  {
  36.                          int error, devno = MKDEV (hello_major, hello_minor);
  37.                          cdev_init (&cdev, &hello_fops);
  38.                          cdev.owner = THIS_MODULE;
  39.                          cdev.ops = &hello_fops;
  40.                          error = cdev_add (&cdev, devno , 1);
  41.                          if (error)
  42.                                  printk (KERN_NOTICE "Error %d adding char_reg_setup_cdev", error);
  43.                          /* creating your own class */
  44.                          my_class =class_create(THIS_MODULE, "farsight_class");
  45.                          if(IS_ERR(my_class)) {
  46.                                  printk("Err: failed in creating class.\n");
  47.                                  return ;
  48.                          }
  49.                          /* register your own device in sysfs, and this will cause udevd to create corresponding device node */
  50.                          device_create(my_class,NULL, devno, NULL,"hello");
  51.                          }
  52.                  static int __init hello_2_init (void)
  53.                  {
  54.                          int result;
  55.                          dev = MKDEV (hello_major, hello_minor);
  56.                          result = register_chrdev_region (dev, number_of_devices, "test");
  57.                          if (result<0) {
  58.                                  printk (KERN_WARNING "hello: can't get major number %d\n", hello_major);
  59.                                  return result;
  60.                          }
  61.                          char_reg_setup_cdev ();
  62.                          printk (KERN_INFO "char device registered\n");
  63.                          return 0;
  64.                  }
  65.                  static void __exit hello_2_exit (void)
  66.                  {
  67.                          dev_t devno = MKDEV (hello_major, hello_minor);
  68.                          cdev_del (&cdev);
  69.                          unregister_chrdev_region (devno, number_of_devices);
  70.                          device_destroy(my_class, devno);
  71.                          class_destroy(my_class);
  72.                    }
  73.                  module_init (hello_2_init);
  74.                module_exit (hello_2_exit);

当加载模块的时候,会在/dev/hello这个设备文件

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