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
/* * device classes */ struct class { const char *name; struct module *owner; struct class_attribute *class_attrs; struct device_attribute *dev_attrs; struct kobject *dev_kobj; int (*dev_uevent)(struct device *dev, struct kobj_uevent_env *env); char *(*devnode)(struct device *dev, mode_t *mode); 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); const struct kobj_ns_type_operations *ns_type; const void *(*namespace)(struct device *dev); const struct dev_pm_ops *pm; struct class_private *p; }; 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 lock_class_key *key) { 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, key); 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, 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:
/* * Copyright (C) 2005 Farsight * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 2 of the License, or * (at your option) any later version. * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA * */ #include <linux/module.h> #include <linux/kernel.h> #include <linux/init.h> #include <linux/fs.h> #include <linux/cdev.h> #include <asm/uaccess.h> #include <linux/device.h> MODULE_LICENSE ("GPL"); int hello_major = 250; int hello_minor = 0; int number_of_devices = 1; struct cdev cdev; dev_t dev = 0; struct file_operations hello_fops = { .owner = THIS_MODULE, }; struct class *my_class; 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); /* creating your own class */ my_class =class_create(THIS_MODULE, "farsight_class"); if(IS_ERR(my_class)) { printk("Err: failed in creating class.\n"); return ; } /* register your own device in sysfs, and this will cause udevd to create corresponding device node */ device_create(my_class,NULL, devno, NULL,"hello"); } static int __init hello_2_init (void) { int result; dev = MKDEV (hello_major, hello_minor); result = register_chrdev_region (dev, number_of_devices, "test"); if (result<0) { printk (KERN_WARNING "hello: can't get major number %d\n", hello_major); return result; } char_reg_setup_cdev (); printk (KERN_INFO "char device registered\n"); return 0; } static void __exit hello_2_exit (void) { dev_t devno = MKDEV (hello_major, hello_minor); cdev_del (&cdev); unregister_chrdev_region (devno, number_of_devices); device_destroy(my_class, devno); class_destroy(my_class); } module_init (hello_2_init); 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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