• 基于linux-2.6.35的class_create(),device_create解析


    基于linux-2.6.35的class_create(),device_create解析

    作者:苗老师,华清远见嵌入式学院讲师。

    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
            /*
            * 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. ");
                            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 ", hello_major);
                            return result;
                    }
                    char_reg_setup_cdev ();
                    printk (KERN_INFO "char device registered ");
                    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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  • 原文地址:https://www.cnblogs.com/cute/p/4643354.html
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