linux-2.6.35核心版本中class_create()和device_create的使用

 

作者：苗德行,華清遠見嵌入式學院（www.embedu.org）講師。

從linux核心2.6的某個版本之後，devfs不複存在，udev成為devfs的替代。提醒一點，udev是應用程式層的，不要試圖在核心的配置選項裡找到它;加入對udev的支援很簡單，以作者所寫的一個字元裝置驅動為例，在驅動初始化的代碼裡調用class_create為該裝置建立一個class，再為每個裝置調用device_create建立對應的裝置。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”);

這樣的module被載入時，udev daemon就會自動在/dev下建立my_device裝置檔案。

我們在剛開始寫Linux裝置驅動程式的時候，很多時候都是利用mknod命令手動建立裝置節點，實際上Linux核心為我們提供了一組函數，可以用來在模組載入的時候自動在 /dev目錄下建立相應裝置節點，並在卸載模組時刪除該節點，當然前提條件是使用者空間移植了udev。

核心中定義了struct class結構體，顧名思義，一個struct class結構體類型變數對應一個類，核心同時提供了class_create(…)函數，可以用它來建立一個類，這個類存放於sysfs下面，一旦建立好了這個類，再調用device_create(…)函數來在/dev目錄下建立相應的裝置節點。這樣，載入模組的時候，使用者空間中的udev會自動響應 device_create(…)函數，去/sysfs下尋找對應的類從而建立裝置節點。

注意，在2.6較早的核心版本中，device_create(…)函數名稱不同，是class_device_create(…)，所以在新的核心中編譯以前的模組程式有時會報錯，就是因為函數名稱 不同，而且裡面的參數設定也有一些變化。

struct class和device_create(…) 以及device_create(…)都定義在/include/linux/device.h中，使用的時候一定要包含這個標頭檔，否則編譯器會報錯。

在2.6.35核心版本中，struct class定義在標頭檔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中實現： 
        /**
        * 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);
        }

第一個參數指定類的所有者是哪個模組，第二個參數指定類名。

在class.c中，還定義了class_destroy(…)函數，用於在模組卸載時刪除類。

device_create(…)函數在/drivers/base/core.c中實現： 
        /**
        * 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;
        }

第一個參數指定所要建立的裝置所從屬的類，第二個參數是這個裝置的父裝置，如果沒有就指定為NULL，第三個參數是裝置號，第四個參數是裝置名稱，第五個參數是從裝置號。

下面以一個簡單字元裝置驅動來展示如何使用這幾個函數：（擴充閱讀：塊裝置和字元裝置驅動比較）
        /* 
        * 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);

當載入模組的時候，會在/dev/hello這個裝置檔案

華清遠見嵌入式linux培訓網，原文地址：http://www.embedu.org/Column/Column476.htm