linux kernel 字符设备详解
有关Linux kernel 字符设备分析：
参考：http://blog.jobbole.com/86531/
一．linux kernel 将设备分为３大类，字符设备，块设备，网络设备．
字符设备是指只能一个字节一个字节读写的设备， 常见的外设基本上都是字符设备．
块设备：常见的存储设备，硬盘，ＳＤ卡都归为块设备，块设备是按一块一块读取的．
网络设备：linux 将对外通信的一个机制抽象成一个设备， 通过套接字对其进行相关的操作．
每一个字符设备或块设备都在/dev目录下对应一个设备文件。linux用户程序通过设备文件（或称设备节点）来使用驱动程序操作字符设备和块设备。
二、字符设备、字符设备驱动与用户空间访问该设备的程序三者之间的关系。
三．字符设备的模型
四．下面讲一下字符设备驱动的编写流程，linux 内核为字符设备的创建提供了一套接口．
首先介绍一下dev_t ， 他是主设备号和次设备号的结构体生成，他就代表了一个主次设备号
通过函数MKDEV （MAJ , MINOR) ; 生成．我们注册一个字符设备可以通过动态注册也可以静态注册 , linux kernel 为我们提供了所需要的接口
首先讲一下静态注册的方法
1 //分配主设备号为200 次设备号从5开始分配5个 设备名字叫MONEY 2 int ret ; 3 DeviceId = MKDEV(MAJ , BASEMINOR); 4 ret = register_chrdev_region(DeviceId , MINORCNT , "MONEY"); 5 if(ret < 0) 6 { 7 return ret ; 8 } 9 10 //******************************************** 11 //方法一 12 //1> 初始化 13 cdev_init(&device, &fops); 14 //2> 添加 domain->probes HASH表上 15 cdev_add(&device,DeviceId , MINORCNT);
通过函数register_chrdev_region() , 我们可以注册主设备号为MAJ , 次设备号BASEMINOR 开始 , 一共注册MINORCNT 个次设备号 , 名字为MONEY 的字符设备.
第二种方法是动态申请主次设备号:
1 //动态分配一个主设备号 2 int ret ; 3 ret = alloc_chrdev_region(&DeviceId , BASEMINOR , MINORCNT,"TONY"); 4 if(ret < 0) 5 { 6 return ret ; 7 } 8 9 printk("major:%d \n" , MAJOR(DeviceId)); 10 11 //******************************************** 12 //方法二 13 //1> 分配空间 14 device = cdev_alloc();
我们可以通过linux kernel 提供的alloc_chrdev_region () 的方法 , 申请一个主设备号和基础设备号 , 一共申请 MINORCNT , 名字为 TONY 的一个字符设备.
这里涉及一个结构体:
1 struct cdev { 2 struct kobject kobj; 3 struct module *owner;
4 const struct file_operations *ops; 5 struct list_head list; 6 dev_t dev; 7 unsigned int count; 8 };
这里的话还要申请一个cdev 结构体的空间
通过cdev_alloc() ;
搞定了主次设备号的问题 , 接下来就是涉及到了初始化和添加到设备列表 .
linux kernel 为我们提供了以下的方法:
1 //2> 初始化 2 cdev_init(device, &fops); 3 //3> 添加 domain->probes HASH表上 4 cdev_add(device,DeviceId , MINORCNT); 5
这里面涉及到了一个&fops 的文件操作结构体
1 static struct file_operations fops = { 2 .owner = THIS_MODULE, 3 .open = myopen, 4 .read = myread , 5 .write = mywrite, 6 .release = myclose, 7 .unlocked_ioctl = myioctl, 8 };
上层的open read write 等函数经过一系列的转换都会到对应的函数
相对应的, 释放主次设备号, 删除在设备列表的节点, linux kernel 为我们提供了一下接口:
1 cdev_del(device); 2 3 unregister_chrdev_region(DeviceId , MINORCNT);
下面的代码是想深入了解里面的代码的看看就行了 , 如果只是向了解接口的 , 上面的就行了
从静态申请注册开始跟起吧
1 #define MKDEV(ma,mi) ((ma)<<8 | (mi))
上面这个是制作一个主次设备号的结构体
1 extern int alloc_chrdev_region(dev_t *, unsigned, unsigned, const char *); 2 extern int register_chrdev_region(dev_t, unsigned, const char *); 3 extern int __register_chrdev(unsigned int major, unsigned int baseminor, 4 unsigned int count, const char *name, 5 const struct file_operations *fops); 6 extern void __unregister_chrdev(unsigned int major, unsigned int baseminor, unsigned int count, const char *name); 7 extern void unregister_chrdev_region(dev_t, unsigned);
这是几个将要用的函数的函数声明 , 它在 include/linux/fs.h 文件中
首先看一下 register_chrdev_region() 函数
1 /** 2 * register_chrdev_region() - register a range of device numbers 3 * @from: the first in the desired range of device numbers; must include 4 * the major number. 5 * @count: the number of consecutive device numbers required 6 * @name: the name of the device or driver. 7 * 8 * Return value is zero on success, a negative error code on failure. 9 */
注释说明: 注册一个范围的设备号 , 原型如下:
1 int register_chrdev_region(dev_t from, unsigned count, const char *name) 2 { 3 struct char_device_struct *cd; 4 dev_t to = from + count; 5 dev_t n, next; 6 7 for (n = from; n < to; n = next) { next = MKDEV(MAJOR(n)+1, 0); 8 if (next > to) 9 next = to; 10 cd = __register_chrdev_region(MAJOR(n), MINOR(n), 11 next - n, name); 12 if (IS_ERR(cd)) 13 goto fail; 14 } 15 return 0; 16 fail: 17 to = n; 18 for (n = from; n < to; n = next) { 19 next = MKDEV(MAJOR(n)+1, 0); 20 kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n)); 21 } 22 return PTR_ERR(cd); 23 }
它是调用了
cd = __register_chrdev_region(MAJOR(n), MINOR(n),
进里面看看
1 /* 2 * Register a single major with a specified minor range. 3 * 4 * If major == 0 this functions will dynamically allocate a major and return 5 * its number. 6 * 7 * If major > 0 this function will attempt to reserve the passed range of 8 * minors and will return zero on success. 9 * 10 * Returns a -ve errno on failure. 11 */
还是看注释: 注册一个指定的主设备号 和一个指定的次设备号范围
判断 主设备号是不是为零 , 如果是零的话 就动态申请一个主设备号 , 这就是后面要讲的那个动态申请 , 它也是调用了这个.
代码如下
1 static struct char_device_struct * 2 __register_chrdev_region(unsigned int major, unsigned int baseminor, 3 int minorct, const char *name) 4 { 5 struct char_device_struct *cd, **cp; 6 int ret = 0; 7 int i; 8
这里面涉及一个结构体没讲:
1 static struct char_device_struct { 2 struct char_device_struct *next; 3 unsigned int major; 4 unsigned int baseminor; 5 int minorct; 6 char name[64]; 7 struct cdev *cdev; /* will die */ 8 } *chrdevs[CHRDEV_MAJOR_HASH_SIZE];
9 cd = kzalloc(sizeof(struct char_device_struct), GFP_KERNEL); //动态申请了一个char_device_struct 结构体 10 if (cd == NULL) 11 return ERR_PTR(-ENOMEM); 12 13 mutex_lock(&chrdevs_lock); //加一个互斥锁 , 防止其他进程并发 14 15 /* temporary */ 16 if (major == 0) { 17 for (i = ARRAY_SIZE(chrdevs)-1; i > 0; i--) { //这里其实就是做了一个动态申请主设备号的功能 18 if (chrdevs[i] == NULL) 19 break; 20 } 21 22 if (i == 0) { //没有申请到主设备号, 直接退出 23 ret = -EBUSY; 24 goto out; 25 } 26 major = i; 27 ret = major; 28 } 29 30 cd->major = major; //对结构体进行初始化 31 cd->baseminor = baseminor; 32 cd->minorct = minorct; 33 strlcpy(cd->name, name, sizeof(cd->name)); 34 35 i = major_to_index(major); // 哈希表的下标生成 36 37 for (cp = &chrdevs[i]; *cp; cp = &(*cp)->next) //进入chdevs[i] 哈希表快速进入 38 if ((*cp)->major > major || 39 ((*cp)->major == major && 40 (((*cp)->baseminor >= baseminor) || 41 ((*cp)->baseminor + (*cp)->minorct > baseminor)))) 42 break; 43 44 /* Check for overlapping minor ranges. */ // 检查次设备号会不会重叠 45 if (*cp && (*cp)->major == major) { 46 int old_min = (*cp)->baseminor; 47 int old_max = (*cp)->baseminor + (*cp)->minorct - 1; 48 int new_min = baseminor; 49 int new_max = baseminor + minorct - 1; 50 51 /* New driver overlaps from the left. */ 52 if (new_max >= old_min && new_max <= old_max) { 53 ret = -EBUSY; 54 goto out; 55 } 56 57 /* New driver overlaps from the right. */ 58 if (new_min <= old_max && new_min >= old_min) { 59 ret = -EBUSY; 60 goto out; 61 } 62 } 63 64 cd->next = *cp; 65 *cp = cd; 66 mutex_unlock(&chrdevs_lock); //解锁 67 return cd; 68 out: 69 mutex_unlock(&chrdevs_lock); 70 kfree(cd); 71 return ERR_PTR(ret); 72 }
到这里一个主次设备号就搞定了 , 并申请一个char_device_struct 结构体, 并对其进行赋值初始化.
第二步就是对cdev 进行init :
1 void cdev_init(struct cdev *, const struct file_operations *);
这里又涉及到一个struct cdev 的结构体:
1 struct cdev { 2 struct kobject kobj; 3 struct module *owner; 4 const struct file_operations *ops; 5 struct list_head list; 6 dev_t dev; 7 unsigned int count; 8 };
进初始化代码一看究竟:
1 /** 2 * cdev_init() - initialize a cdev structure 3 * @cdev: the structure to initialize 4 * @fops: the file_operations for this device 5 * 6 * Initializes @cdev, remembering @fops, making it ready to add to the 7 * system with cdev_add(). 8 */ 9 void cdev_init(struct cdev *cdev, const struct file_operations *fops) 10 { 11 memset(cdev, 0, sizeof *cdev); 12 INIT_LIST_HEAD(&cdev->list); 13 kobject_init(&cdev->kobj, &ktype_cdev_default); 14 cdev->ops = fops; 15 }
看一段代码之前我们尽可能的先看注释, 这样会让我们跟代码轻松很多 , 我们可以顺着代码的作者的思路走
注释: 初始化一个cdev 结构体
进kobject_init() 看看:
1 /** 2 * kobject_init - initialize a kobject structure 初始化一个内核项目结构体 3 * @kobj: pointer to the kobject to initialize 4 * @ktype: pointer to the ktype for this kobject. 5 * 6 * This function will properly initialize a kobject such that it can then 7 * be passed to the kobject_add() call. 8 * 9 * After this function is called, the kobject MUST be cleaned up by a call 10 * to kobject_put(), not by a call to kfree directly to ensure that all of 11 * the memory is cleaned up properly. 12 */
1 void kobject_init(struct kobject *kobj, struct kobj_type *ktype) 2 { 3 char *err_str; 4 5 if (!kobj) { 6 err_str = "invalid kobject pointer!"; 7 goto error; 8 } 9 if (!ktype) { 10 err_str = "must have a ktype to be initialized properly!\n"; 11 goto error; 12 } 13 if (kobj->state_initialized) { 14 /* do not error out as sometimes we can recover */ 15 printk(KERN_ERR "kobject (%p): tried to init an initialized " 16 "object, something is seriously wrong.\n", kobj); 17 dump_stack(); 18 } 19 20 kobject_init_internal(kobj); 21 kobj->ktype = ktype; 22 return; 23 24 error: 25 printk(KERN_ERR "kobject (%p): %s\n", kobj, err_str); 26 dump_stack(); 27 } 28 EXPORT_SYMBOL(kobject_init);
1 struct kobject { 2 const char *name; 3 struct list_head entry; 4 struct kobject *parent; 5 struct kset *kset; 6 struct kobj_type *ktype; 7 struct sysfs_dirent *sd; 8 struct kref kref; 9 unsigned int state_initialized:1; 10 unsigned int state_in_sysfs:1; 11 unsigned int state_add_uevent_sent:1; 12 unsigned int state_remove_uevent_sent:1; 13 unsigned int uevent_suppress:1; 14 };
下一部就是 cdev_add () ;
1 int cdev_add(struct cdev *, dev_t, unsigned);
Read The Fucking Source Code
