linux 字符设备和misc设备

字符设备

2.6版本前使用的结构体和函数

static inline int register_chrdev(unsigned int major, const char *name,const struct file_operations *fops)//注册字符设备到内核中，疑问1
static inline void unregister_chrdev(unsigned int major, const char *name)//注销字符设备
MAJOR(dev_t dev)//计算主设备号
MINOR(dev_t dev)//计算次设备号
MKDEV(int major,int minor)//根据major和minor算得dev_t
struct file_operations{
.....
}

typedef __kernel_dev_t      dev_t;//字符设备的设备号  
static inline int register_chrdev(unsigned int major, const char *name,const struct file_operations *fops)//注册字符设备到内核中，疑问1  
static inline void unregister_chrdev(unsigned int major, const char *name)//注销字符设备  
MAJOR(dev_t dev)//计算主设备号  
MINOR(dev_t dev)//计算次设备号  
MKDEV(int major,int minor)//根据major和minor算得dev_t  
struct file_operations{  
.....  
} 
在使用以上函数注册字符设备的时候比较简单，拿misc的实现函数举例，已知了dev_t的主设备号，所以可以直接调用register_chrdev将该设备注册到内核中。  
	

		

	

	

		
		
static int __init misc_init(void)
{
  int err;

  #ifdef CONFIG_PROC_FS
    proc_create("misc", 0, NULL, &misc_proc_fops);
  #endif
  misc_class = class_create(THIS_MODULE, "misc");//dev下创建misc类，后面在具体设备进行misc_register是再执行device_creat注册具体的misc设备
  err = PTR_ERR(misc_class);
  if (IS_ERR(misc_class))
    goto fail_remove;
  err = -EIO;
  if (register_chrdev(MISC_MAJOR,"misc",&misc_fops))
    goto fail_printk;
  misc_class->devnode = misc_devnode;return 0;
  fail_printk:
    printk("unable to get major %d for misc devices\n", MISC_MAJOR);
  class_destroy(misc_class);
  fail_remove:
    remove_proc_entry("misc", NULL);

  return err;
}
	
static int __init misc_init(void)
{
  int err;
  
  #ifdef CONFIG_PROC_FS
    proc_create("misc", 0, NULL, &misc_proc_fops);
  #endif
  misc_class = class_create(THIS_MODULE, "misc");//dev下创建misc类，后面在具体设备进行misc_register是再执行device_creat注册具体的misc设备
  err = PTR_ERR(misc_class);
  if (IS_ERR(misc_class))
    goto fail_remove;
  err = -EIO;
  if (register_chrdev(MISC_MAJOR,"misc",&misc_fops))
    goto fail_printk;
  misc_class->devnode = misc_devnode;return 0;
  fail_printk:
    printk("unable to get major %d for misc devices\n", MISC_MAJOR);
  class_destroy(misc_class);
  fail_remove:
    remove_proc_entry("misc", NULL);
    
  return err;
}
	

		2.6版本后使用的结构体和函数
	
	

		
		
dev_t;//字符设备的设备号
MAJOR(dev_t dev)//计算主设备号
MINOR(dev_t dev)//计算次设备号
MKDEV(int major,int minor)//根据major和minor算得dev_t
struct cdev {      //新增了cdev结构体，可以看到cdev除了包含了dev_t成员，还有kobj相关成员，2.6正是引入了kobj模型
struct kobject kobj;
struct module *owner;
const struct file_operations *ops;
struct list_head list;
dev_t dev;
unsigned int count;
};
void cdev_init(struct cdev *, const struct file_operations *);//初始化cdev，主要是赋值cdev的ops成员
struct cdev *cdev_alloc(void);//动态分配一个cdev结构体，此时是程序中只定义了一个cdev指针的时候使用，如果定义了一个cdev结构体，使用cdev_init
void cdev_put(struct cdev *p);//释放alloc的cdev
int cdev_add(struct cdev *, dev_t, unsigned);//向系统注册字符设备
void cdev_del(struct cdev *);
extern int alloc_chrdev_region(dev_t *, unsigned, unsigned, const charchar *);//动态向系统申请设备号dev_t
extern int register_chrdev_region(dev_t, unsigned, const charchar *);//已知dev_t时静态申请
struct file_operations{
.....
}
	
dev_t;//字符设备的设备号  
MAJOR(dev_t dev)//计算主设备号  
MINOR(dev_t dev)//计算次设备号  
MKDEV(int major,int minor)//根据major和minor算得dev_t  
struct cdev {      //新增了cdev结构体，可以看到cdev除了包含了dev_t成员，还有kobj相关成员，2.6正是引入了kobj模型  
struct kobject kobj;  
struct module *owner;  
const struct file_operations *ops;  
struct list_head list;  
dev_t dev;  
unsigned int count;  
};  
void cdev_init(struct cdev *, const struct file_operations *);//初始化cdev，主要是赋值cdev的ops成员  
struct cdev *cdev_alloc(void);//动态分配一个cdev结构体，此时是程序中只定义了一个cdev指针的时候使用，如果定义了一个cdev结构体，使用cdev_init  
void cdev_put(struct cdev *p);//释放alloc的cdev  
int cdev_add(struct cdev *, dev_t, unsigned);//向系统注册字符设备  
void cdev_del(struct cdev *);  
extern int alloc_chrdev_region(dev_t *, unsigned, unsigned, const charchar *);//动态向系统申请设备号dev_t extern int register_chrdev_region(dev_t, unsigned, const charchar *);//已知dev_t时静态申请  
struct file_operations{  
.....  
}  
可以看出2.6以后的版本在注册字符设备时稍复杂些，还是举个例子，可以看到在2.6以后的版本中注册字符设备的过程是：申请设备号dev_t->cdev初始化->注册cdev到系统中，对比2.6以前的版本多了针对cdev的操作过程。

	

		
		
static int __init pc8736x_gpio_init(void)
{
    int rc;
    dev_t devid;

......
    if (major) {
        devid = MKDEV(major, 0);
        rc = register_chrdev_region(devid, PC8736X_GPIO_CT, DEVNAME);
    } else {
        rc = alloc_chrdev_region(&devid, 0, PC8736X_GPIO_CT, DEVNAME);
        major = MAJOR(devid);
    }

    if (rc < 0) {
        dev_err(&pdev->dev, "register-chrdev failed: %d\n", rc);
        goto undo_request_region;
    }
    if (!major) {
        major = rc;
        dev_dbg(&pdev->dev, "got dynamic major %d\n", major);
    }

    pc8736x_init_shadow();

    /* ignore minor errs, and succeed */
    cdev_init(&pc8736x_gpio_cdev, &pc8736x_gpio_fileops);
    cdev_add(&pc8736x_gpio_cdev, devid, PC8736X_GPIO_CT);

    return 0;

undo_request_region:
    release_region(pc8736x_gpio_base, PC8736X_GPIO_RANGE);
undo_platform_dev_add:
    platform_device_del(pdev);
undo_platform_dev_alloc:
    platform_device_put(pdev);

    return rc;
}
	
static int __init pc8736x_gpio_init(void)  
{  
    int rc;  
    dev_t devid;  
  
......  
    if (major) {  
        devid = MKDEV(major, 0);  
        rc = register_chrdev_region(devid, PC8736X_GPIO_CT, DEVNAME);  
    } else {  
        rc = alloc_chrdev_region(&devid, 0, PC8736X_GPIO_CT, DEVNAME);  
        major = MAJOR(devid);  
    }  
  
    if (rc < 0) {  
        dev_err(&pdev->dev, "register-chrdev failed: %d\n", rc);  
        goto undo_request_region;  
    }  
    if (!major) {  
        major = rc;  
        dev_dbg(&pdev->dev, "got dynamic major %d\n", major);  
    }  
  
    pc8736x_init_shadow();  
  
    /* ignore minor errs, and succeed */  
    cdev_init(&pc8736x_gpio_cdev, &pc8736x_gpio_fileops);  
    cdev_add(&pc8736x_gpio_cdev, devid, PC8736X_GPIO_CT);  
  
    return 0;  
  
undo_request_region:  
    release_region(pc8736x_gpio_base, PC8736X_GPIO_RANGE);  
undo_platform_dev_add:  
    platform_device_del(pdev);  
undo_platform_dev_alloc:  
    platform_device_put(pdev);  
  
    return rc;  
}
	

		misc设备
	
	

		

	
	

		misc使用的结构体和函数
	
	

		misc设备其实也是字符设备，主不过misc设备驱动在字符设备的基础上又进行了一次封装，使用户可以更方便的使用。
	
	

		

			
			
struct miscdevice  {
    int minor;
    const charchar *name;
    const struct file_operations *fops;//还是字符设备中的文件操作结构体，只不过misc结构体对其又封装了一次
    struct list_head list;
    struct device *parent;
    struct device *this_device;
    const charchar *nodename;
    mode_t mode;
};
int misc_register(struct miscdevice * misc)
		
struct miscdevice  {  
    int minor;  
    const charchar *name;  
    const struct file_operations *fops;//还是字符设备中的文件操作结构体，只不过misc结构体对其又封装了一次  
    struct list_head list;  
    struct device *parent;  
    struct device *this_device;  
    const charchar *nodename;  
    mode_t mode;  
};  
int misc_register(struct miscdevice * misc)  
还是举个例子来看下，用户在注册misc设备的时候只需要：初始化file_operations结构体->初始化miscdevice结构体->调用misc_register将miscdevice注册到系统中就ok了。  

		

			
			
static const struct file_operations mmtimer_fops = {
.owner = THIS_MODULE,
.mmap =mmtimer_mmap,
.unlocked_ioctl = mmtimer_ioctl,
.llseek = noop_llseek,
};
static struct miscdevice mmtimer_miscdev = {
SGI_MMTIMER,
MMTIMER_NAME,
&mmtimer_fops
};
static int __init mmtimer_init(void)
{
    cnodeid_t node, maxn = -1;

......
    if (request_irq(SGI_MMTIMER_VECTOR, mmtimer_interrupt, IRQF_PERCPU, MMTIMER_NAME, NULL)) {
        printk(KERN_WARNING "%s: unable to allocate interrupt.",
            MMTIMER_NAME);
        goto out1;
    }

    if (misc_register(&mmtimer_miscdev)){
        printk(KERN_ERR "%s: failed to register device\n",
               MMTIMER_NAME);
        goto out2;
    }
......

    return 0;

out3:
    kfree(timers);
    misc_deregister(&mmtimer_miscdev);
out2:
    free_irq(SGI_MMTIMER_VECTOR, NULL);
out1:
    return -1;
}
		
static const struct file_operations mmtimer_fops = {  
.owner = THIS_MODULE,  
.mmap =mmtimer_mmap,  
.unlocked_ioctl = mmtimer_ioctl,  
.llseek = noop_llseek,  
};  
static struct miscdevice mmtimer_miscdev = {  
SGI_MMTIMER,  
MMTIMER_NAME,  
&mmtimer_fops  
};  
static int __init mmtimer_init(void)  
{  
    cnodeid_t node, maxn = -1;  
  
......  
    if (request_irq(SGI_MMTIMER_VECTOR, mmtimer_interrupt, IRQF_PERCPU, MMTIMER_NAME, NULL)) {  
        printk(KERN_WARNING "%s: unable to allocate interrupt.",  
            MMTIMER_NAME);  
        goto out1;  
    }  
  
    if (misc_register(&mmtimer_miscdev)){  
        printk(KERN_ERR "%s: failed to register device\n",  
               MMTIMER_NAME);  
        goto out2;  
    }  
......  
  
    return 0;  
  
out3:  
    kfree(timers);  
    misc_deregister(&mmtimer_miscdev);  
out2:  
    free_irq(SGI_MMTIMER_VECTOR, NULL);  
out1:  
    return -1;  
}  
那么到了这里就会有疑问，为什么linux还费劲的又造了一个misc设备呢？为什么不直接都使用字符设备驱动呢？

为什么要有misc设备

首先的好处就是方便，快捷，用户可以只初始化2个结构体，然后调用一个函数就可以创建一个misc设备了，本质上来说他也是个字符设备。

还有应该就是注册misc设备节约了主设备号的占用，linux中只提供了256个主设备号，本身内核中一些驱动已经占用了部分，试想如果有一大波设备都想以字符设备注册进内核，必然导致主设备号不够用的情况，而misc设备在初始化时占用主设备号10，当设备驱动以misc设备注册进内核的时候，只为其分配次设备号，主设备号不变。

为什么要有misc设备的观点从知乎上看来的，自己思考了下，总结下来，后面如果再想到会继续添加。





疑问1：该函数在fs.h中声明为static inline，静态的内联函数，既然是静态的，不是应该不能被其他文件内的函数调用？

其实该函数声明在.h文件中，那么，只要c文件包含了该头文件就可以调用了，也就是说在编译时，调用该函数的地方会被编译器将实际代码直接展开在该c文件中，那么在该c文件中该函数是static的，其他c文件没包含该头文件的是不可以使用该函数的。