字符设备的注册与操作

       相对于块设备来说，字符设备的使用要简单很多。但是简单的东西，也有很多值得一看的东西。比方说，字符设备，与inode如何关联；在打开字符设备的时候，又是如何层层递进，最终执行相应的从设备的实际例程呢？下面拿Mem.c这个文件下面的例子来分析，该字符设备的主设备号为1，文件为/dev/mem，含义是物理内存。

注册：

1、chr_dev_init：

/* 内存字符设备初始化*/static int __init chr_dev_init(void){int i;int err;err = bdi_init(&zero_bdi);if (err)return err;/* key:负责内存操作的字符设备的初始化，将分配器memory_fops赋值给cdev->ops*/if (register_chrdev(MEM_MAJOR,"mem",&memory_fops))  printk("unable to get major %d for memory devs\n", MEM_MAJOR);/* 创建class对象 ，为sysfs系统使用 */mem_class = class_create(THIS_MODULE, "mem");for (i = 0; i < ARRAY_SIZE(devlist); i++)device_create(mem_class, NULL,      MKDEV(MEM_MAJOR, devlist[i].minor),      devlist[i].name);return 0;}

2、register_chrdev：

/** * register_chrdev() - Register a major number for character devices. * register_chrdev() - 为字符设备注册一个主设备号 * @major: major device number or 0 for dynamic allocation * @name: name of this range of devices * @fops: file operations associated with this devices * * If @major == 0 this functions will dynamically allocate a major and return * its number. * * If @major > 0 this function will attempt to reserve a device with the given * major number and will return zero on success. * * Returns a -ve errno on failure. * * The name of this device has nothing to do with the name of the device in * /dev. It only helps to keep track of the different owners of devices. If * your module name has only one type of devices it's ok to use e.g. the name * of the module here. * * This function registers a range of 256 minor numbers. The first minor number * is 0. * 该函数注册一个从设备范围，有256个从设备号。第一个 * 从设备号是0. */int register_chrdev(unsigned int major, const char *name,    const struct file_operations *fops){struct char_device_struct *cd;struct cdev *cdev;char *s;int err = -ENOMEM;cd = __register_chrdev_region(major, 0, 256, name);if (IS_ERR(cd))return PTR_ERR(cd);cdev = cdev_alloc();  /* 分配内存：一个cdev结构实例*/if (!cdev)goto out2;cdev->owner = fops->owner; cdev->ops = fops; /* 这里的实际效果是，刚刚申请的cdev实例代表mem设备，其中的cdev->ops指针指向相应的memory_fops，在实际操作时，先根据主设备号选择cdev，在根据从设备号从memory_fops里选择相应的实际例程*/kobject_set_name(&cdev->kobj, "%s", name);for (s = strchr(kobject_name(&cdev->kobj),'/'); s; s = strchr(s, '/'))*s = '!';err = cdev_add(cdev, MKDEV(cd->major, 0), 256); /* 将初始化完毕的cdev内存设备添加到字符设备数据库，即散列表中*/if (err)goto out;cd->cdev = cdev;return major ? 0 : cd->major;out:kobject_put(&cdev->kobj);out2:kfree(__unregister_chrdev_region(cd->major, 0, 256));return err;}

打开设备文件

1、在linux下，一切皆为文件，所以，我们这里讨论的mem字符设备也要与系统的文件系统结合。关键的概念在于inode：在打开一个设备文件时，各种文件系统的实现会调用init_special_inode函数，为mem设备创建它在文件系统中的表示inode：

void init_special_inode(struct inode *inode, umode_t mode, dev_t rdev){inode->i_mode = mode;if (S_ISCHR(mode)) { /* 在这里显然是进入这个分支，根据mode选择文件类型。*/inode->i_fop = &def_chr_fops;/* 给定inode相关的文件操作指针，至此，inode->i_fop->open = chrdev_open */inode->i_rdev = rdev;/* 给定inode设备的主设备号*/} else if (S_ISBLK(mode)) {inode->i_fop = &def_blk_fops;inode->i_rdev = rdev;} else if (S_ISFIFO(mode))inode->i_fop = &def_fifo_fops;else if (S_ISSOCK(mode))inode->i_fop = &bad_sock_fops;elseprintk(KERN_DEBUG "init_special_inode: bogus i_mode (%o)\n",       mode);}

2、chrdev_open：

/* * Called every time a character special file is opened * 每次当一个字符设备文件被打开时，会调用该函数 */int chrdev_open(struct inode * inode, struct file * filp){struct cdev *p;struct cdev *new = NULL;int ret = 0;spin_lock(&cdev_lock);p = inode->i_cdev;  /* 获取与mem相关的cdev，即在注册的时候分配的那个结构体实例 */if (!p) {/* 如果设备文件的inode此前没有被打开过*/struct kobject *kobj;int idx;spin_unlock(&cdev_lock);/* 根据主设备号查询字符设备数据库，即从散列表中查*/kobj = kobj_lookup(cdev_map, inode->i_rdev, &idx);if (!kobj)return -ENXIO;new = container_of(kobj, struct cdev, kobj); /* 获取struct cdev 实例*/spin_lock(&cdev_lock);p = inode->i_cdev;if (!p) {inode->i_cdev = p = new;  /* 初始化inode实例*/inode->i_cindex = idx;list_add(&inode->i_devices, &p->list);/* 将该inode添加到cdev->list中，即inode中的i_devices用作链表元素*/new = NULL;} else if (!cdev_get(p))ret = -ENXIO;} else if (!cdev_get(p))ret = -ENXIO;spin_unlock(&cdev_lock);cdev_put(new);if (ret)return ret; /* 找到指定于设备的file_operations ，这里就通过inode找到相应的cdev->ops，即memory_fops*/filp->f_op = fops_get(p->ops);if (!filp->f_op) {cdev_put(p);return -ENXIO;}/* 执行打开操作*/if (filp->f_op->open) {lock_kernel();ret = filp->f_op->open(inode,filp);/* 执行打开操作，这里filp->f_op->open执行的其实是memory_open函数*/unlock_kernel();}if (ret)cdev_put(p);return ret;}

3、memory_open函数：作为一个分配器，根据从设备号，执行更加具体的不同的字符操作

static int memory_open(struct inode * inode, struct file * filp){switch (iminor(inode)) { /*根据从设备号区分各个设备，并且选择适当的文件操作：mem_fops、kmem_fops等等，这里选择mem_fops*/case 1:filp->f_op = &mem_fops;filp->f_mapping->backing_dev_info =&directly_mappable_cdev_bdi;break;case 2:filp->f_op = &kmem_fops;filp->f_mapping->backing_dev_info =&directly_mappable_cdev_bdi;break;case 3:filp->f_op = &null_fops;break;#ifdef CONFIG_DEVPORTcase 4:filp->f_op = &port_fops;break;#endifcase 5:filp->f_mapping->backing_dev_info = &zero_bdi;filp->f_op = &zero_fops;break;case 7:filp->f_op = &full_fops;break;case 8:filp->f_op = &random_fops;break;case 9:filp->f_op = &urandom_fops;break;case 11:filp->f_op = &kmsg_fops;break;#ifdef CONFIG_CRASH_DUMPcase 12:filp->f_op = &oldmem_fops;break;#endifdefault:return -ENXIO;}if (filp->f_op && filp->f_op->open)return filp->f_op->open(inode,filp);return 0;}

读操作等：

在打开设备文件之后，实际上文件指针file指向的是mem_ops(代码体现：filp->f_op = &mem_fops;)，所以再执行其他的操作如下，就顺理成章了，成为简单的函数调用了：

static const struct file_operations mem_fops = {.llseek= memory_lseek,.read= read_mem,.write= write_mem,.mmap= mmap_mem,.open= open_mem,.get_unmapped_area = get_unmapped_area_mem,};

总结：最初只知道打开字符设备的一般函数，然后由打开与内存相关的设备文件的具体函数所替代。接下来根据选择的从设备号，进一步细化函数指针。