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驅(qū)動(dòng)之cdev

前面寫到如何向系統(tǒng)申請(qǐng)一個(gè)設(shè)備號(hào)，設(shè)備號(hào)就像我們的身份證號(hào)一樣，號(hào)本身并沒有什么特殊的意義，只有把這個(gè)號(hào)和人對(duì)應(yīng)才有意義，通用設(shè)備號(hào)也需要和一個(gè)特殊的東西對(duì)于，這就是cdev, cdev是linux下抽象出來(lái)的一個(gè)用來(lái)描述一個(gè)字符設(shè)備的結(jié)構(gòu)體，在linux下定義如下：

struct cdev {
                struct kobject kobj;
                struct module *owner;
                const struct file_operations *ops;
                struct list_head list;
                dev_t dev;
                unsigned int count;
        };

結(jié)構(gòu)體中有幾個(gè)成員事我們寫驅(qū)動(dòng)的時(shí)候必須關(guān)心的：

dev 類型是dev_t,也就是我們的設(shè)備號(hào)

ops是一個(gè)同樣也是一個(gè)結(jié)構(gòu)體并且是一個(gè)字符驅(qū)動(dòng)實(shí)現(xiàn)的主體，字符驅(qū)動(dòng)通常需要和應(yīng)用程序交互，在學(xué)linux系統(tǒng)編程的時(shí)候，都會(huì)講到linux 應(yīng)用程序通過(guò)系統(tǒng)調(diào)用陷入到內(nèi)核空間，從而執(zhí)行內(nèi)核代碼，而驅(qū)動(dòng)作為內(nèi)核的一部分同樣也是需要在內(nèi)核空間執(zhí)行的，ops也就是file_operations這個(gè)結(jié)構(gòu)體就是我們的驅(qū)動(dòng)為應(yīng)用程序調(diào)用驅(qū)動(dòng)而實(shí)現(xiàn)的一個(gè)操作的集合，它的定義如下：

struct file_operations {
                struct module *owner;
                loff_t (*llseek) (struct file *, loff_t, int);
                ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
                ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
                ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
                ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
                int (*readdir) (struct file *, void *, filldir_t);
                unsigned int (*poll) (struct file *, struct poll_table_struct *);
                long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
                long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
                int (*mmap) (struct file *, struct vm_area_struct *);
                int (*open) (struct inode *, struct file *);
                int (*flush) (struct file *, fl_owner_t id);
                int (*release) (struct inode *, struct file *);
                int (*fsync) (struct file *, loff_t, loff_t, int datasync);
                int (*aio_fsync) (struct kiocb *, int datasync);
                int (*fasync) (int, struct file *, int);
                int (*lock) (struct file *, int, struct file_lock *);
                ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
                unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
                int (*check_flags)(int);
                int (*flock) (struct file *, int, struct file_lock *);
                ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
                ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
                int (*setlease)(struct file *, long, struct file_lock **);
                long (*fallocate)(struct file *file, int mode, loff_t offset,loff_t len);
        };

我們?cè)隍?qū)動(dòng)中要做的事情就是申請(qǐng)一個(gè)cdev并把cdev注冊(cè)到系統(tǒng)中去，操作cdev的函數(shù)有：

void cdev_init(struct cdev *, const struct file_operations *);
        struct cdev *cdev_alloc(void);
        int cdev_add(struct cdev *, dev_t, unsigned);
        void cdev_del(struct cdev *);

1、cdev的定義

cdev的定義有兩種方式一種是：struct cdev cdev；另外一種是：strcut cdev cdev;cdev = cdev_alloc();

2、cdev的初始化

cdev_init實(shí)現(xiàn)cdev的初始化，主要的工作是將我們定義好的file_operaionts與cdev關(guān)聯(lián)起來(lái)，file_operations的實(shí)現(xiàn)根據(jù)實(shí)際需求來(lái)實(shí)現(xiàn)，后面詳細(xì)介紹。

3、cdev的注冊(cè)

cdev_add實(shí)現(xiàn)cdev的注冊(cè)，linux內(nèi)核里維護(hù)了一個(gè)cdev_map的表，所謂cdev的注冊(cè)就是把我們的cdev注冊(cè)到cdev_map表上,cdev_map表結(jié)構(gòu)如圖：

4、設(shè)備的刪除

cdev_del 將我們的cdev從cdev_map中移除。