分类： LINUX
2014-04-09 14:04:04
Linux驱动开发庖丁解牛之三
——揭开字符设备驱动程序的面纱
By:dreamice
我们知道，在Linux设备驱动开发中，包括三大设备类：字符设备，块设备和网络设备。而字符设备，作为最简单的设备类，为此，我们将从最简单的字符设备开始，走进Linux驱动程序设计的神秘殿堂。
——我们已经踏上了真正的设备驱动开发的道路了！
有志者，事竟成。付出越多，而上苍定会以同等的收获回馈于你，当然，最重要的一点是：我们必须走上正确的道路，做正确的付出。开始吧……
参考书目：
《Linux Device Driver》第三版
《Understanding the linux kernel》第三版
《Linux设备驱动开发详解》
盖大楼，得预先准备好砖头。同样的道理，要写好驱动程序，我们也必须准备好自己的“砖头”，拿好这些砖头，便会真正如庖丁解牛般，游刃于Linux驱动程序设计的神奇艺术之中。
在Linux的设计之初，曾提出：一切皆文件，如果一个东西不是文件，那就是进程。由此可见，文件的概念在Linux系统中可谓是根深蒂固，以至于它深入到对驱动程序的控制，这也是情理之中的事。
下图描述了Linux系统中虚拟文件系统和进程之间的关系：
图表 1进程和文件系统的关系
在上图中，我们看到了Process，File object，dentry object，inode object以及Sperblock object等概念。Process就是指一个特定的进程，而File obeject对应于进程打开的一个文件；dentry object描述了一个目录项；inode object则对应于磁盘上一个特定的文件；Sperblock object描述了文件系统的相关信息。从这个图中，可以看到进程到磁盘上一个文件实体的路径及对应关系。下面，我们一次看看这些实体结构在内核中的定义。
File结构代表一个打开的文件，系统中每个打开的文件，在内核空间都对应一个file结构。它由内核在调用open时创建，并传递给在该文件上操作的所有函数，直到最后的close函数。在文件的所有实例都被关闭以后，内核才会释放这个结构。
在内核中，通常以filp来代表指向file结构的指针。File结构的详细定义如下：
//linux/fs.h
779 struct file {
780 /*
781 * fu_list becomes invalid after file_free is called and queued via
782 * fu_rcuhead for RCU freeing
783 */
784 union {
785 struct list_head fu_list;
786 struct rcu_head fu_rcuhead;
787 } f_u;
788 struct path f_path;
789 #define f_dentry f_path.dentry
790 #define f_vfsmnt f_path.mnt
791 const struct file_operations *f_op; //与文件操作相关的函数指针结构
792 atomic_t f_count;
793 unsigned int f_flags;
794 mode_t f_mode;
795 loff_t f_pos;
796 struct fown_struct f_owner;
797 unsigned int f_uid, f_gid;
798 struct file_ra_state f_ra;
799
800 u
801 #ifdef CONFIG_SECURITY
802 void *f_security;
803 #endif
804 /* needed for tty driver, and maybe others */
805 void *private_data;
806
807 #ifdef CONFIG_EPOLL
808 /* Used by fs/eventpoll.c to link all the hooks to this file */
809 struct list_head f_ep_links;
810 spinlock_t f_ep_lock;
811 #endif /* #ifdef CONFIG_EPOLL */
812 struct address_space *f_mapping;
813 };
1166 /*
1167 * NOTE:
1168 * read, write, poll, fsync, readv, writev, unlocked_ioctl and compat_ioctl
1169 * can be called without the big kernel lock held in all filesystems.
1170 */
1171 struct file_operations {
1172 struct module *owner;
1173 loff_t (*llseek) (struct file *, loff_t, int);
1174 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
1175 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
1176 ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
1177 ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
1178 int (*readdir) (struct file *, void *, filldir_t);
1179 unsigned int (*poll) (struct file *, struct poll_table_struct *);
1180 int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
1181 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
1182 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
1183 int (*mmap) (struct file *, struct vm_area_struct *);
1184 int (*open) (struct inode *, struct file *);
1185 int (*flush) (struct file *, fl_owner_t id);
1186 int (*release) (struct inode *, struct file *);
1187 int (*fsync) (struct file *, struct dentry *, int datasync);
1188 int (*aio_fsync) (struct kiocb *, int datasync);
1189 int (*fasync) (int, struct file *, int);
1190 int (*lock) (struct file *, int, struct file_lock *);
1191 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
1192 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
1193 int (*check_flags)(int);
1194 int (*dir_notify)(struct file *filp, unsigned long arg);
1195 int (*flock) (struct file *, int, struct file_lock *);
1196 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
1197 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
1198 int (*setlease)(struct file *, long, struct file_lock **);
1199 };
其中，蓝色字体标出部分，为与驱动程序最为密切的部分。由于很多书中都对这些结构体做了详细的阐述，这里就不再赘述了。
内核用inode结构在内部表示文件，它和file结构的不同之处在于：file表示打开的文件描述符，对单个文件，可能有多个表示打开的文件描述符的file结构，但他们都指向同一个inode结构。
Inode结构的详细定义如下：
593 struct inode {
594 struct hlist_node i_hash;
595 struct list_head i_list;
596 struct list_head i_sb_list;
597 struct list_head i_dentry;
598 unsigned long i_ino;
599 atomic_t i_count;
600 unsigned int i_nlink;
601 uid_t i_uid;
602 gid_t i_gid;
603 dev_t i_rdev;
604 u64 i_version;
605 loff_t i_size;
606 #ifdef __NEED_I_SIZE_ORDERED
607 seqcount_t i_size_seqcount;
608 #endif
609 struct timespec i_atime;
610 struct timespec i_mtime;
611 struct timespec i_ctime;
612 unsigned int i_blkbits;
613 blkcnt_t i_blocks;
614 unsigned short i_bytes;
615 umode_t i_mode;
616 spinlock_t i_lock; /* i_blocks, i_bytes, maybe i_size */
617 struct mutex i_mutex;
618 struct rw_semaphore i_alloc_sem;
619 const struct inode_operations *i_op;//inode操作函数集合
620 const struct file_operations *i_fop; /* former ->i_op->default_file_ops */
621 struct super_block *i_sb;
622 struct file_lock *i_flock;
623 struct address_space *i_mapping;
624 struct address_space i_data;
625 #ifdef CONFIG_QUOTA
626 struct dquot *i_dquot[MAXQUOTAS];
627 #endif
628 struct list_head i_devices;
629 union {
630 struct pipe_inode_info *i_pipe;
631 struct block_device *i_bdev;
632 struct cdev *i_cdev;
633 };
634 int i_cindex;
635
636 __u32 i_generation;
637
638 #ifdef CONFIG_DNOTIFY
639 unsigned long i_dnotify_mask; /* Directory notify events */
640 struct dnotify_struct *i_dnotify; /* for directory notifications */
641 #endif
642
643 #ifdef CONFIG_INOTIFY
644 struct list_head inotify_watches; /* watches on this inode */
645 struct mutex inotify_mutex; /* protects the watches list */
646 #endif
647
648 unsigned long i_state;
649 unsigned long dirtied_when; /* jiffies of first dirtying */
650
651 unsigned int i_flags;
652
653 atomic_t i_writecount;
654 #ifdef CONFIG_SECURITY
655 void *i_security;
656 #endif
657 void *i_private; /* fs or device private pointer */
658 };
Super block object对应于一个特定的文件系统，通常对应于存放在磁盘扇区中的文件系统超级块或文件系统控制块，而对于非基于文件系统的文件，他们会在使用现场创建超级块，并将其保存到内存中。
一下是结构体的详细描述：
981 struct super_block {
982 struct list_head s_list; /* Keep this first */
983 dev_t s_dev; /* search index; _not_ kdev_t */
984 unsigned long s_blocksize;
985 unsigned char s_blocksize_bits;
986 unsigned char s_dirt;
987 unsigned long long s_maxbytes; /* Max file size */
988 struct file_system_type *s_type;
989 const struct super_operations *s_op;
990 struct dquot_operations *dq_op;
991 struct quotactl_ops *s_qcop;
992 const struct export_operations *s_export_op;
993 unsigned long s_flags;
994 unsigned long s_magic;
995 struct dentry *s_root;
996 struct rw_semaphore s_umount;
997 struct mutex s_lock;
998 int s_count;
999 int s_syncing;
1000 int s_need_sync_fs;
1001 atomic_t s_active;
1002 #ifdef CONFIG_SECURITY
1003 void *s_security;
1004 #endif
1005 struct xattr_handler **s_xattr;
1006
1007 struct list_head s_inodes; /* all inodes */
1008 struct list_head s_dirty; /* dirty inodes */
1009 struct list_head s_io; /* parked for writeback */
1010 struct list_head s_more_io; /* parked for more writeback */
1011 struct hlist_head s_anon; /* anonymous dentries for (nfs) exporting */
1012 struct list_head s_files;
1013
1014 struct block_device *s_bdev;
1015 struct mtd_info *s_mtd;
1016 struct list_head s_instances;
1017 struct quota_info s_dquot; /* Diskquota specific options */
1018
1019 int s_frozen;
1020 wait_queue_head_t s_wait_unfrozen;
1021
1022 char s_id[32]; /* Informational name */
1023
1024 void *s_fs_info; /* Filesystem private info */
1025
1026 /*
1027 * The next field is for VFS *only*. No filesystems have any business
1028 * even looking at it. You had been warned.
1029 */
1030 struct mutex s_vfs_rename_mutex; /* Kludge */
1031
1032 /* Granularity of c/m/atime in ns.
1033 Cannot be worse than a second */
1034 u32 s_time_gran;
1035
1036 /*
1037 * Filesystem subtype. If non-empty the filesystem type field
1038 * in /proc/mounts will be "type.subtype"
