字符设备驱动在内核源码中的分析

设备号相关：位于kdev_t.h中

#define MINORBITS	20
#define MINORMASK	((1U << MINORBITS) - 1)

#define MAJOR(dev)	((unsigned int) ((dev) >> MINORBITS))
#define MINOR(dev)	((unsigned int) ((dev) & MINORMASK))
#define MKDEV(ma,mi)	(((ma) << MINORBITS) | (mi))

#define print_dev_t(buffer, dev)					\
sprintf((buffer), "%u:%u\n", MAJOR(dev), MINOR(dev))

#define format_dev_t(buffer, dev)					\
({								\
sprintf(buffer, "%u:%u", MAJOR(dev), MINOR(dev));	\
buffer;							\
})

/* acceptable for old filesystems */
static inline int old_valid_dev(dev_t dev)
{
return MAJOR(dev) < 256 && MINOR(dev) < 256;
}

static inline u16 old_encode_dev(dev_t dev)
{
return (MAJOR(dev) << 8) | MINOR(dev);
}

static inline dev_t old_decode_dev(u16 val)
{
return MKDEV((val >> 8) & 255, val & 255);
}

static inline int new_valid_dev(dev_t dev)
{
return 1;
}

static inline u32 new_encode_dev(dev_t dev)
{
unsigned major = MAJOR(dev);
unsigned minor = MINOR(dev);
return (minor & 0xff) | (major << 8) | ((minor & ~0xff) << 12);
}

static inline dev_t new_decode_dev(u32 dev)
{
unsigned major = (dev & 0xfff00) >> 8;
unsigned minor = (dev & 0xff) | ((dev >> 12) & 0xfff00);
return MKDEV(major, minor);
}

static inline int huge_valid_dev(dev_t dev)
{
return 1;
}

static inline u64 huge_encode_dev(dev_t dev)
{
return new_encode_dev(dev);
}

static inline dev_t huge_decode_dev(u64 dev)
{
return new_decode_dev(dev);
}

static inline int sysv_valid_dev(dev_t dev)
{
return MAJOR(dev) < (1<<14) && MINOR(dev) < (1<<18);
}

static inline u32 sysv_encode_dev(dev_t dev)
{
return MINOR(dev) | (MAJOR(dev) << 18);
}

static inline unsigned sysv_major(u32 dev)
{
return (dev >> 18) & 0x3fff;
}

static inline unsigned sysv_minor(u32 dev)
{
return dev & 0x3ffff;
}

#else /* __KERNEL__ */

/*
Some programs want their definitions of MAJOR and MINOR and MKDEV
from the kernel sources. These must be the externally visible ones.
*/
#define MAJOR(dev)	((dev)>>8)
#define MINOR(dev)	((dev) & 0xff)
#define MKDEV(ma,mi)	((ma)<<8 | (mi))

字符设备用一个cdev数据结构表示，是它的一个实例。通过全局数组cdev_map映射为内核的哈希表，cdev_map是数据结构kobj_map的实例。

struct kobj_map {
struct probe {
struct probe *next;
dev_t dev;
unsigned long range;
struct module *owner;
kobj_probe_t *get;
int (*lock)(dev_t, void *);
void *data;
} *probes[255];
struct mutex *lock;
};

字符设备驱动的结构体：位于char_dev.c中

static struct char_device_struct {
struct char_device_struct *next;
unsigned int major;
unsigned int baseminor;
int minorct;
char name[64];
struct cdev *cdev;		/* will die */
} *chrdevs[CHRDEV_MAJOR_HASH_SIZE];

注册过程涉及的文件有fs.h   cdev.h   video1394.h

linux遵从一切皆文件的哲学，因此定义于fs.h中的inode结构体中有关于设备文件的成员，其中包括文件类型i_mode（字符或者块），主从设备号dev_t，文件操作函数指针i_fop。

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（位于inode.c），为字符设备或块设备创建一个inode。

字符设备操作

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

接下来，按照下图