register_chrdev，register_chrdev_region和alloc_chrdev_region的关系和区别
register_chrdev，register_chrdev_region和alloc_chrdev_region最终都是调用的__register_chrdev_region函数。
而与之相关的主设备号可以通过/proc/devices来查看。
相关代码如下：
register_chrdev源码解析：
从0-255中查找可用的设备号，并返回。
static inline int register_chrdev(unsigned int major, const char *name,
const struct file_operations *fops)
{
return __register_chrdev(major, 0, 256, name, fops);
}
int __register_chrdev(unsigned int major, unsigned int baseminor,
unsigned int count, const char *name,
const struct file_operations *fops)
{
struct char_device_struct *cd;
struct cdev *cdev;
int err = -ENOMEM;
cd = __register_chrdev_region(major, baseminor, count, name);
if (IS_ERR(cd))
return PTR_ERR(cd);
cdev = cdev_alloc();
if (!cdev)
goto out2;
cdev->owner = fops->owner;
cdev->ops = fops;
kobject_set_name(&cdev->kobj, "%s", name);
err = cdev_add(cdev, MKDEV(cd->major, baseminor), count);
if (err)
goto out;
cd->cdev = cdev;
return major ? 0 : cd->major;
out:
kobject_put(&cdev->kobj);
out2:
kfree(__unregister_chrdev_region(cd->major, baseminor, count));
return err;
}
完整代码：
1.自动获取主设备号:
/* hello.c */
#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/cdev.h>
// MODULE_LICENSE("leo BSD/GPL");
static const struct file_operations hello_fops = {
.owner = THIS_MODULE,
.unlocked_ioctl = NULL,
.open = NULL,
};
int major;
#define HELLO_MAJOR (66)
static int hello_init(void)
{
printk(KERN_ALERT "Hello, world\n");
printk("Hello, world\n");
major = register_chrdev(0, "hello_world", &hello_fops);
printk(KERN_WARNING "hello_world: register character device %d.", major);
if (major < 0)
printk(KERN_WARNING "hello_world: Failed to register character device.");
return 0;
}
static void hello_exit(void)
{
printk(KERN_ALERT "Goodbye, Hello world\n");
printk("Goodbye, Hello world\n");
printk(KERN_WARNING "hello_world: unregister character device %d.", major);
unregister_chrdev(major, "hello_world");
}
module_init(hello_init);
module_exit(hello_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("bigcatio");
MODULE_DESCRIPTION("A good start");
MODULE_VERSION("0.0.1");
2.指定主设备号：
/* hello.c */
#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/cdev.h>
// MODULE_LICENSE("leo BSD/GPL");
static const struct file_operations hello_fops = {
.owner = THIS_MODULE,
.unlocked_ioctl = NULL,
.open = NULL,
};
int major;
#define HELLO_MAJOR (66)
static int hello_init(void)
{
printk(KERN_ALERT "Hello, world\n");
printk("Hello, world\n");
major = register_chrdev(HELLO_MAJOR, "hello_world", &hello_fops);
printk(KERN_WARNING "hello_world: register character device %d.", HELLO_MAJOR);
if (major < 0)
printk(KERN_WARNING "hello_world: Failed to register character device.");
return 0;
}
static void hello_exit(void)
{
printk(KERN_ALERT "Goodbye, Hello world\n");
printk("Goodbye, Hello world\n");
printk(KERN_WARNING "hello_world: unregister character device %d.", HELLO_MAJOR);
unregister_chrdev(HELLO_MAJOR, "hello_world");
}
module_init(hello_init);
module_exit(hello_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("bigcatio");
MODULE_DESCRIPTION("A good start");
MODULE_VERSION("0.0.1");
register_chrdev_region源码解析：
从from开始往后的count个设备号中，查找n个可用的设备号并返回。
int register_chrdev_region(dev_t from, unsigned count, const char *name)
{
struct char_device_struct *cd;
dev_t to = from + count;
dev_t n, next;
for (n = from; n < to; n = next) {
next = MKDEV(MAJOR(n)+1, 0);
if (next > to)
next = to;
cd = __register_chrdev_region(MAJOR(n), MINOR(n),
next - n, name);
if (IS_ERR(cd))
goto fail;
}
return 0;
fail:
to = n;
for (n = from; n < to; n = next) {
next = MKDEV(MAJOR(n)+1, 0);
kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n));
}
return PTR_ERR(cd);
}
完整代码：
/* hello.c */
#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/cdev.h>
// MODULE_LICENSE("leo BSD/GPL");
static const struct file_operations hello_fops = {
.owner = THIS_MODULE,
.unlocked_ioctl = NULL,
.open = NULL,
};
int major;
#define HELLO_MAJOR (66)
static struct cdev hello_device_cdev;
#define MAX_MINORS 256
static int hello_init(void)
{
int retval;
dev_t dev = MKDEV(HELLO_MAJOR, 0);
if ((retval = register_chrdev_region(dev, MAX_MINORS, "hello_world")) != 0)
{
pr_err("hello_world: unable to get major %d\n", HELLO_MAJOR);
return retval;
}
cdev_init(&hello_device_cdev, &hello_fops);
if ((retval = cdev_add(&hello_device_cdev, dev, MAX_MINORS)) != 0)
{
pr_err("hello_world: unable register character device\n");
}
return 0;
}
static void hello_exit(void)
{
printk(KERN_ALERT "Goodbye, Hello world\n");
printk("Goodbye, Hello world\n");
printk(KERN_WARNING "hello_world: unregister character device %d.", HELLO_MAJOR);
cdev_del(&hello_device_cdev);
unregister_chrdev_region(MKDEV(HELLO_MAJOR, 0), MAX_MINORS);
}
module_init(hello_init);
module_exit(hello_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("bigcatio");
MODULE_DESCRIPTION("A good start");
MODULE_VERSION("0.0.1");
alloc_chrdev_region源码解析：
int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,
const char *name)
{
struct char_device_struct *cd;
cd = __register_chrdev_region(0, baseminor, count, name);
if (IS_ERR(cd))
return PTR_ERR(cd);
*dev = MKDEV(cd->major, cd->baseminor);
return 0;
}
完整代码：
/* hello.c */
#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/cdev.h>
// MODULE_LICENSE("leo BSD/GPL");
static const struct file_operations hello_fops = {
.owner = THIS_MODULE,
.unlocked_ioctl = NULL,
.open = NULL,
};
int major;
static struct cdev hello_device_cdev;
#define MAX_MINORS 256
dev_t dev = 0;
static int hello_init(void)
{
int ret;
int retval;
/* Get a range of minor numbers (starting with 0) to work with */
ret = alloc_chrdev_region(&dev, 0, MAX_MINORS, "hello_world");
if (ret < 0)
{
pr_err("hello_world: alloc_chrdev_region() failed\n");
return 0;
}
cdev_init(&hello_device_cdev, &hello_fops);
if ((retval = cdev_add(&hello_device_cdev, dev, MAX_MINORS)) != 0)
{
pr_err("hello_world: unable register character device\n");
}
printk(KERN_WARNING "hello_world: register character device %d.", MAJOR(dev));
return 0;
}
static void hello_exit(void)
{
printk(KERN_ALERT "Goodbye, Hello world\n");
printk("Goodbye, Hello world\n");
printk(KERN_WARNING "hello_world: unregister character device %d.", MAJOR(dev));
cdev_del(&hello_device_cdev);
unregister_chrdev_region(dev, MAX_MINORS);
}
module_init(hello_init);
module_exit(hello_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("bigcatio");
MODULE_DESCRIPTION("A good start");
MODULE_VERSION("0.0.1");
下面进一步剖析_register_chrdev_region:
_register_chrdev_region通过调用find_dynamic_major找到可用的主设备号major，
/*
* Register a single major with a specified minor range.
*
* If major == 0 this functions will dynamically allocate a major and return
* its number.
*
* If major > 0 this function will attempt to reserve the passed range of
* minors and will return zero on success.
*
* Returns a -ve errno on failure.
*/
static struct char_device_struct *
__register_chrdev_region(unsigned int major, unsigned int baseminor,
int minorct, const char *name)
{
struct char_device_struct *cd, **cp;
int ret = 0;
int i;
cd = kzalloc(sizeof(struct char_device_struct), GFP_KERNEL);
if (cd == NULL)
return ERR_PTR(-ENOMEM);
mutex_lock(&chrdevs_lock);
if (major == 0) {
ret = find_dynamic_major();
if (ret < 0) {
pr_err("CHRDEV \"%s\" dynamic allocation region is full\n",
name);
goto out;
}
major = ret;
}
if (major >= CHRDEV_MAJOR_MAX) {
pr_err("CHRDEV \"%s\" major requested (%u) is greater than the maximum (%u)\n",
name, major, CHRDEV_MAJOR_MAX-1);
ret = -EINVAL;
goto out;
}
cd->major = major;
cd->baseminor = baseminor;
cd->minorct = minorct;
strlcpy(cd->name, name, sizeof(cd->name));
i = major_to_index(major);
for (cp = &chrdevs[i]; *cp; cp = &(*cp)->next)
if ((*cp)->major > major ||
((*cp)->major == major &&
(((*cp)->baseminor >= baseminor) ||
((*cp)->baseminor + (*cp)->minorct > baseminor))))
break;
/* Check for overlapping minor ranges. */
if (*cp && (*cp)->major == major) {
int old_min = (*cp)->baseminor;
int old_max = (*cp)->baseminor + (*cp)->minorct - 1;
int new_min = baseminor;
int new_max = baseminor + minorct - 1;
/* New driver overlaps from the left. */
if (new_max >= old_min && new_max <= old_max) {
ret = -EBUSY;
goto out;
}
/* New driver overlaps from the right. */
if (new_min <= old_max && new_min >= old_min) {
ret = -EBUSY;
goto out;
}
}
cd->next = *cp;
*cp = cd;
mutex_unlock(&chrdevs_lock);
return cd;
out:
mutex_unlock(&chrdevs_lock);
kfree(cd);
return ERR_PTR(ret);
}
主设备号为0时，会动态寻找可用的空闲的主设备号。
内核中字符设备号最多可以到512，但实际最多能够容纳255个设备，这是由*chrdevs[CHRDEV_MAJOR_HASH_SIZE]决定的，chrdevs中的每一个非空元素都对应着一个字符设备。
查找可用的主设备号：
find_dynamic_major会优先从254开始向CHRDEV_MAJOR_DYN_END（234）寻找可用的主设备号，当返现可用的设备号后则返回该值，否则继续利用哈希值（对255%）从511向384，共127个设备号进行查询，若未使用，则返回该设备号，否则返回失败。
可知，Linux下字符设备的主设备号的范围｛129······254｝
#define CHRDEV_MAJOR_HASH_SIZE 255
static struct char_device_struct {
struct char_device_struct *next; // 指向散列冲突链表中的下一个元素的指针
unsigned int major; // 主设备号
unsigned int baseminor; // 起始次设备号
int minorct; // 设备编号的范围大小
char name[64]; / 处理该设备编号范围内的设备驱动的名称
struct cdev *cdev; /* will die */
} *chrdevs[CHRDEV_MAJOR_HASH_SIZE];
/* index in the above */
static inline int major_to_index(unsigned major)
{
return major % CHRDEV_MAJOR_HASH_SIZE;
}
/* fs/char_dev.c */
#define CHRDEV_MAJOR_MAX 512
/* Marks the bottom of the first segment of free char majors */
#define CHRDEV_MAJOR_DYN_END 234
/* Marks the top and bottom of the second segment of free char majors */
#define CHRDEV_MAJOR_DYN_EXT_START 511
#define CHRDEV_MAJOR_DYN_EXT_END 384
static int find_dynamic_major(void)
{
int i;
struct char_device_struct *cd;
for (i = ARRAY_SIZE(chrdevs)-1; i >= CHRDEV_MAJOR_DYN_END; i--) {
if (chrdevs[i] == NULL)
return i;
}
for (i = CHRDEV_MAJOR_DYN_EXT_START;
i >= CHRDEV_MAJOR_DYN_EXT_END; i--) {
for (cd = chrdevs[major_to_index(i)]; cd; cd = cd->next)
if (cd->major == i)
break;
if (cd == NULL)
return i;
}
return -EBUSY;
}
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