linux字符设备解析

字符设备的表示当然是指这个struct cdev结构。let us see。

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

kobj用来表示sys下的目录，owner模块使用者指针，一般赋值为THIS_MODULE，第三个参数很关键file_operations,是用户层调用open、release、write、read函数时。传到内核层处理，第四个结构list表示的是一个链表节点。第五个是设备号，没什么讲的。第六个是count

怎么分配结构体cdev呢，有两种方式，一种是静态定义一个 struct cdev my_cdev，然后在使用cdev_init函数去初始化该结构，第二种方式是利用cdev_alloc函数去动态申请。

有的人会先使用cdev_alloc函数去动态分配，在使用函数cdev_init去释放，其实这种用法是不太正确的，为什么呢？且看下面分析。

void cdev_init(struct cdev *cdev, const struct file_operations *fops)
{
    memset(cdev, 0, sizeof *cdev);
    INIT_LIST_HEAD(&cdev->list);
    kobject_init(&cdev->kobj, &ktype_cdev_default);
    cdev->ops = fops;
}

struct cdev *cdev_alloc(void)
{
    struct cdev *p = kzalloc(sizeof(struct cdev), GFP_KERNEL);
    if (p) {
        INIT_LIST_HEAD(&p->list);
        kobject_init(&p->kobj, &ktype_cdev_dynamic);
    }
    return p;
}

看出相同之处和不同之处了吗，yes kobject_init这个函数是关键，关键在于ktype类型不同、

static struct kobj_type ktype_cdev_default = {
    .release    = cdev_default_release,
};

static struct kobj_type ktype_cdev_dynamic = {
    .release    = cdev_dynamic_release,
};

很明显，再看

static void cdev_default_release(struct kobject *kobj)
{
    struct cdev *p = container_of(kobj, struct cdev, kobj);
    cdev_purge(p);
}

static void cdev_dynamic_release(struct kobject *kobj)
{
    struct cdev *p = container_of(kobj, struct cdev, kobj);
    cdev_purge(p);
    kfree(p);
}

对，多了一个kfree，也就是说alloc申请出来的cdev占用的内存空间可以在设备被卸载时自动释放。

下面来看linux 内核怎么管理字符设备号。
内核提供了两种方式，一种是alloc_chrdev_region另一个是register_chrdev_region,两个的底层实现如出一辙。下面来分析alloc_chrdev_region。

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;
}

主要的函数是__register_chrdev_region(0, baseminor, count, name);

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);//分配设备号相关的信息。
    if (cd == NULL)
        return ERR_PTR(-ENOMEM);

    mutex_lock(&chrdevs_lock);

    /* temporary */
    if (major == 0) {//动态申请
        for (i = ARRAY_SIZE(chrdevs)-1; i > 0; i--) {
            if (chrdevs[i] == NULL)
                break;
        }

        if (i == 0) {
            ret = -EBUSY;
            goto out;
        }
        major = i;
        ret = major;
    }

    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;//次设备号的挂接是从小到大，

    /* Check for overlapping minor ranges.  */
    if (*cp && (*cp)->major == major) {//检测是否有设备号重合冲突
        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;//挂接到哈希表里面
    *cp = cd;
    mutex_unlock(&chrdevs_lock);
    return cd;
out:
    mutex_unlock(&chrdevs_lock);
    kfree(cd);
    return ERR_PTR(ret);
}

分析到这里，就该看一个有意思的数据结构了，也就是所谓的哈希表。

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];

这就是哈希表结构，chrdevs是一个指针数组，每一个元素都存储着一个字符设备，一看是static类型的，就知道这个chrdevs的每一项初始化后都为NULL，当一个新的设备来了之后通过主设备号来挂接到相应的chrdevs里面，主设备号就是哈希表的表头，而冲突域就用next指针来挂接。比如我注册了一个major = 254 baseminor= 3 的一个设备和major = 254 baseminor= 6的一个设备，这时候chrdevs[254]就应该指向major= 254 baseminor= 3的char_device_struct结构然后该结构的next指针又指向 major= 254 baseminor= 6的char_device_struct结构。