设备树 aliases

 / {
    #address-cells = <1>;
    #size-cells = <1>;
 
    aliases {
        fimc0 = &fimc0;
        fimc1 = &fimc1;
        fimc2 = &fimc2;     
        i2c0 = &i2c0;
        i2c1 = &i2c1;
        i2c2 = &i2c2;
           ······
    };
    
    ......
};

start_kernel
    setup_arch
        unflatten_device_tree
            of_alias_scan
void of_alias_scan(void * (*dt_alloc)(u64 size, u64 align))
{
    struct property *pp;
 
    /* 找到/aliases节点对应的device_node */
    of_aliases = of_find_node_by_path("/aliases");
 
    /*  找到/chosen节点对应的device_node */
    of_chosen = of_find_node_by_path("/chosen");
 
    /* 如果没有/chosen的话，就找/chosen@0节点 */
    if (of_chosen == NULL)
        of_chosen = of_find_node_by_path("/chosen@0");
 
    /* 遍历/aliases节点的属性 */
    for_each_property_of_node(of_aliases, pp) {
        const char *start = pp->name;            // 属性的名字
        const char *end = start + strlen(start); // 名字的结尾，*end是'\0'
        struct device_node *np;
        struct alias_prop *ap;
        int id, len;
 
        /* Skip those we do not want to proceed */
        /* 忽略名字是name、phandle、linux,phandle的属性 */
        if (!strcmp(pp->name, "name") ||
            !strcmp(pp->name, "phandle") ||
            !strcmp(pp->name, "linux,phandle"))
            continue;
 
        /* 根据属性的值获得这个值对应的节点 */
        /* 比如这个这个属性,fimc2 = "/soc/camera/fimc@fb400000"通过路径来找到节点 */
        np = of_find_node_by_path(pp->value);
        if (!np)
            continue;
 
        /* 正常一个属性名字是fimc1,i2c0,这样,这里是把end指针指向数字前面那一个位置,
         * 比如fimc2的c字符的地址 */
        while (isdigit(*(end-1)) && end > start)
            end--;
        len = end - start;    /* 这个len就是名字除去数字的长度 */
 
        /* 把end字符串后面的数字,由字符串转换成数字复制给id,比如fimc2的,数子2 */
        if (kstrtoint(end, 10, &id) < 0)        
            continue;
 
        /* Allocate an alias_prop with enough space for the stem */
        /* 分配内存，多分配的"len+1"用于存放stem的名字 */
        ap = dt_alloc(sizeof(*ap) + len + 1, __alignof__(*ap));
        if (!ap)
            continue;
        memset(ap, 0, sizeof(*ap) + len + 1);
        ap->alias = start;        /*  ap->alias指向字符串"fimc2 */
        of_alias_add(ap, np, id, start, len);
    }
}
 
static void of_alias_add(struct alias_prop *ap, struct device_node *np,
             int id, const char *stem, int stem_len)
{
    ap->np = np;        // np是"/soc/camera/fimc@fb400000";"对应的节点device_node
    ap->id = id;        // id的值是2,代表设备序号,使用的时候要用到
 
    //由于stem_len是名字除去数字的长度，所以ap->stem被赋值为"fimc"
    strncpy(ap->stem, stem, stem_len);    
    ap->stem[stem_len] = 0;
    list_add_tail(&ap->link, &aliases_lookup);    //将这个ap加入到全局aliases_lookup链表中
    pr_debug("adding DT alias:%s: stem=%s id=%i node=%pOF\n",
         ap->alias, ap->stem, ap->id, np);
}

int i2c_add_adapter(struct i2c_adapter *adapter)
{
    struct device *dev = &adapter->dev;
    int id;
 
    if (dev->of_node) {
//获取与这个device_node对应的alias_prop的id。如果以 i2c2 = &i2c2;节点为例，这里得到的id就是2
        id = of_alias_get_id(dev->of_node, "i2c");
        if (id >= 0) {
            adapter->nr = id;
            return __i2c_add_numbered_adapter(adapter);
        }
    }
 
    id = idr_alloc(&i2c_adapter_idr, adapter,
               __i2c_first_dynamic_bus_num, 0, GFP_KERNEL);
 
    adapter->nr = id;
    return i2c_register_adapter(adapter);
}
 
int of_alias_get_id(struct device_node *np, const char *stem)
{
    struct alias_prop *app;
    int id = -ENODEV;
 
    mutex_lock(&of_mutex);
    /* 遍历全局链表aliases_lookup */
    list_for_each_entry(app, &aliases_lookup, link) {
        /* 找到 stem 是 "i2c" 的alias_prop */
        if (strcmp(app->stem, stem) != 0)
            continue;
        /* 所有的i2c控制的stem都叫"i2c",所以要保证设备节点一致 */
        if (np == app->np) {
            id = app->id;        /* 最终返回找到的编号名字 */
            break;
        }
    }
    mutex_unlock(&of_mutex);
 
    return id;
}