轉自http://www.cnblogs.com/pengdonglin137/p/5252348.html

在設備樹中有一個叫做aliases的節點：

   1: / {
   2:     ... ...
   3:  
   4:     chosen {
   5:         stdout-path = "/serial@13800000";
   6:         bootargs = "root=/dev/ram0 rw rootfstype=ext4 console=ttySAC0,115200 ethmac=1C:6F:65:34:51:7E init=/linuxrc";
   7:     };
   8:  
   9:     aliases {
  10:         spi0 = "/spi@13920000";
  11:         spi1 = "/spi@13930000";
  12:         spi2 = "/spi@13940000";
  13:         i2c0 = "/i2c@13860000";
  14:         i2c1 = "/i2c@13870000";
  15:         i2c2 = "/i2c@13880000";
  16:         i2c3 = "/i2c@13890000";
  17:         ... ...
  18:     };
  19: ... ...
  20: };
在Linux內核啟動的時候會解析這個節點：
start_kernel    ---> setup_arch            ---> unflatten_device_tree                    ---> of_alias_scan
在of_alias_scan中會掃描這個節點：
of_alias_scan：
   1: void of_alias_scan(void * (*dt_alloc)(u64 size, u64 align))
   2: {
   3:     struct PRoperty *pp;
   4:  
   5:     of_aliases = of_find_node_by_path("/aliases"); // 找到/aliases節點對應的device_node
   6:     of_chosen = of_find_node_by_path("/chosen"); // 找到/chosen節點對應的device_node
   7:     if (of_chosen == NULL) // 如果沒有/chosen的話，就找/chosen@0節點
   8:         of_chosen = of_find_node_by_path("/chosen@0");
   9:  
  10:     if (of_chosen) {
  11:         /* linux,stdout-path and /aliases/stdout are for legacy compatibility */
  12:         const char *name = of_get_property(of_chosen, "stdout-path", NULL);
  13:         if (!name)
  14:             name = of_get_property(of_chosen, "linux,stdout-path", NULL);
  15:         if (IS_ENABLED(CONFIG_PPC) && !name)
  16:             name = of_get_property(of_aliases, "stdout", NULL);
  17:         if (name)
  18:             of_stdout = of_find_node_opts_by_path(name, &of_stdout_options);
  19:     }
  20:  
  21:     if (!of_aliases)
  22:         return;
  23:  
  24:     for_each_property_of_node(of_aliases, pp) { // 遍歷/aliases節點的屬性，以屬性i2c2 = "/i2c@13880000";為例
  25:         const char *start = pp->name; // 屬性的名字，如"i2c2"
  26:         const char *end = start + strlen(start); // 名字的結尾，*end是'/0'
  27:         struct device_node *np;
  28:         struct alias_prop *ap;
  29:         int id, len;
  30:  
  31:         /* 不處理名字是name、phandle、linux,phandle的屬性 */
  32:         if (!strcmp(pp->name, "name") || 
  33:             !strcmp(pp->name, "phandle") ||
  34:             !strcmp(pp->name, "linux,phandle"))
  35:             continue;
  36:  
  37:         np = of_find_node_by_path(pp->value); 
  38:         /*
  39:             根據屬性的值（如"/i2c@13880000"）獲得這個值對應的節點
  40:             i2c@13880000 {
  41:                 #address-cells = <0x1>;
  42:                 #size-cells = <0x0>;
  43:                 compatible = "samsung,s3c2440-i2c";
  44:                 reg = <0x13880000 0x100>;
  45:                 interrupts = <0x0 0x3c 0x0>;
  46:                 clocks = <0x7 0x13f>;
  47:                 clock-names = "i2c";
  48:                 pinctrl-names = "default";
  49:                 pinctrl-0 = <0x22>;
  50:                 status = "disabled";
  51:             };        
  52:         */
  53:         if (!np)
  54:             continue;
  55:  
  56:         /* walk the alias backwards to extract the id and work out
  57:          * the 'stem' string */
  58:         while (isdigit(*(end-1)) && end > start) //對于"i2c2"，end最終會指向字符'2'的地址
  59:             end--;
  60:         len = end - start; // 獲得"i2c"的長度(不包含結尾的數字2)，就是3
  61:  
  62:         if (kstrtoint(end, 10, &id) < 0) // 將end指向的字符'2'轉化為數字2，賦值給id
  63:             continue;
  64:  
  65:         /* Allocate an alias_prop with enough space for the stem */
  66:         ap = dt_alloc(sizeof(*ap) + len + 1, 4); // 分配內存，多分配的"len+1"用于存放stem的名字
  67:         if (!ap)
  68:             continue;
  69:         memset(ap, 0, sizeof(*ap) + len + 1);
  70:         ap->alias = start; // ap->alias指向字符串"i2c2"
  71:         of_alias_add(ap, np, id, start, len);
  72:     }
  73: }
of_alias_add：
   1: static void of_alias_add(struct alias_prop *ap, struct device_node *np,
   2:              int id, const char *stem, int stem_len)
   3: {
   4:     ap->np = np; // np是"/i2c@13880000"對應的節點device_node
   5:     ap->id = id; // id的值是2
   6:     strncpy(ap->stem, stem, stem_len); // 由于stem_len是3，所以ap->stem被賦值為"i2c"
   7:     ap->stem[stem_len] = 0;
   8:     list_add_tail(&ap->link, &aliases_lookup); // 將這個ap加入到全局aliases_lookup鏈表中
   9:     pr_debug("adding DT alias:%s: stem=%s id=%i node=%s/n",
  10:          ap->alias, ap->stem, ap->id, of_node_full_name(np));
  11: }
使用：
在drivers/i2c/i2c-core.c中：
   1: int i2c_add_adapter(struct i2c_adapter *adapter)
   2: {
   3:     struct device *dev = &adapter->dev;
   4:     int id;
   5:  
   6:     if (dev->of_node) {
   7:         id = of_alias_get_id(dev->of_node, "i2c");
   8:         if (id >= 0) {
   9:             adapter->nr = id;
  10:             return __i2c_add_numbered_adapter(adapter);
  11:         }
  12:     }
  13:     ... ...
  14: }
第7行調用of_alias_get_id獲得與這個device_node（即/i2c@13880000節點）對應的alias_prop的id，如果以/i2c@13880000節點為例，這里得到的id就是2。
of_alias_get_id：
   1: int of_alias_get_id(struct device_node *np, const char *stem)
   2: {
   3:     struct alias_prop *app;
   4:     int id = -ENODEV;
   5:  
   6:     mutex_lock(&of_mutex);
   7:     list_for_each_entry(app, &aliases_lookup, link) { // 遍歷全局鏈表aliases_lookup
   8:         if (strcmp(app->stem, stem) != 0) // 找到 stem 是 "i2c" 的alias_prop
   9:             continue;
  10:  
  11:         if (np == app->np) { // 判斷這個alias_prop指向的device_node是不是跟傳入的匹配
  12:             id = app->id;  // 獲得 id，2
  13:             break;
  14:         }
  15:     }
  16:     mutex_unlock(&of_mutex);
  17:  
  18:     return id;
  19: }
 
從上面的分析就可以知道alias節點的作用了：
比如SoC上有如果多個i2c控制器，alias的相當於給每個i2c控制器分配一個唯一的編號，如上面的i2c@13880000對應的alias是i2c2，那麼這個編號就是2，將來就可以在/dev下看到名爲i2c-2的設備節點。
在內核中可以看到很多地方都會調用of_alias_get_id，他的作用就是根據傳入的device node，在alias中找到對應的唯一編號，如：
of_alias_get_id(pdev->dev.of_node, "spi")
of_alias_get_id(node, "fimc")
of_alias_get_id(pdev->dev.of_node, "serial")
of_alias_get_id(pdev->dev.of_node, "uart")
of_alias_get_id(dev->of_node, "gpio")
... ...
完。