of_alias_get_id 函数与设备树中aliases节点的关系【转】

转自：https://blog.csdn.net/qq_30145093/article/details/78053823?locationNum=10&fps=1

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

在设备树中有一个叫做aliases的节点：

   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内核启动的时候会解析这个节点：

start_kernel
    ---> setup_arch
            ---> unflatten_device_tree
                    ---> of_alias_scan

在of_alias_scan中会扫描这个节点：

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节点对应的device_node

   6:     of_chosen = of_find_node_by_path("/chosen"); // 找到/chosen节点对应的device_node

   7:     if (of_chosen == NULL) // 如果没有/chosen的话，就找/chosen@0节点

   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) { // 遍历/aliases节点的属性，以属性i2c2 = "/i2c@13880000";为例

  25:         const char *start = pp->name; // 属性的名字，如"i2c2"

  26:         const char *end = start + strlen(start); // 名字的结尾，*end是'\0'

  27:         struct device_node *np;

  28:         struct alias_prop *ap;

  29:         int id, len;

  30:  

  31:         /* 不处理名字是name、phandle、linux,phandle的属性 */

  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:             根据属性的值（如"/i2c@13880000"）获得这个值对应的节点

  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) //对于"i2c2"，end最终会指向字符'2'的地址

  59:             end--;

  60:         len = end - start; // 获得"i2c"的长度(不包含结尾的数字2)，就是3

  61:  

  62:         if (kstrtoint(end, 10, &id) < 0) // 将end指向的字符'2'转化为数字2，赋值给id

  63:             continue;

  64:  

  65:         /* Allocate an alias_prop with enough space for the stem */

  66:         ap = dt_alloc(sizeof(*ap) + len + 1, 4); // 分配内存，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"对应的节点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被赋值为"i2c"

   7:     ap->stem[stem_len] = 0;

   8:     list_add_tail(&ap->link, &aliases_lookup); // 将这个ap加入到全局aliases_lookup链表中

   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行调用of_alias_get_id获得与这个device_node（即/i2c@13880000节点）对应的alias_prop的id，如果以/i2c@13880000节点为例，这里得到的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) { // 遍历全局链表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) { // 判断这个alias_prop指向的device_node是不是跟传入的匹配

  12:             id = app->id;  // 获得 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