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在设备树中有一个叫做aliases的节点：
/ {
#address-cells = <1>;
#size-cells = <1>;
aliases {
csis0 = &csis0;
fimc0 = &fimc0;
fimc1 = &fimc1;
fimc2 = &fimc2;
i2c0 = &i2c0;
i2c1 = &i2c1;
i2c2 = &i2c2;
i2s0 = &i2s0;
i2s1 = &i2s1;
i2s2 = &i2s2;
pinctrl0 = &pinctrl0;
spi0 = &spi0;
spi1 = &spi1;
};
......
};
对这个dtb文件反编译后的信息如下
dtc -I dtb -O dts -o s5pv210-x210_temp.dts s5pv210-x210.dtb / {
#address-cells = <0x1>;
#size-cells = <0x1>;
aliases {
csis0 = "/soc/camera/csis@fa600000";
fimc0 = "/soc/camera/fimc@fb200000";
fimc1 = "/soc/camera/fimc@fb300000";
fimc2 = "/soc/camera/fimc@fb400000";
i2c0 = "/soc/i2c@e1800000";
i2c1 = "/soc/i2c@fab00000";
i2c2 = "/soc/i2c@e1a00000";
i2s0 = "/soc/audio-subsystem/i2s@eee30000";
i2s1 = "/soc/i2s@e2100000";
i2s2 = "/soc/i2s@e2a00000";
pinctrl0 = "/soc/pinctrl@e0200000";
spi0 = "/soc/spi@e1300000";
spi1 = "/soc/spi@e1400000";
};
};
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/**
* unflatten_device_tree - create tree of device_nodes from flat blob
*
* unflattens the device-tree passed by the firmware, creating the
* tree of struct device_node. It also fills the "name" and "type"
* pointers of the nodes so the normal device-tree walking functions
* can be used.
*/
void __init unflatten_device_tree(void)
{
__unflatten_device_tree(initial_boot_params, NULL, &of_root,
early_init_dt_alloc_memory_arch, false);
/* Get pointer to "/chosen" and "/aliases" nodes for use everywhere */
of_alias_scan(early_init_dt_alloc_memory_arch);
unittest_unflatten_overlay_base();
}
/**
* of_alias_scan - Scan all properties of the 'aliases' node
*
* The function scans all the properties of the 'aliases' node and populates
* the global lookup table with the properties. It returns the
* number of alias properties found, or an error code in case of failure.
*
* @dt_alloc: An allocator that provides a virtual address to memory
* for storing the resulting tree
*/
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");
if (of_chosen) {
/* linux,stdout-path and /aliases/stdout are for legacy compatibility */
const char *name = NULL;
if (of_property_read_string(of_chosen, "stdout-path", &name))
of_property_read_string(of_chosen, "linux,stdout-path",
&name);
if (IS_ENABLED(CONFIG_PPC) && !name)
of_property_read_string(of_aliases, "stdout", &name);
if (name)
of_stdout = of_find_node_opts_by_path(name, &of_stdout_options);
}
if (!of_aliases)
return;
/* 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;
/* walk the alias backwards to extract the id and work out
* the 'stem' string */
/* 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);
}
使用：
假设在i2c总选注册为时
int i2c_add_adapter(struct i2c_adapter *adapter)
{
struct device *dev = &adapter->dev;
int id;
if (dev->of_node) {
id = of_alias_get_id(dev->of_node, "i2c");
if (id >= 0) {
adapter->nr = id;
return __i2c_add_numbered_adapter(adapter);
}
}
mutex_lock(&core_lock);
id = idr_alloc(&i2c_adapter_idr, adapter,
__i2c_first_dynamic_bus_num, 0, GFP_KERNEL);
mutex_unlock(&core_lock);
if (WARN(id < 0, "couldn't get idr"))
return id;
adapter->nr = id;
return i2c_register_adapter(adapter);
}id = of_alias_get_id(dev->of_node, "i2c");
调用of_alias_get_id获得与这个device_node对应的alias_prop的id。如果以 i2c2 = "/soc/i2c@e1a00000";节点为例，这里得到的id就是2。
/**
* of_alias_get_id - Get alias id for the given device_node
* @np: Pointer to the given device_node
* @stem: Alias stem of the given device_node
*
* The function travels the lookup table to get the alias id for the given
* device_node and alias stem. It returns the alias id if found.
*/
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;
}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的相当于給每個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(np, "serial");
of_alias_get_id(np, "ssc");
of_alias_get_id(dev->of_node, "rtc");
of_alias_get_id(dev->of_node, "fimc");
of_alias_get_id(pdev->dev.of_node, "gpio");
of_alias_get_id(pdev->dev.of_node, "spi");