device_node转换成platform_device

设备树替换了平台总线模型当中对硬件资源描述的device部分。所以设备树也是对硬件资源进行描述的文件。

在平台总线模型中，device部分是用platform_device结构体来描述硬件资源的。所以内核最终会将内核认识的device_node树转换platform_device。
但是并不是所有的节点都会被转换成platform_device，只有满足要求的才会转换成platform_device，
转换成platformdevice的节点可以在/sys/bus/platform/devices下查看。

节点要满足什么要求才会被转换成platform_device呢？

转换规则

1.根节点下包含compatible属性的子节点。
2．节点中compatible属性包含simple-bus,simple-mfd，isa其中之一的节点下包含compatible属性的子节。
3．如果节点的compatible属性包含arm,primecell值，则对应的节点会被转换成不会被转换成amba设各。不会被转换成platform_device。

主要核心函数

of_default_bus_match_table

const struct of_device_id of_default_bus_match_table[] = {{ .compatible = "simple-bus", },{ .compatible = "simple-mfd", },
#ifdef CONFIG_ARM_AMBA{ .compatible = "arm,amba-bus", },
#endif /* CONFIG_ARM_AMBA */{} /* Empty terminated list */
};

arm

static int __init customize_machine(void)
{/ customizes platform devices, or adds new ones* On DT based machines, we fall back to populating the* machine from the device tree, if no callback is provided,* otherwise we would always need an init_machine callback.*/of_iommu_init();if (machine_desc->init_machine)machine_desc->init_machine();
#ifdef CONFIG_OFelseof_platform_populate(NULL, of_default_bus_match_table,NULL, NULL);
#endifreturn 0;
}
arch_initcall(customize_machine);

arm64

在内核启动的时候会执行of_platform_default_populate_init函数，这个函数是用arch_initcall_sync来修饰的。

arch_initcall_sync(of_platform_default_populate_init);

所以系统启动的时候会调用of_platform_default_populate_init函数，然后调用一系列函数进行转换，转换流程如下，

arch/arm64/kernel/setup.c

static int __init arm64_device_init(void)
{
of_iommu_init();
of_platform_populate(NULL, of_default_bus_match_table, NULL, NULL);
return 0;
}
arch_initcall_sync(arm64_device_init);

of_platform_register_reconfig_notifier

int __init platform_bus_init(void)
{int error;early_platform_cleanup();error = device_register(&platform_bus);if (error)return error;error =  bus_register(&platform_bus_type);if (error)device_unregister(&platform_bus);of_platform_register_reconfig_notifier();return error;
}

of_platform_notify函数是Linux内核中的一个通知机制，主要作用是把动态加载的设备树的节点解析出来，以便在驱动模型的基础上构建实际的设备驱动。在platform总线上的控制器模块都是挂在这条总线上，而of_platform_notify函数则是把这些控制器模块挂载到内核中。
在这个函数中，当动态加载一个设备树节点时，会先检查其父节点是否为一个总线，如果不是则返回NOTIFY_OK；如果已经被挂载，则返回NOTIFY_OK；否则会创建一个platform_device并将其挂载到内核中。当卸载一个设备树节点时，会先检查其是否已经被卸载，如果没有则会找到该设备并将其从内核中卸载。

static int of_platform_notify(struct notifier_block *nb,unsigned long action, void *arg)
{struct of_reconfig_data *rd = arg;struct platform_device *pdev_parent, *pdev;bool children_left;switch (of_reconfig_get_state_change(action, rd)) {case OF_RECONFIG_CHANGE_ADD:/* verify that the parent is a bus */if (!of_node_check_flag(rd->dn->parent, OF_POPULATED_BUS))return NOTIFY_OK;    /* not for us *//* already populated? (driver using of_populate manually) */if (of_node_check_flag(rd->dn, OF_POPULATED))return NOTIFY_OK;/* pdev_parent may be NULL when no bus platform device */pdev_parent = of_find_device_by_node(rd->dn->parent);pdev = of_platform_device_create(rd->dn, NULL,pdev_parent ? &pdev_parent->dev : NULL);of_dev_put(pdev_parent);if (pdev == NULL) {pr_err("%s: failed to create for '%s'\\n",__func__, rd->dn->full_name);/* of_platform_device_create tosses the error code */return notifier_from_errno(-EINVAL);}break;case OF_RECONFIG_CHANGE_REMOVE:/* already depopulated? */if (!of_node_check_flag(rd->dn, OF_POPULATED))return NOTIFY_OK;/* find our device by node */pdev = of_find_device_by_node(rd->dn);if (pdev == NULL)return NOTIFY_OK;    /* no? not meant for us *//* unregister takes one ref away */of_platform_device_destroy(&pdev->dev, &children_left);/* and put the reference of the find */of_dev_put(pdev);break;}return NOTIFY_OK;
}static struct notifier_block platform_of_notifier = {.notifier_call = of_platform_notify,
};void of_platform_register_reconfig_notifier(void)
{WARN_ON(of_reconfig_notifier_register(&platform_of_notifier));
}
#endif /* CONFIG_OF_DYNAMIC */#endif /* CONFIG_OF_ADDRESS */

Linux内核是如何将device_node转换成platform_device

of_platform_populate函数

该函数主要调用了of_platform_populate函数
drivers/of/platform.c
of_platform_populate函数是Linux内核中的一个函数，主要作用是将设备树中的device node创建成platform device，为后续和各类驱动的platform driver匹配做准备。在Linux内核启动时，内核通过of_platform_populate函数，将dts中的device node创建成platform device。
of_platform_populate函数定义如下所示：

int of_platform_populate(struct device_node *root,const struct of_device_id *matches,const struct of_dev_auxdata *lookup,struct device *parent)

其中，root是第一级要探测的父节点或NULL表示树的根；matches是匹配表，NULL表示使用默认值；lookup是用于匹配id和platform_data与设备节点的auxdata表；parent是要挂载设备的父节点，NULL表示顶层。详细内容如下：

int of_platform_populate(struct device_node *root,const struct of_device_id *matches,const struct of_dev_auxdata *lookup,struct device *parent)
{struct device_node *child;int rc = 0;root = root ? of_node_get(root) : of_find_node_by_path("/");if (!root)return -EINVAL;for_each_child_of_node(root, child) {rc = of_platform_bus_create(child, matches, lookup, parent, true);if (rc)break;}of_node_set_flag(root, OF_POPULATED_BUS);of_node_put(root);return rc;
}of_platform_populate
of_platform_bus_create
static int of_platform_bus_create(struct device_node *bus,const struct of_device_id *matches,const struct of_dev_auxdata *lookup,struct device *parent, bool strict)
{const struct of_dev_auxdata *auxdata;struct device_node *child;struct platform_device *dev;const char *bus_id = NULL;void *platform_data = NULL;int rc = 0;/* Make sure it has a compatible property */if (strict && (!of_get_property(bus, "compatible", NULL))) {pr_debug("%s() - skipping %s, no compatible prop\\n",__func__, bus->full_name);return 0;}auxdata = of_dev_lookup(lookup, bus);if (auxdata) {bus_id = auxdata->name;platform_data = auxdata->platform_data;}if (of_device_is_compatible(bus, "arm,primecell")) {/ Don't return an error here to keep compatibility with older* device tree files.*/of_amba_device_create(bus, bus_id, platform_data, parent);return 0;}dev = of_platform_device_create_pdata(bus, bus_id, platform_data, parent);if (!dev || !of_match_node(matches, bus))return 0;for_each_child_of_node(bus, child) {pr_debug("   create child: %s\\n", child->full_name);rc = of_platform_bus_create(child, matches, lookup, &dev->dev, strict);if (rc) {of_node_put(child);break;}}of_node_set_flag(bus, OF_POPULATED_BUS);return rc;
}

转换规则1

if (strict && (!of_get_property(bus, “compatible”, NULL))) {
pr_debug("%s() - skipping %s, no compatible prop\\n",
func, bus->full_name);
return 0;
}

转换规则3

  if (of_device_is_compatible(bus, "arm,primecell")) {/ Don't return an error here to keep compatibility with older* device tree files.*/of_amba_device_create(bus, bus_id, platform_data, parent);return 0;}

处理根节点下的子节点（匹配规则2）

dev = of_platform_device_create_pdata(bus, bus_id, platform_data, parent);
if (!dev || !of_match_node(matches, bus))
return 0;

for_each_child_of_node(bus, child) {
pr_debug(" create child: %s\\n", child->full_name);
rc = of_platform_bus_create(child, matches, lookup, &dev->dev, strict);
if (rc) {
of_node_put(child);
break;
}
}
of_node_set_flag(bus, OF_POPULATED_BUS);

static int of_platform_bus_create(struct device_node *bus,const struct of_device_id *matches,const struct of_dev_auxdata *lookup,struct device *parent, bool strict)
{const struct of_dev_auxdata *auxdata;struct device_node *child;struct platform_device *dev;const char *bus_id = NULL;void *platform_data = NULL;int rc = 0;/* Make sure it has a compatible property */if (strict && (!of_get_property(bus, "compatible", NULL))) {pr_debug("%s() - skipping %s, no compatible prop\\n",__func__, bus->full_name);return 0;}auxdata = of_dev_lookup(lookup, bus);if (auxdata) {bus_id = auxdata->name;platform_data = auxdata->platform_data;}if (of_device_is_compatible(bus, "arm,primecell")) {/ Don't return an error here to keep compatibility with older* device tree files.*/of_amba_device_create(bus, bus_id, platform_data, parent);return 0;}dev = of_platform_device_create_pdata(bus, bus_id, platform_data, parent);if (!dev || !of_match_node(matches, bus))return 0;for_each_child_of_node(bus, child) {pr_debug("   create child: %s\\n", child->full_name);rc = of_platform_bus_create(child, matches, lookup, &dev->dev, strict);if (rc) {of_node_put(child);break;}}of_node_set_flag(bus, OF_POPULATED_BUS);return rc;
}

设备树转换成paltform_device以后就和平台总线模型一样啦。所以驱动和设备也会根据名字来匹配。匹配成功以后会执行驱动中的probe函数。
匹配优先级：name<id_table<of_match_table