来看一个cortina Gemini ethernet driver的例子。

DTS file路径 /arch/arm/boot/dts/gemini/gemini.dtsi.

Driver 路径 drivers/net/ethernet/cortina/gemini.c

 {soc {#address-cells = <1>;#size-cells = <1>;ranges;compatible = "simple-bus";....ethernet: ethernet@60000000 {compatible = "cortina,gemini-ethernet";reg = <0x60000000 0x4000>, /* Global registers, queue */<0x60004000 0x2000>, /* V-bit */<0x60006000 0x2000>; /* A-bit */pinctrl-names = "default";pinctrl-0 = <&gmii_default_pins>;status = "disabled";#address-cells = <1>;#size-cells = <1>;ranges;gmac0: ethernet-port@0 {compatible = "cortina,gemini-ethernet-port";reg = <0x60008000 0x2000>, /* Port 0 DMA/TOE */<0x6000a000 0x2000>; /* Port 0 GMAC */interrupt-parent = <&intcon>;interrupts = <1 IRQ_TYPE_LEVEL_HIGH>;resets = <&syscon GEMINI_RESET_GMAC0>;clocks = <&syscon GEMINI_CLK_GATE_GMAC0>;clock-names = "PCLK";};gmac1: ethernet-port@1 {compatible = "cortina,gemini-ethernet-port";reg = <0x6000c000 0x2000>, /* Port 1 DMA/TOE */<0x6000e000 0x2000>; /* Port 1 GMAC */interrupt-parent = <&intcon>;interrupts = <2 IRQ_TYPE_LEVEL_HIGH>;resets = <&syscon GEMINI_RESET_GMAC1>;clocks = <&syscon GEMINI_CLK_GATE_GMAC1>;clock-names = "PCLK";};};};};

有之前的博文分析， 根节点下的含有compatible属性的节点在内核启动时会为其创建platform device, 因此 soc节点会有一个platform device与其对应。而soc节点的compatible为已知的特殊属性“simple-bus”， 因此也会为其下面的子节点创建platform device， 故 “ethernet”也会有platform device 与其对应。那么“ethernet”厂商驱动只需写自己的平台设备驱动即可。对应以太网驱动，“ethernet”下的子节点 “gmac0” 通常是在ethernet 驱动下解析子节点然后为其创建netdevice。

但是我们看gemini driver， 其对于每个ethernet port 即“gmac”子节点也单独设计的platform driver. 那么ethernet port子节点是如何创建成平台devices的呢？

我们看其处理, 在“ethernet” driver probe函数的结尾使用 devm_of_platform_populate 为其子节点创建platfrom device.

static int gemini_ethernet_port_probe(struct platform_device *pdev)
{struct gemini_ethernet_port *port;struct gemini_ethernet *geth;parent = dev->parent; //获取其parent “ethernet”devicegeth = dev_get_drvdata(parent);  // 获取 “etherent” private data.....port = netdev_priv(netdev);SET_NETDEV_DEV(netdev, dev);port->netdev = netdev;port->id = id;port->geth = geth; // ethernet port private与 ethernet private建立关系
}static int gemini_ethernet_probe(struct platform_device *pdev)
{..../* Spawn child devices for the two ports */return devm_of_platform_populate(dev); //为 ethernet port 子节点创建平台devices}static struct platform_driver gemini_ethernet_driver = {.driver = {.name = DRV_NAME,.of_match_table = gemini_ethernet_of_match,},.probe = gemini_ethernet_probe,.remove_new = gemini_ethernet_remove,
};static int __init gemini_ethernet_module_init(void)
{int ret;ret = platform_driver_register(&gemini_ethernet_port_driver);if (ret)return ret;ret = platform_driver_register(&gemini_ethernet_driver);if (ret) {platform_driver_unregister(&gemini_ethernet_port_driver);return ret;}return 0;
}

 思考： 如果dts设计成如下, 如何将ethernet-port 节点创建成platform devices呢？

             ethernet: ethernet@60000000 {compatible = "cortina,gemini-ethernet";......ethernet-ports {#address-cells = <1>#size-cells = <0>;gmac0: ethernet-port@0 {compatible = "cortina,gemini-ethernet-port";reg = <0x60008000 0x2000>, /* Port 0 DMA/TOE */<0x6000a000 0x2000>; /* Port 0 GMAC */...};gmac1: ethernet-port@1 {compatible = "cortina,gemini-ethernet-port";reg = <0x6000c000 0x2000>, /* Port 1 DMA/TOE */<0x6000e000 0x2000>; /* Port 1 GMAC */...};};};

方法一：为“ethernet-ports”创建平台device

const struct of_device_id of_ethernet_ports_match_table[] = {

        {.compatible = "cortina,gemini-ethernet-ports", },

        {}

};
of_platform_populate(pdev->dev.of_node, of_ethernet_ports_match_table, NULL, &pdev->dev);

在 ethernet probe 函数结尾使用上述， 同时DTS中为“ethernet-ports” 节点添加compatible 属性“cortina,gemini-ethernet-ports”

这样 of_platform_populate(pdev->dev.of_node, of_ethernet_ports_match_table, NULL, &pdev->dev)函数会为其子节点“ethernet-ports” 创建platform device. 同时为“ethernet-ports”的子节点“ethernet-port”创建platform device.

# cat /sys/devices/platform/soc/60000000.ethernet/60000000.ethernet:ethernet-ports/
60000000.ethernet:ethernet-ports:ethernet-port@0/
60000000.ethernet:ethernet-ports:ethernet-port@1/

方法二： 绕过 “ethernet-ports” 为 “etherne-port”创建平台device

ethernet_ports_node = of_get_child_by_name(ppe->dev->of_node, "ethernet-ports");
of_platform_populate(ethernet_ports_node, NULL, NULL, &pdev->dev ); 

在ethernet driver probe结尾使用上述，即 ethernet driver中parse “ethernet-ports”自己点然后调用populate函数为 “ethernet-ports”自己点创建平台设备。注意这里parent device参数仍然使用ethernet devie, 因为“ethernet-ports”并没有创建任何device, “ethernet-port” device hook在 “ethernet” device上。

# cat /sys/devices/platform/soc/60000000.ethernet/
60000000.ethernet:ethernet-ports:ethernet-port@0/
60000000.ethernet:ethernet-ports:ethernet-port@1/ 

 再回顾popuate相关函数的实现， 可以看出最终都归结到of_platform_populate函数，第一个参数root为device node类型， 表示要为改device node下的node穿件平台device, 如果为NULL表示根节点（kernel 启动时就是指定的NULL）。第二个参数 matches 可以指定compatible属性，表示 root的子节点如果有子节点match该属性列表就位子节点的子节点继续创建平台device，为NULL则只会创建一级device。最后一个参数struct device *parent表示 root下第一级子节点device的parent device.

/*** of_platform_populate() - Populate platform_devices from device tree data* @root: parent of the first level to probe or NULL for the root of the tree* @matches: match table, NULL to use the default* @lookup: auxdata table for matching id and platform_data with device nodes* @parent: parent to hook devices from, NULL for toplevel** Similar to of_platform_bus_probe(), this function walks the device tree* and creates devices from nodes.  It differs in that it follows the modern* convention of requiring all device nodes to have a 'compatible' property,* and it is suitable for creating devices which are children of the root* node (of_platform_bus_probe will only create children of the root which* are selected by the @matches argument).** New board support should be using this function instead of* of_platform_bus_probe().** Return: 0 on success, < 0 on failure.*/
int of_platform_populate(struct device_node *root,const struct of_device_id *matches,const struct of_dev_auxdata *lookup,struct device *parent)
{......
}const struct of_device_id of_default_bus_match_table[] = {{ .compatible = "simple-bus", },{ .compatible = "simple-mfd", },{ .compatible = "isa", },
#ifdef CONFIG_ARM_AMBA{ .compatible = "arm,amba-bus", },
#endif /* CONFIG_ARM_AMBA */{} /* Empty terminated list */
};int of_platform_default_populate(struct device_node *root,const struct of_dev_auxdata *lookup,struct device *parent)
{return of_platform_populate(root, of_default_bus_match_table, lookup,parent);
}static int __init of_platform_default_populate_init(void)
{/* Populate everything else. */of_platform_default_populate(NULL, NULL, NULL);
}int devm_of_platform_populate(struct device *dev)
{struct device **ptr;int ret;if (!dev)return -EINVAL;ptr = devres_alloc(devm_of_platform_populate_release,sizeof(*ptr), GFP_KERNEL);if (!ptr)return -ENOMEM;ret = of_platform_populate(dev->of_node, NULL, NULL, dev);if (ret) {devres_free(ptr);} else {*ptr = dev;devres_add(dev, ptr);}return ret;
}