内核版本：Linux-3.18.2

在3.x版本内核中platform_device不再静态定义，而是通过device tree来动态生成，例如（arch/arm/mach-s3c24xx/mach-sc2416-dt.c）：

static void __init s3c2416_dt_machine_init(void)
{
	of_platform_populate(NULL, of_default_bus_match_table, NULL, NULL);
	s3c_pm_init();
}

在s3c2416_dt_machine_init函数中调用of_platform_populate函数，而of_platform_populate函数是of子系统提供的函数，在drivers/of/platform.c中：

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_put(root);
	return rc;
}

该函数前面的的注释已经说的很明白了，of_platform_populate函数根据device tree生成platform_device。该函数同of_platform_bus_probe函数有点类似，在新的board中最好使用of_platform_populate来代替of_platform_bus_probe函数。

如果传递进来的参数root为NULL，那么需要通过of_find_node_by_path函数找到device tree中的根节点。

得到根节点之后呢，就需要通过这个根节点来遍历device tree中的节点了。得到一个子节点之后，调用of_platform_bus_create函数：

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;
}

在of_platform_bus_create函数中，首先是需要确定节点是否有"compatible"属性，如果没有"compatible"属性，则直接返回，即不会创建platform设备的。

所幸在mach-sc2416-dt.c中传递进来的lookup参数为NULL，所以of_dev_lookup这部分也就不去看了。

如果"compatible"属性值有"arm,primecell"，则会调用of_amba_device_create函数去创建amba_device，这个设备暂时也不知道是一个什么设备，那么这里还是先忽略。

继续，调用of_platform_device_create_pdata函数：

static struct platform_device *of_platform_device_create_pdata(
					struct device_node *np,
					const char *bus_id,
					void *platform_data,
					struct device *parent)
{
	struct platform_device *dev;

	if (!of_device_is_available(np) ||
	    of_node_test_and_set_flag(np, OF_POPULATED))
		return NULL;

	dev = of_device_alloc(np, bus_id, parent);
	if (!dev)
		goto err_clear_flag;

	of_dma_configure(&dev->dev);
	dev->dev.bus = &platform_bus_type;
	dev->dev.platform_data = platform_data;

	/* We do not fill the DMA ops for platform devices by default.
	 * This is currently the responsibility of the platform code
	 * to do such, possibly using a device notifier
	 */

	if (of_device_add(dev) != 0) {
		platform_device_put(dev);
		goto err_clear_flag;
	}

	return dev;

err_clear_flag:
	of_node_clear_flag(np, OF_POPULATED);
	return NULL;
}

可以看到，这个函数才是真正创建platform_device的。

首先调用of_device_is_available函数，这个函数主要是用于检测"status"属性的，如果没有"status"属性，那还好说直接返回true。如果有"status"属性，而它的值又不是"okay"或"ok"，那么不好意思，返回false，否则还是返回true。所以"status"属性就是用来检测是否可用（有点拗口，其实就是用来确认是否需要创建platform设备）。

"status"属性检测完毕了，则要调用of_device_alloc函数来为platform_device分配内存了。

struct platform_device *of_device_alloc(struct device_node *np,
				  const char *bus_id,
				  struct device *parent)
{
	struct platform_device *dev;
	int rc, i, num_reg = 0, num_irq;
	struct resource *res, temp_res;

	dev = platform_device_alloc("", -1);
	if (!dev)
		return NULL;

	/* count the io and irq resources */
	while (of_address_to_resource(np, num_reg, &temp_res) == 0)
		num_reg++;
	num_irq = of_irq_count(np);

	/* Populate the resource table */
	if (num_irq || num_reg) {
		res = kzalloc(sizeof(*res) * (num_irq + num_reg), GFP_KERNEL);
		if (!res) {
			platform_device_put(dev);
			return NULL;
		}

		dev->num_resources = num_reg + num_irq;
		dev->resource = res;
		for (i = 0; i < num_reg; i++, res++) {
			rc = of_address_to_resource(np, i, res);
			WARN_ON(rc);
		}
		if (of_irq_to_resource_table(np, res, num_irq) != num_irq)
			pr_debug("not all legacy IRQ resources mapped for %s\n",
				 np->name);
	}

	dev->dev.of_node = of_node_get(np);
	dev->dev.parent = parent;

	if (bus_id)
		dev_set_name(&dev->dev, "%s", bus_id);
	else
		of_device_make_bus_id(&dev->dev);

	return dev;
}

调用platform中的platform_device_alloc函数来分配内存。

内存申请了之后，还会对platform_device做一些初始化，例如IO、中断资源等等。首先是调用of_address_to_resource和of_irq_count去计算io和中断资源的个数（有注释说明）。

int of_address_to_resource(struct device_node *dev, int index,
			   struct resource *r)
{
	const __be32	*addrp;
	u64		size;
	unsigned int	flags;
	const char	*name = NULL;

	addrp = of_get_address(dev, index, &size, &flags);
	if (addrp == NULL)
		return -EINVAL;

	/* Get optional "reg-names" property to add a name to a resource */
	of_property_read_string_index(dev, "reg-names",	index, &name);

	return __of_address_to_resource(dev, addrp, size, flags, name, r);
}

首先调用of_get_address获取地址信息。

const __be32 *of_get_address(struct device_node *dev, int index, u64 *size,
		    unsigned int *flags)
{
	const __be32 *prop;
	unsigned int psize;
	struct device_node *parent;
	struct of_bus *bus;
	int onesize, i, na, ns;

	/* Get parent & match bus type */
	parent = of_get_parent(dev);
	if (parent == NULL)
		return NULL;
	bus = of_match_bus(parent);
	bus->count_cells(dev, &na, &ns);
	of_node_put(parent);
	if (!OF_CHECK_ADDR_COUNT(na))
		return NULL;

	/* Get "reg" or "assigned-addresses" property */
	prop = of_get_property(dev, bus->addresses, &psize);
	if (prop == NULL)
		return NULL;
	psize /= 4;

	onesize = na + ns;
	for (i = 0; psize >= onesize; psize -= onesize, prop += onesize, i++)
		if (i == index) {
			if (size)
				*size = of_read_number(prop + na, ns);
			if (flags)
				*flags = bus->get_flags(prop);
			return prop;
		}
	return NULL;
}

首先是找到它的parent，然后根据parent去找bus。

static struct of_bus *of_match_bus(struct device_node *np)
{
	int i;

	for (i = 0; i < ARRAY_SIZE(of_busses); i++)
		if (!of_busses[i].match || of_busses[i].match(np))
			return &of_busses[i];
	BUG();
	return NULL;
}

of_busses定义如下：

static struct of_bus of_busses[] = {
#ifdef CONFIG_OF_ADDRESS_PCI
	/* PCI */
	{
		.name = "pci",
		.addresses = "assigned-addresses",
		.match = of_bus_pci_match,
		.count_cells = of_bus_pci_count_cells,
		.map = of_bus_pci_map,
		.translate = of_bus_pci_translate,
		.get_flags = of_bus_pci_get_flags,
	},
#endif /* CONFIG_OF_ADDRESS_PCI */
	/* ISA */
	{
		.name = "isa",
		.addresses = "reg",
		.match = of_bus_isa_match,
		.count_cells = of_bus_isa_count_cells,
		.map = of_bus_isa_map,
		.translate = of_bus_isa_translate,
		.get_flags = of_bus_isa_get_flags,
	},
	/* Default */
	{
		.name = "default",
		.addresses = "reg",
		.match = NULL,
		.count_cells = of_bus_default_count_cells,
		.map = of_bus_default_map,
		.translate = of_bus_default_translate,
		.get_flags = of_bus_default_get_flags,
	},
};

在of_match_bus函数中，如果前面的bus不匹配，则使用默认的"default" bus，注意它的addresses字段为"reg"。

回到of_get_address函数中，调用of_get_property函数去读取哪个属性呢，就是前面的addresses值的属性，即reg属性，所以reg属性就是用来定义io地址地址信息的。而io地址的长度是通过of_get_address中的of_read_number去读取完成的，最后返回这个io地址。

回到of_address_to_resource函数中，在得到这个io地址之后，调用__of_address_to_resource函数将io地址转换成struct resource资源信息。

然后是中断资源。

int of_irq_count(struct device_node *dev)
{
	struct of_phandle_args irq;
	int nr = 0;

	while (of_irq_parse_one(dev, nr, &irq) == 0)
		nr++;

	return nr;
}

int of_irq_parse_one(struct device_node *device, int index, struct of_phandle_args *out_irq)
{
	struct device_node *p;
	const __be32 *intspec, *tmp, *addr;
	u32 intsize, intlen;
	int i, res = -EINVAL;

	pr_debug("of_irq_parse_one: dev=%s, index=%d\n", of_node_full_name(device), index);

	/* OldWorld mac stuff is "special", handle out of line */
	if (of_irq_workarounds & OF_IMAP_OLDWORLD_MAC)
		return of_irq_parse_oldworld(device, index, out_irq);

	/* Get the reg property (if any) */
	addr = of_get_property(device, "reg", NULL);

	/* Try the new-style interrupts-extended first */
	res = of_parse_phandle_with_args(device, "interrupts-extended",
					"#interrupt-cells", index, out_irq);
	if (!res)
		return of_irq_parse_raw(addr, out_irq);

	/* Get the interrupts property */
	intspec = of_get_property(device, "interrupts", &intlen);
	if (intspec == NULL)
		return -EINVAL;

	intlen /= sizeof(*intspec);

	pr_debug(" intspec=%d intlen=%d\n", be32_to_cpup(intspec), intlen);

	/* Look for the interrupt parent. */
	p = of_irq_find_parent(device);
	if (p == NULL)
		return -EINVAL;

	/* Get size of interrupt specifier */
	tmp = of_get_property(p, "#interrupt-cells", NULL);
	if (tmp == NULL)
		goto out;
	intsize = be32_to_cpu(*tmp);

	pr_debug(" intsize=%d intlen=%d\n", intsize, intlen);

	/* Check index */
	if ((index + 1) * intsize > intlen)
		goto out;

	/* Copy intspec into irq structure */
	intspec += index * intsize;
	out_irq->np = p;
	out_irq->args_count = intsize;
	for (i = 0; i < intsize; i++)
		out_irq->args[i] = be32_to_cpup(intspec++);

	/* Check if there are any interrupt-map translations to process */
	res = of_irq_parse_raw(addr, out_irq);
 out:
	of_node_put(p);
	return res;
}

我们看在of_irq_parse_one函数中，是查找的"interrupts"属性值。

回到of_device_alloc函数，还是通过前面的of_address_to_resource函数将io地址资源赋值给平台设备，通过of_irq_to_resource_table函数将中断号转换成中断资源信息并赋值给平台设备。

int of_irq_to_resource_table(struct device_node *dev, struct resource *res,
		int nr_irqs)
{
	int i;

	for (i = 0; i < nr_irqs; i++, res++)
		if (!of_irq_to_resource(dev, i, res))
			break;

	return i;
}

int of_irq_to_resource(struct device_node *dev, int index, struct resource *r)
{
	int irq = irq_of_parse_and_map(dev, index);

	/* Only dereference the resource if both the
	 * resource and the irq are valid. */
	if (r && irq) {
		const char *name = NULL;

		memset(r, 0, sizeof(*r));
		/*
		 * Get optional "interrupt-names" property to add a name
		 * to the resource.
		 */
		of_property_read_string_index(dev, "interrupt-names", index,
					      &name);

		r->start = r->end = irq;
		r->flags = IORESOURCE_IRQ | irqd_get_trigger_type(irq_get_irq_data(irq));
		r->name = name ? name : of_node_full_name(dev);
	}

	return irq;
}

我们可以看出在of_device_alloc函数中除了为平台设备分配内存之外，还为平台设备找到了io地址资源和中断资源。

回到of_platform_device_create_pdata函数中，平台设备已经申请好了，然后对平台设备继续进行赋值操作，例如平台设备的总线赋值为平台总线，平台设备的私有数据赋值为platform_data，最后调用of_device_add函数将平台设备注册到内核中。

总结，涉及到的属性有：
"compatible"	必须
"status"	可选属性
"reg"		io资源
"interrupts"	中断资源