device_for_each_child_node 函数和 device_get_child_node_count 函数说明

Linux version: 4.14

Code link: Linux source code (v4.14) - Bootlin

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1 函数 device_for_each_child_node 

（1）对于函数 device_for_each_child_node 

#define device_for_each_child_node(dev, child)				\
	for (child = device_get_next_child_node(dev, NULL); child;	\
	     child = device_get_next_child_node(dev, child))

（2）其中调用了函数 device_get_next_child_node

struct fwnode_handle *device_get_next_child_node(struct device *dev,
						 struct fwnode_handle *child)
{
	struct acpi_device *adev = ACPI_COMPANION(dev);
	struct fwnode_handle *fwnode = NULL;

	if (dev->of_node)
		fwnode = &dev->of_node->fwnode;
	else if (adev)
		fwnode = acpi_fwnode_handle(adev);

	return fwnode_get_next_child_node(fwnode, child);
}
EXPORT_SYMBOL_GPL(device_get_next_child_node);

（3）在 device_get_next_child_node 中调用了 fwnode_get_next_child_node

struct fwnode_handle *
fwnode_get_next_child_node(const struct fwnode_handle *fwnode,
			   struct fwnode_handle *child)
{
	return fwnode_call_ptr_op(fwnode, get_next_child_node, child);
}
EXPORT_SYMBOL_GPL(fwnode_get_next_child_node);

（4）fwnode_get_next_child_node 函数中 fwnode_call_ptr_op 调用了 fwnode 中 fwnode_operations 的 get_next_child_node ，并把参数传进 get_next_child_node。 

#define fwnode_has_op(fwnode, op)				\
	((fwnode) && (fwnode)->ops && (fwnode)->ops->op)
 
#define fwnode_call_ptr_op(fwnode, op, ...)		\
	(fwnode_has_op(fwnode, op) ?			\
	 (fwnode)->ops->op(fwnode, ## __VA_ARGS__) : NULL)

另外，通过 of_fwnode_ops 结构体可以看出 get_next_child_node 对应的是 of_fwnode_get_next_child_node

const struct fwnode_operations of_fwnode_ops = {
	.get = of_fwnode_get,
	.put = of_fwnode_put,
	.device_is_available = of_fwnode_device_is_available,
	.property_present = of_fwnode_property_present,
	.property_read_int_array = of_fwnode_property_read_int_array,
	.property_read_string_array = of_fwnode_property_read_string_array,
	.get_parent = of_fwnode_get_parent,
	.get_next_child_node = of_fwnode_get_next_child_node,
	.get_named_child_node = of_fwnode_get_named_child_node,
	.get_reference_args = of_fwnode_get_reference_args,
	.graph_get_next_endpoint = of_fwnode_graph_get_next_endpoint,
	.graph_get_remote_endpoint = of_fwnode_graph_get_remote_endpoint,
	.graph_get_port_parent = of_fwnode_graph_get_port_parent,
	.graph_parse_endpoint = of_fwnode_graph_parse_endpoint,
};
EXPORT_SYMBOL_GPL(of_fwnode_ops);

（5）of_fwnode_get_next_child_node 函数

static struct fwnode_handle *
of_fwnode_get_next_child_node(const struct fwnode_handle *fwnode,
			      struct fwnode_handle *child)
{
	return of_fwnode_handle(of_get_next_available_child(to_of_node(fwnode),
							    to_of_node(child)));
}

 对于 of_get_next_available_child 函数

struct device_node *of_get_next_available_child(const struct device_node *node,
	struct device_node *prev)
{
	struct device_node *next;
	unsigned long flags;

	if (!node)
		return NULL;

	raw_spin_lock_irqsave(&devtree_lock, flags);
	next = prev ? prev->sibling : node->child;
	for (; next; next = next->sibling) {
		if (!__of_device_is_available(next))
			continue;
		if (of_node_get(next))
			break;
	}
	of_node_put(prev);
	raw_spin_unlock_irqrestore(&devtree_lock, flags);
	return next;
}
EXPORT_SYMBOL(of_get_next_available_child);

如果传入的prev为NULL则返回node节点的第一个子节点，否则返回prev节点的下一个兄弟节点。注意这里返回的类型为device_node，而of_fwnode_handle函数将device_node对应的节点转换为fwnode_handle类型返回。

#define of_fwnode_handle(node)						\
	({								\
		typeof(node) __of_fwnode_handle_node = (node);		\
									\
		__of_fwnode_handle_node ?				\
			&__of_fwnode_handle_node->fwnode : NULL;	\
	})

因此，device_for_each_child_node(dev, child); 函数的功能是遍历 dev 设备对应的 fwnode_handle 节点的每一个子节点并通过 child 变量返回。至此，函数解析完毕。 

2 函数 device_get_child_node_count 

unsigned int device_get_child_node_count(struct device *dev)
{
	struct fwnode_handle *child;
	unsigned int count = 0;

	device_for_each_child_node(dev, child)
		count++;

	return count;
}
EXPORT_SYMBOL_GPL(device_get_child_node_count);

从函数定义可以看出，device_get_child_node_count 的功能是统计 dev 设备子节点的个数。