RFC: Should we have a device_for_each_available_child_node()?

[Jonathan Cameron <jic23@kernel.org>]

Hi All,

This came up in review of
https://lore.kernel.org/linux-iio/20210725172458.487343-1-jic23@kernel.org/
which is a series converting a dt only driver over to generic properties.
I'm sending a separate email to raise the profile of the question rather
higher than it was buried in a driver review.

The original code used for_each_available_child_of_node(np, child) 
and the patch converted it to device_for_each_child_node().

Andy raised the question of whether it should have been
device_for_each_available_child_node() but that doesn't exist currently.

Things get more interesting when you look at the implementation of
device_for_each_child_node() which uses device_get_next_child_node()
which in turn calls fwnode_get_next_child_node() which calls
the get_next_child_node() op and for of that is
of_fwnode_get_next_child_node() which uses of_get_next_available_child()
rather than of_get_next_child().

So I think under the hood device_for_each_child_node() on of_ is going to
end up checking the node is available anyway.

So this all seemed a little odd given there were obvious calls to use
if we wanted to separate the two cases for device tree and they weren't
the ones used.  However, if we conclude that there is a bug here and
the two cases should be handled separately then it will be really hard
to be sure no driver is relying on this behaviour.

So, ultimately the question is:  Should I add a
device_for_each_available_child_node()?  It will be something like:

struct fwnode_handle *device_get_next_child_node(struct device *dev,
						 struct fwnode_handle *child)
{
	const struct fwnode_handle *fwnode = dev_fwnode(dev);
	struct fwnode_handle *next;

	/* Try to find a child in primary fwnode */
	next = fwnode_get_next_available_child_node(fwnode, child);
	if (next)
		return next;

	/* When no more children in primary, continue with secondary */
	if (fwnode && !IS_ERR_OR_NULL(fwnode->secondary))
		next = fwnode_get_next_available_child_node(fwnode->secondary, child);

	return next;
}

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

As far as I can tell it doesn't make any difference for my particular bit
of refactoring in the sense of I won't break anything that currently
works by using device_for_each_child_node() but it may cause issues with
other firmware by enumerating disabled child nodes.

Jonathan