Re: LED subsystem child DT node ref counting

[Rob Herring <robherring2@gmail.com>]

On Fri, Apr 17, 2015 at 6:03 AM, Jacek Anaszewski
<j.anaszewski@samsung.com> wrote:
> Hi,
>
> I'd like to clarify whether LED subsystem drivers behave correctly
> or not, regarding child DT nodes reference counting.

In general, the DT reference counting is broken. It is really only
used on pSeries and only for certain nodes on it.

> Single LED controller can have connected more then one LED to it.
> The LEDs are represented by child DT nodes of the node representing
> the controller (see Documentation/devicetree/bindings/leds).
>
> LED subsystem drivers parse child DT nodes and use the node name,
> or 'label' property string as the LED class device name.
>
> This is usually accomplished like below:
>
> for_each_child_of_node(np, child) {
>     ...
>     led.name = of_get_property(child, "label", NULL) ? : child->name;
>
>
> The question is whether reference count of the child node shouldn't
> be increased here with of_node_get(child). Whereas intuitively it could
> be thought of as a right thing to do, empirical experiments don't
> necessary confirm that.
>
> When I print the value of child_node->kobj.kref.refcount.counter
> inside for_each loop it is 3 and and after leaving the loop it gets
> decreased to 2. On driver removal the value is also 2. It means that
> label is available all the time, without increasing child node ref
> counter.

I believe the ref count is only left incremented if you break from the
loop. Otherwise, it is only incremented during the loop one child at a
time.

Rob