From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rob Herring Subject: Re: LED subsystem child DT node ref counting Date: Mon, 27 Apr 2015 08:57:21 -0500 Message-ID: References: <5530E891.20707@samsung.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Return-path: In-Reply-To: <5530E891.20707@samsung.com> Sender: linux-leds-owner@vger.kernel.org To: Jacek Anaszewski Cc: "devicetree@vger.kernel.org" , Linux LED Subsystem , Sakari Ailus , Bryan Wu , Grant Likely , Rob Herring List-Id: devicetree@vger.kernel.org On Fri, Apr 17, 2015 at 6:03 AM, Jacek Anaszewski 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