Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753601AbcDNKLS (ORCPT ); Thu, 14 Apr 2016 06:11:18 -0400 Received: from foss.arm.com ([217.140.101.70]:41151 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753003AbcDNKLP (ORCPT ); Thu, 14 Apr 2016 06:11:15 -0400 Date: Thu, 14 Apr 2016 11:10:54 +0100 From: Mark Rutland To: Masahiro Yamada Cc: devicetree@vger.kernel.org, Linux Kernel Mailing List , linux-arm-kernel , Rob Herring , Arnd Bergmann , Frank Rowand , pantelis.antoniou@konsulko.com Subject: Re: [Question] refcount of DT node Message-ID: <20160414101053.GC10273@leverpostej> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2169 Lines: 73 On Thu, Apr 14, 2016 at 04:47:57PM +0900, Masahiro Yamada wrote: > Hi experts. > > My understanding of refcount of DT node is poor. > Please help me understand it correctly. > > Sorry if I am asking stupid questions. > > > [1] Does this reference count exist for Overlay? > Is a node freed when its refcount becomes zero? I'm not familiar with the way that overlays are intended to work, but generally this is true, and I believe the same applies. Pantelis, please correct me if I am wrong on that front. > [2] When of_node_put() should be called, > or should not be called? > > > Shouldn't of_node_put() be called > when we are still referencing to any of its properties? > > For example, cpu_read_enable_method() > in arch/arm64/kernel/cpu_ops.c > returns a pointer to the property value > instead of creating a copy of it. > > In this case, of_node_put() should not be called > because we are still referencing the DT property > (in other words, referencing to the DT node indirectly). > > Am I right? Yes, the node should not be freed while its data is referred to. We are leaking a ref there, though, as we no longer refer to that data after cpu_read_ops(). Fixing that will require some restructuring. We don't expect a CPU node to need to disappear, so while it's currently not strictly correct the code shouldn't lead to any adverse behaviour. > [3] Is the following code correct? > > np = of_find_compatible_node(NULL, NULL,"foo-node"); > of_node_put(np); > ret = of_address_to_resource(np, 0, &res); > if (ret) { > pr_err("failed to get resource\n"); > return ret; > } > > Actually I wrote the code above, and it was applied. > > But, the node is still referenced while of_address_to_resource() is being run. > > So the correct code should be as follows? > > np = of_find_compatible_node(NULL, NULL,"foo-node"); > ret = of_address_to_resource(np, 0, &res); > of_node_put(np); > if (ret) { > pr_err("failed to get resource\n"); > return ret; > } It is correctly balanced, yes. If you don't need to keep the node for future use, this is fine. Mark.