Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752982Ab2KJDg3 (ORCPT ); Fri, 9 Nov 2012 22:36:29 -0500 Received: from mail-qc0-f174.google.com ([209.85.216.174]:61684 "EHLO mail-qc0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751005Ab2KJDg1 (ORCPT ); Fri, 9 Nov 2012 22:36:27 -0500 MIME-Version: 1.0 In-Reply-To: <559B8433-67C3-4A1A-A5D6-859907655176@antoniou-consulting.com> References: <02FF5400-9F97-4B8A-AEF0-267B01C8099F@antoniou-consulting.com> <-4237940489086529028@unknownmsgid> <559B8433-67C3-4A1A-A5D6-859907655176@antoniou-consulting.com> Date: Fri, 9 Nov 2012 21:36:26 -0600 Message-ID: Subject: Re: [RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices to mach-omap2) From: Joel A Fernandes To: Pantelis Antoniou Cc: Grant Likely , Rob Herring , Deepak Saxena , Benjamin Herrenschmidt , Scott Wood , Tony Lindgren , Russ Dill , Felipe Balbi , Benoit Cousson , linux-kernel , Koen Kooi , Matt Porter , linux-omap@vger.kernel.org, Kevin Hilman , Paul Walmsley , devicetree-discuss@lists.ozlabs.org Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3219 Lines: 68 Hi Pantelis, On Fri, Nov 9, 2012 at 2:13 AM, Pantelis Antoniou wrote: >>>> Option C: U-Boot loads both the base and overlay FDT files, merges them, >>>> and passes the resolved tree to the kernel. >>>> >>> >>> Could be made to work. Only really required if Joanne wants the >>> cape interface to work for u-boot too. For example if the cape has some >>> kind of network interface that u-boot will use to boot from. >>> >> >> I love Grant's hashing idea a lot keeping the phandle problem for >> compile time and not requiring fixups. >> >> IMO it is still a cleaner approach if u-boot does the tree merging for >> all cases, and not the kernel. >> >> That way from a development standpoint, very little or nothing will >> have to be changed in kernel (except for scripts/dtc) considering we >> are moving forward with hashing. >> >> Also this discussed a while back but at some point is going to brought >> up again- loading of dt fragment directly from EEPROM and merging at >> run time. If we were to implement this in kernel, we would have to add >> cape specific EEPROM reading code, merge the tree before it is >> unflattened and parse. I think doing tree merging in kernel is messy >> and we should do it in uboot. Ideally reading the fragment from the >> EEPROM for all capes and merging without worrying about version >> detection, Doing the merge and passing the merged blob to the kernel >> which (kernel) works the same way it does today. > > Not going to work, for a lot of cases. Doing it in the kernel seems to be > the cleaner option. There are valid use cases for doing in u-boot too. True, if dynamic runtime stuff from userspace is what we're talking about, then yeah I see the important need for kernel to do the merge. >> Alternatively to hashing, reading david gibsons paper I followed, >> phandle is supposed to 'uniquely' identity node. I wonder why the node >> name itself is not sufficient to unquiely identify. The code that does >> the tree walking can then just strcmp the node name while it walks the >> tree instead of having to find a node with a phandle number. I guess >> the reason is phandles are small to store as data values. Another >> approach can be to arrange the string block in alphabetical order >> (unless it already is), and store phandle as index of the node name >> referenced relative to the starting of the strong block. This will not >> affect nodes in dtb being moved around since they will still have the >> same index value. the problem being adding or removing nodes Changes >> the offsets of all other nodes in the string block as well.. Hmm. >> > > This is pretty radical change to the DT format, no? Yes, true and the only way hypothetically to replace the phandle tree-walking mechanism is to store node paths instead of phandle which David pointed is too long to store, so I guess this wont work after all. Anyway this was an interesting exercise, thanks. Regards, Joel -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/