Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933286Ab2KEVly (ORCPT ); Mon, 5 Nov 2012 16:41:54 -0500 Received: from [216.32.180.188] ([216.32.180.188]:48093 "EHLO co1outboundpool.messaging.microsoft.com" rhost-flags-FAIL-FAIL-OK-OK) by vger.kernel.org with ESMTP id S932162Ab2KEVlw convert rfc822-to-8bit (ORCPT ); Mon, 5 Nov 2012 16:41:52 -0500 X-Forefront-Antispam-Report: CIP:70.37.183.190;KIP:(null);UIP:(null);IPV:NLI;H:mail.freescale.net;RD:none;EFVD:NLI X-SpamScore: -1 X-BigFish: VS-1(zz98dI9371Izz1de0h1202h1d1ah1d2ahzzz2dh2a8h668h839h8e2h8e3hd25hf0ah1288h12a5h12a9h12bdh137ah13b6h1441h1504h1537h153bhbe9i1155h) From: Tabi Timur-B04825 To: Grant Likely CC: Pantelis Antoniou , Rob Herring , Deepak Saxena , Benjamin Herrenschmidt , Wood Scott-B07421 , Tony Lindgren , Kevin Hilman , Matt Porter , Koen Kooi , linux-kernel , Felipe Balbi , Russ Dill , "linux-omap@vger.kernel.org" , "devicetree-discuss@lists.ozlabs.org" Subject: Re: [RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices to mach-omap2) Thread-Topic: [RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices to mach-omap2) Thread-Index: AQHNu54kuqdNMyRX6kaXWyq1/nNWiQ== Date: Mon, 5 Nov 2012 21:40:16 +0000 Message-ID: <6AE080B68D46FC4BA2D2769E68D765B708174B7D@039-SN2MPN1-023.039d.mgd.msft.net> References: In-Reply-To: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [70.112.97.129] Content-Type: text/plain; charset=US-ASCII Content-ID: <6932259F391B4F4AB9A0C0DF5FBDE793@mgd.freescale.com> Content-Transfer-Encoding: 7BIT MIME-Version: 1.0 X-OriginatorOrg: freescale.com Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2271 Lines: 45 On Mon, Nov 5, 2012 at 2:40 PM, Grant Likely wrote: > Jane is building custom BeagleBone expansion boards called 'capes'. She > can boot the system with a stock BeagleBoard device tree, but additional > data is needed before a cape can be used. She could replace the FDT file > used by U-Boot with one that contains the extra data, but she uses the > same Linux system image regardless of the cape, and it is inconvenient > to have to select a different device tree at boot time depending on the > cape. What's wrong with having the boot loader detect the presence of the Cape and update the device tree accordingly? We do this all the time in U-Boot. Doing stuff like reading EEPROMs and testing for the presence of hardware is easier in U-Boot than in Linux. For configurations that can be determined by the boot loader, I'm not sure overlays are practical. > Nigel is building a real-time video processing system around a MIPS SoC > and a Virtex FPGA. Video data is streamed through the FPGA for post > processing operations like motion tracking or compression. The FPGA is > configured via the SPI bus, and is also connected to GPIO lines and the > memory mapped peripheral bus. Nigel has designed several FPGA > configurations for different video processing tasks. The user will > choose which configuration to load which can even be reprogrammed at > runtime to switch tasks. Now this, on the other hand, makes more sense. If the hardware configuration is literally user-configurable, then okay. However, I'm not sure I see the need to update the device tree. The device tree is generally for hardware that cannot be discovered/probed by the device driver. If we're loading a configuration from user space, doesn't the driver already know what the hardware's capabilities are, since it's the one doing the uploading of a new FPGA code? Why not skip the device tree update and just tell the driver what the new capabilities are? -- Timur Tabi Linux kernel developer at Freescale -- 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/