Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752681Ab2KFAHR (ORCPT ); Mon, 5 Nov 2012 19:07:17 -0500 Received: from mail-we0-f174.google.com ([74.125.82.174]:49647 "EHLO mail-we0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751527Ab2KFAHO (ORCPT ); Mon, 5 Nov 2012 19:07:14 -0500 User-Agent: K-9 Mail for Android In-Reply-To: <6AE080B68D46FC4BA2D2769E68D765B708174B7D@039-SN2MPN1-023.039d.mgd.msft.net> References: <6AE080B68D46FC4BA2D2769E68D765B708174B7D@039-SN2MPN1-023.039d.mgd.msft.net> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Subject: Re: [RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices to mach-omap2) From: Grant Likely Date: Tue, 06 Nov 2012 00:07:03 +0000 To: Tabi Timur-B04825 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" Message-ID: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2815 Lines: 69 Tabi Timur-B04825 wrote: >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. >From the discussion in the previous thread, I'm sufficiently convinced that it is an important use case. I certainly disagree with the assertion that it is always easier to do it in U-Boot. Sometimes the kernel is the better place. > >> 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? Not if the driver is only responsible for loading the bitstream. There is already a xilinx driver that does things this way. > Why not skip the >device tree update and just tell the driver what the new capabilities >are? How? What format will that data be in if not device tree? g. -- Sent from my Android phone with K-9 Mail. Please excuse my brevity. -- 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/