linux-kernel - Re: [RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap

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Message-ID: <509AE247.3000404@gmail.com>
Date:	Thu, 08 Nov 2012 09:35:51 +1100
From:	Ryan Mallon <rmallon@...il.com>
To:	Tabi Timur-B04825 <B04825@...escale.com>
CC:	Grant Likely <grant.likely@...retlab.ca>,
	Pantelis Antoniou <panto@...oniou-consulting.com>,
	Rob Herring <robherring2@...il.com>,
	Deepak Saxena <dsaxena@...aro.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Wood Scott-B07421 <B07421@...escale.com>,
	Tony Lindgren <tony@...mide.com>,
	Kevin Hilman <khilman@...com>, Matt Porter <mporter@...com>,
	Koen Kooi <koen@...inion.thruhere.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Felipe Balbi <balbi@...com>, Russ Dill <Russ.Dill@...com>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>
Subject: Re: [RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices
 to mach-omap2)

On 06/11/12 08:40, Tabi Timur-B04825 wrote:
> On Mon, Nov 5, 2012 at 2:40 PM, Grant Likely <grant.likely@...retlab.ca> 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.

This is probably okay for some hardware, but doesn't work in the general
case. Not all hardware is detectable, for example a cape which just adds
a set of LEDs for GPIO pins. Also, some hardware might not easily be
detectable without adding additional complexity to the boot loader.

~Ryan

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