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

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Message-ID: <CACxGe6s1ksem-6=fxVMeCoGw=vLN8n_j1CmfNJMdPS0e3fUxWw@mail.gmail.com>
Date:	Fri, 9 Nov 2012 12:06:02 +0000
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Tony Lindgren <tony@...mide.com>
Cc:	Tabi Timur-B04825 <B04825@...escale.com>,
	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>,
	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 Mon, Nov 5, 2012 at 11:22 PM, Tony Lindgren <tony@...mide.com> wrote:
> Hi,
>
> * Tabi Timur-B04825 <B04825@...escale.com> [121105 13:42]:
>> 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.
>>
>> For configurations that can be determined by the boot loader, I'm not
>> sure overlays are practical.
>
> I guess the beaglebone capes could be stackable and hotpluggable if
> handled carefully enough.

And even if it can't on the beaglebone, it will happen somewhere else.
I don't want to exclude that use-case just because nobody thought
about it.

g.
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