[RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices to mach-omap2)

[Grant Likely]

On Mon, Nov 5, 2012 at 11:22 PM, Tony Lindgren <tony at atomide.com> wrote:
> Hi,
>
> * Tabi Timur-B04825 <B04825 at freescale.com> [121105 13:42]:
>> On Mon, Nov 5, 2012 at 2:40 PM, Grant Likely <grant.likely at secretlab.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.