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

[Ryan Mallon]

On 06/11/12 08:40, Tabi Timur-B04825 wrote:
> 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.

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