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

[Koen Kooi]

Op 7 nov. 2012, om 23:35 heeft Ryan Mallon <rmallon at gmail.com> het volgende geschreven:

> 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.

And as Pantelis mentioned before, I really don't want my users to change the bootloader whenever they add a new LED. Touching the bootloader is just too accident prone, we had a ton of RMA requests for older versions of the beagleboard from people trying to upgrade u-boot.

Apart from the above I'd like to have fewer points of failure. Right now I need to keep uImage and foo.dtb in sync and I hate to add u-boot to that equasion as well.

regards,

Koen