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

[Mitch Bradley]

On 11/8/2012 3:28 AM, Koen Kooi wrote:
> 
> 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.

One possibility for dynamic device tree mods would be to run Open
Firmware from u-boot and have it generate the device tree and possibly
modify it either interactively or from a script loaded from a file or
the network.  OFW could then either load Linux directly or return to
u-boot, which would proceed with loading.

> 
> 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
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