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

[Stephen Warren]

On 11/09/2012 09:28 AM, Grant Likely wrote:
> On Tue, Nov 6, 2012 at 10:37 PM, Stephen Warren <swarren at wwwdotorg.org> wrote:
...
>> I do rather suspect this use-case is quite common. NVIDIA certainly has
>> a bunch of development boards with pluggable
>> PMIC/audio/WiFi/display/..., and I believe there's some ability to
>> re-use the pluggable components with a variety of base-boards.
>>
>> Given people within NVIDIA started talking about this recently, I asked
>> them to enumerate all the boards we have that support pluggable
>> components, and how common it is that some boards support being plugged
>> into different main boards. I don't know when that enumeration will
>> complete (or even start) but hopefully I can provide some feedback on
>> how common the use-case is for us once it's done.
> 
> From your perspective, is it important to use the exact same .dtb
> overlays for those add-on boards, or is it okay to have a separate
> build of the overlay for each base tree?

I certainly think it'd be extremely beneficial to use the exact same
child board .dtb with arbitrary base boards.

Consider something like the Arduino shield connector format, which I
/believe/ has been re-used across a wide variety of Arduino boards and
other compatible or imitation boards. Now consider a vendor of an
Arduino shield. The shield vendor probably wants to publish a single
.dtb file that works for users irrespective of which board they're using
it with.

(Well, I'm not sure that Arduino can run Linux; perhaps that's why you
picked BeagleBone capes for your document!)

I suppose it would be acceptable for the shield vendor to ship the .dts
file rather than the .dtb, and hence need to build the shield .dtb for a
specific base board.

However, I think the process for an end-user needs to be as simple as
"drop this .dts/.dtb file into some standard directory", and I imagine
it'll be much easier for distros/... to make that process work if
they're dealing with a .dtb that they can just blast into the kernel's
firmware loader interface, rather than having to also locate the
base-board .dts/.dtb file, and run dtc and/or other tools on both .dts
files together.