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

[Grant Likely <grant.likely@secretlab.ca>]

On Fri, Nov 9, 2012 at 11:23 PM, Stephen Warren <swarren@wwwdotorg.org> wrote:
> On 11/09/2012 09:28 AM, Grant Likely wrote:
>> On Tue, Nov 6, 2012 at 10:37 PM, Stephen Warren <swarren@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!)

Correct, the Arduino is only an AVR with a tiny amount of ram. No Linux there.

However, Arduino shields are a good example of a use case. I think
there are even some Arduino shield compatible Linux boards out there.

> 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.

That would be better I think than relying on a binary. However, some
though needs to go into how to handle base boards that /aren't/ mostly
equivalent. Such as if they have a different GPIO controller. It may
be that for gpios and irqs, the solution really is to use
interrupt-map and create a gpio-map. i2c, spi and others still would
need to become children of the correct bus.

> 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.

The base-board .dts is unnecessary. dtc is fully capable of using
/proc/device-tree as the source material.  :-)

g.

-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.