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

[Pantelis Antoniou]

Hi Stephen,

On Nov 7, 2012, at 6:18 PM, Stephen Warren wrote:

> On 11/07/2012 01:47 AM, Pantelis Antoniou wrote:
>> Hi Stephen,
>> 
>> On Nov 6, 2012, at 11:37 PM, Stephen Warren wrote:
>> 
>>> On 11/05/2012 01:40 PM, Grant Likely wrote:
>>>> Hey folks,
>>>> 
>>>> As promised, here is my early draft to try and capture what device
>>>> tree overlays need to do and how to get there. Comments and
>>>> suggestions greatly appreciated.
>>> 
>>> Interesting. This just came up internally at NVIDIA within the last
>>> couple weeks, and was discussed on the U-Boot mailing list very recently
>>> too:
>>> 
>>> http://lists.denx.de/pipermail/u-boot/2012-October/thread.html#138227
>>> (it spills into the November archive too)
>> 
>> I am aware of this discussion. For our use case u-boot DT manipulation was
>> tried, but then abandoned. Asking our user base to modify anything in u-boot
>> was ruled out.
>> 
>>> 
>>>> For these cases it is proposed to implement an overlay feature for the
>>>> so that the initial device tree data can be modified by userspace at
>>> 
>>> I don't know if you're maintaining this as a document and taking patches
>>> to it, but if so:
>>> 
>>> "for the so" split across those two lines.
>>> 
>>>> Jane solves this problem by storing an FDT overlay for each cape in the
>>>> root filesystem. When the kernel detects that a cape is installed it
>>>> reads the cape's eeprom to identify it and uses request_firmware() to
>>>> obtain the appropriate overlay. Userspace passes the overlay to the
>>>> kernel in the normal way. If the cape doesn't have an eeprom, then the
>>>> kernel will still use firmware_request(), but userspace needs to already
>>>> know which cape is installed.
>>> 
>>> As mentioned by Pantelis, multiple versions of a board is also very
>>> common. We already have the following .dts files in the kernel where
>>> this applies, for the main board even:
>>> 
>>> arch/arm/boot/dts/tegra30-cardhu.dtsi
>>> arch/arm/boot/dts/tegra30-cardhu-a02.dts
>>> arch/arm/boot/dts/tegra30-cardhu-a04.dts
>> 
>> Exactly. I've made this point in another email, but IMHO board-revision
>> management is exactly the same with cape revision management.
>> 
>> Ideally you'd like to get rid of those three, and replace it with only
>> one that's properly versioned.
> 
> I don't expect we would ever get rid of some of those .dts files; there
> is after all a common subset that applies to all boards, and an
> incremental difference that applies to only A02/3, and another for
> A04/5/... Representing those as separate source files seems appropriate
> to me. If we try and dump all the multiple versions into a single file
> with some markup indicating which version of the board some sub-sections
> of the .dts apply to, I think we'll end up with rather complex .dts
> files. In this case, the simple overlay model works extremely well.

If that's your use case, fine. We're not really trying to impose any
special format for the DT files. We think that for our use cases
a single DT works better, not so much from the developer point of view
but from the users. A single DT is much easier to support, especially
for users that aren't so great at boot-loader option juggling.

Regards

-- Pantelis