[yocto] Move device tree generation from include file to bbclass

[Nikolay Dimitrov]

Hi Bruce,

On 04/15/2015 06:26 PM, Bruce Ashfield wrote:
> On Wed, Apr 15, 2015 at 11:22 AM, Nikolay Dimitrov
> <picmaster at mail.bg> wrote:
>> Hi Bruce,
>>
>>
>> On 04/15/2015 04:13 PM, Bruce Ashfield wrote:
>>>
>>> On 2015-04-15 08:33 AM, Bach, Pascal wrote:
>>>>
>>>> Hi
>>>>
>>>
>>> Adding oe-core, since that's the right place to have a discussion
>>> like this.
>>>
>>>> As ARM now also moved to device tree it look like in future we
>>>> will have more kernels that are using device tree then ones
>>>> that are not.
>>>
>>>
>>> True, but it has been like this for quite some time now :)
>>>
>>>> As far as I understand currently the generation of device
>>>> trees is controlled via KERNEL_DEVICETREE and is handled in via
>>>> an include file recipes-kernel/linux/linux-dtb.inc.
>>>>
>>>> I was thinking about moving this include into a class so it
>>>> becomes easier to use. Before I dive into implementing
>>>> something I would like some feedback from the community.
>>>
>>>
>>> The big trick with changing anything like this is compatibility
>>> with existing recipes. Whatever we do, existing recipes and
>>> layers shouldn't be broken .. or if they are broken, there
>>> should be a compelling technical reason to do so.
>>>
>>>>
>>>> I have the following variant in mind.
>>>>
>>>> Add the device tree generation to the current kernel.bbclass
>>>> (or let kernel.bblcass inherit from a kernel-dtb.bbclass).
>>>> This way all kernels would automatically be DT enabled. The
>>>> class would check if KERNEL_DEVICETREE is set and generate
>>>> device trees based on this information. For boards that don't
>>>> have KERNEL_DEVICETREE set the class would do nothing and the
>>>> behavior is like before. The advantage I see with this
>>>> approach is that the only thing a user needs to do is to set
>>>> KERNEL_DEVICETREE in the board and make sure the device trees
>>>> are available in the kernel they like to build.
>>>
>>>
>>> That's pretty much the experience that most users have now, since
>>> there's nearly always a kernel recipe created, that recipe
>>> includes linux-dtb.inc, and sets KERNEL_DEVICETREE.
>>
>>
>> As far as I understood, Pascal's idea is to remove the need for
>> user recipes to include linux-dtb.inc, and provide this
>> functionality via inheritance.
>
> That is obvious. My questions are around "why". There's no big
> technical advantage, and if you remove that existing file, you break
>  existing recipes. Which means you need to leave a stub in place.
>
> So without a technical advantage, it's churn for the sake of churn.

Well, removing redundancy and simplifying users' recipes could be
considered an advantage. Also, as the contents of linux-dtb.inc are
going to be moved to bbclass, the file can be left empty, later
maintainers remove the extra line from all users' recipes in following
commits. I don't see breaking as an option.

> Bruce
>
>>
>>> Everything else happens to build and package the device tree.
>>>
>>> Was there something specifically that was causing issues with the
>>> current way of building them ?
>>>
>>> Cheers,
>>>
>>> Bruce
>>>
>>>>
>>>> I appreciate your feedback?
>>>>
>>>> Regards Pascal

Kind regards,
Nikolay