Re: [Buildroot] [PATCH v2 1/1] linux: make out-of-tree DTS work with newest kernels

[Romain Naour via buildroot <buildroot@buildroot.org>]

Hello Fiona, All,

Le 28/01/2025 à 14:33, Fiona Klute a écrit :
> Am 24.12.24 um 13:22 schrieb Edgar Bonet:
>> Hello!
>>
>> On 2024-12-18, Romain Naour wrote:
>>> Hello Edgar,
>>>
>>> Le 17/12/2024 à 10:55, Edgar Bonet a écrit :
>>>> Since Linux 6.12 (commit e7e2941300d2, "kbuild: split device tree build
>>>> rules into scripts/Makefile.dtbs"), the kernel build process does not
>>>> build the DTBs from the source files listed in
>>>> BR2_LINUX_KERNEL_CUSTOM_DTS_PATH.[1] This is because it expects all the
>>>> DTS files to live in vendor-specific subdirectories of
>>>> arch/$ARCH/boot/dts/ , whereas Buildroot drops them in the top dts
>>>> directory.
>>>
>>> Maybe we could add some additional explanation:
>>>
>>> BR2_LINUX_KERNEL_CUSTOM_DTS_PATH was introduced in Buildroot 2012.08 [1] at the
>>> time the kernel didn't have any vendor-specific subdirectory in
>>> arch/$ARCH/boot/dts/.
>>>
>>> vendor-specific subdirectory was introduced in kernel v3.19 [2] for ARM64 and in
>>> kernel v6.5 for ARM [3]. Note, other arch than ARM and ARM64 doesn't actually
>>> use a vendor-specific subdirectory (not even powerpc).
>>
>> I hadn't noticed that this varies among archs. Now that I look at it, it
>> appears mips and riscv also use vendor-specific subdirectories.
>>
>>> Your comment doesn't seems to apply for non ARM{,64} arch:
>>> "This is because it expects all the DTS files to live in vendor-specific
>>> subdirectories of arch/$ARCH/boot/dts/"
>>
>> Indeed, it is architecture-dependent.
>>
>>> [1]
>>> https://gitlab.com/buildroot.org/buildroot/-/
>>> commit/69fc497df0ae51bcc8a067c849447abdeb3cb2be
>>> [2]
>>> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?
>>> id=ca5b34100c571658e605c5554aac374649593327
>>> [3]
>>> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?
>>> id=724ba6751532055db75992fc6ae21c3e322e94a7
>>>
>>>
>>>>
>>>> Fulfilling this kernel expectation would require a new configuration
>>>> parameter for the name of the appropriate vendor subdirectory. It would
>>>> also require changing the relative paths to included .dtsi files.
>>>>
>>>> Let's add instead a rule to the top dts/Makefile for each DTB we want to
>>>> build. This avoids breaking user's out-of-tree configurations.
>>>>
>>>> [1] https://lists.buildroot.org/pipermail/buildroot/2024-October/765463.html
>>>
>>> Note: Yocto doesn't rely on the kernel build infrastructure to build user
>>> provided devicetree [4].
>>>
>>> Maybe we should consider adding a new package infrastructure like kernel-module?
>>
>> This seems like an interesting option. It is, however, beyond my
>> capabilities.
> 
> Bit late to the party, but: I currently have a package in an external
> tree that takes a list of .dts/.dtso files from config and calls cpp &
> dtc to build .dtb/.dtbo files, with include paths into the Linux build
> dir added. The build commands are naturally modeled on how the kernel
> build builds DTBs, but it doesn't rely on the kernel Makefiles.

AFAIK, the current BR2_LINUX_KERNEL_CUSTOM_DTS_PATH option doesn't seems to take
into account any external devicetree overlay files (dtso).

I've send a new version of this patch (but dtso are not supported):

https://lore.kernel.org/buildroot/20250129095716.2511434-1-romain.naour@smile.fr/T/#t

> 
> Would something like that be interesting for Buildroot, whether as a
> generic DTB build package or infrastructure?

While reviewing this patch, I had to take a look at how to build a dtb file
outside of the kernel build infrastructure (like Yocto does) but it's not easy.
Actually we are happy to keep this build process hidden in the kernel Makefiles.

Since the next Buildroot meeting is next week, this is probably a topic we can
discuss. Feel free to send a patch (even a draft).

Best regards,
Romain


> 
> Best regards,
> Fiona
> 

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