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

[Fiona Klute via buildroot <buildroot@buildroot.org>]

Am 29.01.25 um 11:10 schrieb Romain Naour via buildroot:
> 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).
I've just sent a patch, and would be happy to hear what you think. I'm
not at the meeting, but available on IRC if there's anything to discuss. :-)

See:
https://patchwork.ozlabs.org/project/buildroot/patch/20250203154651.2790784-1-fiona.klute@gmx.de/

Best regards,
Fiona

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