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

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

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).

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/"

[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?

[4]
https://github.com/openembedded/openembedded-core/blob/f642edb006a8c16dbe45681afe547eabfae17073/meta/classes-recipe/devicetree.bbclass#L59C1-L59C9

Best regards,
Romain


> 
> Reported-by: Chris Packham <judge.packham@gmail.com>
> Signed-off-by: Edgar Bonet <bonet@grenoble.cnrs.fr>
> ---
> Changes v1 -> v2:
>   - define a hook for LINUX_POST_PATCH_HOOKS and LINUX_POST_RSYNC_HOOKS
>     instead of modifying LINUX_BUILD_CMDS
>   - use $(patsubst %.dts,%.dtb,$(dts)) instead of $(dts:%.dts=%.dtb)
> both changes suggested by Arnout Vandecappelle.
> 
> I ran the folowing build tests:
> 
>     # Test with an old kernel: v6.6.30
>     make acmesystems_acqua_a5_512mb_defconfig
>     make linux
> 
>     # Test with a new kernel: v6.12.5
>     make linux-dirclean
>     git cherry-pick 0938346cde  # bump Acqua's kernel to v6.12.5
>     make acmesystems_acqua_a5_512mb_defconfig
>     make linux
> 
>     # Test the OVERRIDE_SRCDIR case.
>     make linux-dirclean
>     git -C $HOME/src/linux checkout v6.12.5
>     echo "LINUX_OVERRIDE_SRCDIR = $HOME/src/linux" > local.mk
>     make acmesystems_acqua_a5_512mb_defconfig
>     make linux
> 
>  linux/linux.mk | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/linux/linux.mk b/linux/linux.mk
> index 1d3d8fffa6..eb509fd747 100644
> --- a/linux/linux.mk
> +++ b/linux/linux.mk
> @@ -327,6 +327,17 @@ define LINUX_TRY_PATCH_TIMECONST
>  endef
>  LINUX_POST_PATCH_HOOKS += LINUX_TRY_PATCH_TIMECONST
>  
> +# Kernels 6.12+ won't compile the custom DTS files we drop into
> +# arch/$ARCH/boot/dts unless we add specific rules to that directory's
> +# Makefile.
> +define LINUX_ADD_DTB_MAKE_RULES
> +	$(foreach dts,$(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_DTS_PATH)), \
> +		echo "dtb-y += $(patsubst %.dts,%.dtb,$(notdir $(dts)))" >> $(LINUX_ARCH_PATH)/boot/dts/Makefile
> +	)
> +endef
> +LINUX_POST_PATCH_HOOKS += LINUX_ADD_DTB_MAKE_RULES
> +LINUX_POST_RSYNC_HOOKS += LINUX_ADD_DTB_MAKE_RULES
> +
>  LINUX_KERNEL_CUSTOM_LOGO_PATH = $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_LOGO_PATH))
>  ifneq ($(LINUX_KERNEL_CUSTOM_LOGO_PATH),)
>  LINUX_DEPENDENCIES += host-imagemagick

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