linux: introduce BR2_LINUX_KERNEL_CUSTOM_DTS_DIR

Since Linux 6.12, the Buildroot option BR2_LINUX_KERNEL_CUSTOM_DTS_PATH
does not work as expected on arm, arm64, mips and riscv[1]. These are
the architectures that store the in-tree DTS files in vendor-specific
subdirectories of arch/$ARCH/boot/dts/.

BR2_LINUX_KERNEL_CUSTOM_DTS_PATH was introduced in Buildroot 2012.08
(commit 69fc497df0ae "Rework support for the device tree"). At the time,
the kernel kept all in-tree DTS files directly in arch/$ARCH/boot/dts/,
and this is where Buildroot drops the user's custom DTS. Vendor-specific
subdirectories appeared in Linux v3.19 for the arm64 architecture, and
this scheme was later adopted by mips, riscv and arm.

For these architectures, Linux 6.12 (commit e7e2941300d2, "kbuild: split
device tree build rules into scripts/Makefile.dtbs") made the DTB build
infrastructure incompatible with the way
BR2_LINUX_KERNEL_CUSTOM_DTS_PATH is implemented. This infrastructure now
expects all DTS files to be in vendor-specific subdirectories on the
architectures that use this scheme.

We can't update easily the current behavior of
BR2_LINUX_KERNEL_CUSTOM_DTS_PATH since it expect a list of files but
can also be used "unexpectedly" with directories [2].

  BR2_LINUX_KERNEL_INTREE_DTS_NAME="st/stm32mp135f-dk-mx"
  BR2_LINUX_KERNEL_CUSTOM_DTS_PATH="$(BR2_EXTERNAL_ST_PATH)/[...]/linux-dts/st"

In this case, the st directory is copied into the arch/$ARCH/boot/dts/
and BR2_LINUX_KERNEL_INTREE_DTS_NAME is used to build stm32mp135f-dk-mx dtb
as if it was intree.

Introduce BR2_LINUX_KERNEL_CUSTOM_DTS_DIR configuration
parameter to specify a list of directories that are copied as-is over
the arch/<arch>/boot/dts/ directory before building the device tree
blob:

  board/acmesystems/acqua-a5/dts/
  └── microchip
      └── at91-sama5d3_acqua.dts

defconfig:
  BR2_LINUX_KERNEL_CUSTOM_DTS_DIR="board/acmesystems/acqua-a5/dts"

Each dts file found is automatically added to the list of devicetree
to build.

BR2_LINUX_KERNEL_CUSTOM_DTS_DIR can also be used for external
devicetree overlays files:

  board/ti/am574x-idk/dts/
  └── ti
      └── omap
          └── am57xx-evm.dtso

With this new option, BR2_LINUX_KERNEL_CUSTOM_DTS_PATH is now deprecated.

Note: We want to create a list of dts files (LINUX_DTS_LIST) present in
diectrories listed by BR2_LINUX_KERNEL_CUSTOM_DTS_DIR. But
LINUX_DTS_LIST must not contain BR2_LINUX_KERNEL_CUSTOM_DTS_DIR
paths. Use GNU 'find' print format %P to print each dts file path
without their respective dts overlay directory path.
Do the same for LINUX_DTSO_LIST.

Thanks to Edgar Bonet for the initial contribution [3].

[1] https://lists.buildroot.org/pipermail/buildroot/2024-October/765463.html
[2] https://github.com/bootlin/buildroot-external-st/blob/541ba7d96330cd7da5674b46806b549a7ccf9da8/configs/st_stm32mp135f_dk_demo_defconfig#L21
[3] https://lore.kernel.org/buildroot/93f83afb-6987-441c-8e06-dab4d43b828f@grenoble.cnrs.fr/

Cc: Michael Walle <michael@walle.cc>
Cc: Gaël PORTAY <gael.portay+rtone@gmail.com>
Reported-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Edgar Bonet <bonet@grenoble.cnrs.fr>
[Romain:
  - Rework the initial contribution by Edgar Bonet by
    BR2_LINUX_KERNEL_CUSTOM_DTS_DIR following
    Michael Walle comments and Gaël PORTAY review.
  - Reword the commit log accordingly.
]
Signed-off-by: Romain Naour <romain.naour@smile.fr>
Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

linux/Config.in

@@ -435,6 +435,7 @@ config BR2_LINUX_KERNEL_INTREE_DTSO_NAMES
 
 config BR2_LINUX_KERNEL_CUSTOM_DTS_PATH
 	string "Out-of-tree Device Tree Source file paths"
+	depends on BR2_LINUX_KERNEL_CUSTOM_DTS_DIR = ""
 	help
 	  Paths to out-of-tree Device Tree Source (.dts), Device Tree
 	  Source Include (.dtsi) and Device Tree Overlay Source (.dtso)
@@ -442,6 +443,34 @@ config BR2_LINUX_KERNEL_CUSTOM_DTS_PATH
 	  kernel sources and the .dts files will
 	  be compiled from there.
 
+	  Due to a kernel build system changes in 6.12,
+	  BR2_LINUX_KERNEL_CUSTOM_DTS_PATH is now deprecated and
+	  replaced by BR2_LINUX_KERNEL_CUSTOM_DTS_DIR
+
+config BR2_LINUX_KERNEL_CUSTOM_DTS_DIR
+	string "Out-of-tree Device Tree Source overlay directories"
+	help
+	  Specify a list of directories that are copied as-is over the
+	  arch/<arch>/boot/dts/ directory before building the device
+	  tree blob.
+
+	  This overlay can contain dts, dtso and dtsi files.
+
+	  BR2_LINUX_KERNEL_CUSTOM_DTS_DIR should point to one or more
+	  directories containing a vendor subdirectory (e.g. rockchip)
+	  which contains the dts files. This vendor subdirectory should
+	  match the vendor subdirectory used by the board in the kernel
+	  (e.g. arch/arm64/boot/dts/rockchip/).
+
+	  While most architechtures make use of vendor subdirectories,
+	  like arm, arm64 and riscv, some architectures like powerpc
+	  and xtensa do not.
+	  In this case, BR2_LINUX_KERNEL_CUSTOM_DTS_DIR should point to
+	  a directory containing the dts files directly.
+
+	  Since the 6.12 release, each out-of-tree Device Tree Source
+	  file must be copied into their corresponding sub-directory.
+
 config BR2_LINUX_KERNEL_DTB_KEEP_DIRNAME
 	bool "Keep the directory name of the Device Tree"
 	help

linux/linux.mk

@@ -213,6 +213,22 @@ LINUX_CUSTOM_DTS_PATH = $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_DTS_PATH))
 LINUX_DTS_NAME += $(basename $(filter %.dts,$(notdir $(LINUX_CUSTOM_DTS_PATH))))
 LINUX_DTSO_NAMES += $(basename $(filter %.dtso,$(notdir $(LINUX_CUSTOM_DTS_PATH))))
 
+LINUX_KERNEL_CUSTOM_DTS_DIR = $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_DTS_DIR))
+ifneq ($(LINUX_KERNEL_CUSTOM_DTS_DIR),)
+# Use evaluation-during-assignment using := to avoid any re-evaluation
+# of LINUX_DTS_LIST when LINUX_DTS_NAME is used.
+LINUX_DTS_LIST := $(shell find $(LINUX_KERNEL_CUSTOM_DTS_DIR) -name '*.dts' -printf '%P\n' 2>/dev/null)
+LINUX_DTSO_LIST := $(shell find $(LINUX_KERNEL_CUSTOM_DTS_DIR) -name '*.dtso' -printf '%P\n' 2>/dev/null)
+LINUX_DTS_NAME += $(basename $(LINUX_DTS_LIST))
+LINUX_DTSO_NAMES += $(basename $(LINUX_DTSO_LIST))
+
+define LINUX_COPY_CUSTOM_DTS_FILES
+	$(foreach d, $(LINUX_KERNEL_CUSTOM_DTS_DIR), \
+		@$(call MESSAGE,"Copying devicetree overlay $(d)")$(sep) \
+		$(Q)$(call SYSTEM_RSYNC,$(d),$(LINUX_ARCH_PATH)/boot/dts/)$(sep))
+endef
+endif
+
 LINUX_DTBS = $(addsuffix .dtb,$(LINUX_DTS_NAME)) $(addsuffix .dtbo,$(LINUX_DTSO_NAMES))
 
 ifeq ($(BR2_LINUX_KERNEL_IMAGE_TARGET_CUSTOM),y)
@@ -527,6 +543,7 @@ define LINUX_BUILD_CMDS
 	$(foreach dts,$(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_DTS_PATH)), \
 		cp -f $(dts) $(LINUX_ARCH_PATH)/boot/dts/
 	)
+	$(LINUX_COPY_CUSTOM_DTS_FILES)
 	$(LINUX_MAKE_ENV) $(BR2_MAKE) $(LINUX_MAKE_FLAGS) -C $(@D) all
 	$(LINUX_MAKE_ENV) $(BR2_MAKE) $(LINUX_MAKE_FLAGS) -C $(@D) $(LINUX_TARGET_NAME)
 	$(LINUX_BUILD_DTB)