How OpenWrt compiles DTS files?

arsz6733 · June 23, 2020, 7:28am

Hi
I'm fairly new to OpenWrt development and I have a question regarding dts(device tree) files.
I've checked that there are some folders in target/linux which consist dts files but I didn't find out how OpenWrt uses them and more specific how it compiles them and where the output dtb file is generated.
I previously knew that dtb files may be passed to kernel by bootloader or they may be appended to kernel image itself(Not sure).
I want to know which statement is correct and also which config determines this?
Thank you

johnth · June 24, 2020, 4:26am

In the devices I've looked at, append-dtb or KERNEL_DTB are used in the target makefile:

github.com

openwrt/openwrt/blob/8f95220bcb554b1b668114e5264ebce4028c5f93/target/linux/ramips/image/mt7621.mk#L391


TARGET_DEVICES += iodata_wn-ax1167gr

define Device/iodata_nand
  DEVICE_VENDOR := I-O DATA
  BLOCKSIZE := 128k
  PAGESIZE := 2048
  UBINIZE_OPTS := -E 5
  KERNEL_SIZE := 4096k
  IMAGE_SIZE := 51200k
  LOADER_TYPE := bin
  KERNEL := kernel-bin | append-dtb | lzma | loader-kernel | lzma | uImage lzma
  IMAGE/sysupgrade.bin := sysupgrade-tar | append-metadata
endef

define Device/iodata_wn-ax1167gr2
  $(Device/iodata_nand)
  UIMAGE_MAGIC := 0x434f4d42
  DEVICE_MODEL := WN-AX1167GR2
  KERNEL_INITRAMFS := $(KERNEL_DTB) | loader-kernel | lzma | \
	custom-initramfs-uimage 3.10(XBC.1)b10 | iodata-mstc-header
  DEVICE_PACKAGES := kmod-mt7615e kmod-mt7615-firmware wpad-basic

The dtb is built from here:

github.com

openwrt/openwrt/blob/8f95220bcb554b1b668114e5264ebce4028c5f93/include/image-commands.mk#L141


		-B $(TPLINK_BOARD_ID) \
		-V $(REVISION) \
		-k $(IMAGE_KERNEL) \
		-r $@ \
		-o $@.new \
		-j \
		$(wordlist 2,$(words $(1)),$(1)) \
		$(if $(findstring sysupgrade,$(word 1,$(1))),-S) && mv $@.new $@ || rm -f $@
endef

define Build/append-dtb
	cat $(KDIR)/image-$(firstword $(DEVICE_DTS)).dtb >> $@
endef

define Build/install-dtb
	$(call locked, \
		$(foreach dts,$(DEVICE_DTS), \
			$(CP) \
				$(DTS_DIR)/$(dts).dtb \
				$(BIN_DIR)/$(IMG_PREFIX)-$(dts).dtb; \
		), \

github.com

openwrt/openwrt/blob/8f95220bcb554b1b668114e5264ebce4028c5f93/include/image.mk#L190


	dd if=$(1) of=$(1).new bs=$(2) conv=sync
	mv $(1).new $(1)
endef

ROOTFS_PARTSIZE=$(shell echo $$(($(CONFIG_TARGET_ROOTFS_PARTSIZE)*1024*1024)))

define Image/pad-root-squashfs
	$(call Image/pad-to,$(KDIR)/root.squashfs,$(if $(1),$(1),$(ROOTFS_PARTSIZE)))
endef

# $(1) source dts file
# $(2) target dtb file
# $(3) extra CPP flags
# $(4) extra DTC flags
define Image/BuildDTB
	$(TARGET_CROSS)cpp -nostdinc -x assembler-with-cpp \
		-I$(DTS_DIR) \
		-I$(DTS_DIR)/include \
		-I$(LINUX_DIR)/include/ \
		-undef -D__DTS__ $(3) \
		-o $(2).tmp $(1)

The build process for my devices compiles the dts files to here:
ls build_dir/target-*/linux-*/*.dt*

On a device which uses dts, you can normally see the device tree here (but this can be configured off):
ls /proc/device-tree/
hexdump -C /proc/device-tree/compatible

https://elinux.org/Device_Tree_Reference is a great resource.

The linux dtc utilities are very useful for investigating the dts / dtb.