dtc: Add Kconfig option to pad device tree blob

Eric Schikschneit · trini · commit 5ba5cbf5a29d · 2025-06-19T19:00:09.000-06:00
This will allow arch(s) that use device tree blobs to pad the end of the
device tree so they can be modified by board files at run time. This will
help prevent errors such as FDT_ERR_NOSPACE from occurring.

Signed-off-by: Eric Schikschneit &lt;eric.schikschneit@novatechautomation.com&gt;
[trini: Change default order so that X86 &amp;&amp; EFI_APP works correctly]
diff --git a/arch/Kconfig b/arch/Kconfig
@@ -63,6 +63,13 @@ config SYS_CACHELINE_SIZE
 	default 64 if RISCV
 	default 32 if MIPS
 
+config SYS_DTC_PAD_BYTES
+	int "Size in bytes to pad device tree blob"
+	default 32768 if X86 && EFI_APP
+	default 4096 if ARC || ARM64 || M68K || MICROBLAZE || NIOS2 \
+		|| RISCV || SANDBOX || X86
+	default 0
+
 config LINKER_LIST_ALIGN
 	int
 	default 32 if SANDBOX
diff --git a/arch/arc/dts/Makefile b/arch/arc/dts/Makefile
@@ -11,4 +11,4 @@ dtb-$(CONFIG_TARGET_IOT_DEVKIT) +=  iot_devkit.dtb
 include $(srctree)/scripts/Makefile.dts
 
 # Add any required device tree compiler flags here
-DTC_FLAGS += -R 4 -p 0x1000
+DTC_FLAGS += -R 4
diff --git a/arch/m68k/dts/Makefile b/arch/m68k/dts/Makefile
@@ -21,4 +21,4 @@ dtb-$(CONFIG_TARGET_STMARK2) += stmark2.dtb
 include $(srctree)/scripts/Makefile.dts
 
 # Add any required device tree compiler flags here
-DTC_FLAGS += -R 4 -p 0x1000
+DTC_FLAGS += -R 4
diff --git a/arch/microblaze/dts/Makefile b/arch/microblaze/dts/Makefile
@@ -5,4 +5,4 @@ dtb-y += $(shell echo $(CONFIG_DEFAULT_DEVICE_TREE)).dtb
 include $(srctree)/scripts/Makefile.dts
 
 # Add any required device tree compiler flags here
-DTC_FLAGS += -R 4 -p 0x1000
+DTC_FLAGS += -R 4
diff --git a/arch/nios2/dts/Makefile b/arch/nios2/dts/Makefile
@@ -5,4 +5,4 @@ dtb-y += $(CONFIG_DEFAULT_DEVICE_TREE:"%"=%).dtb
 include $(srctree)/scripts/Makefile.dts
 
 # Add any required device tree compiler flags here
-DTC_FLAGS += -R 4 -p 0x1000
+DTC_FLAGS += -R 4
diff --git a/arch/riscv/dts/Makefile b/arch/riscv/dts/Makefile
@@ -19,4 +19,4 @@ dtb-$(CONFIG_TARGET_ASPEED_AST2700_IBEX) += ast2700-ibex.dtb
 include $(srctree)/scripts/Makefile.dts
 
 # Add any required device tree compiler flags here
-DTC_FLAGS += -R 4 -p 0x1000
+DTC_FLAGS += -R 4
diff --git a/arch/sandbox/dts/Makefile b/arch/sandbox/dts/Makefile
@@ -11,4 +11,4 @@ dtb-$(CONFIG_CMD_EXTENSION) += overlay0.dtbo overlay1.dtbo
 include $(srctree)/scripts/Makefile.dts
 
 # Add any required device tree compiler flags here
-DTC_FLAGS += -R 4 -p 0x1000
+DTC_FLAGS += -R 4
diff --git a/arch/x86/dts/Makefile b/arch/x86/dts/Makefile
@@ -24,4 +24,4 @@ dtb-y += bayleybay.dtb \
 
 include $(srctree)/scripts/Makefile.dts
 
-DTC_FLAGS += -R 4 -p $(if $(CONFIG_EFI_APP),0x8000,0x1000)
+DTC_FLAGS += -R 4
diff --git a/doc/board/broadcom/bcm7xxx.rst b/doc/board/broadcom/bcm7xxx.rst
@@ -149,10 +149,8 @@ and with a generic ARMv7 root file system.
 				 * to the Linux source tree as a .dts file.
 				 *
 				 * To support modifications to the device tree
-				 * in-place in U-Boot, add to Linux's
-				 * arch/arm/boot/dts/Makefile:
-				 *
-				 * DTC_FLAGS ?= -p 4096
+				 * in-place in U-Boot, set the config variable
+				 * CONFIG_SYS_DTC_PAD_BYTES as needed.
 				 *
 				 * This will leave some padding in the DTB and
 				 * thus reserve room for node additions.
diff --git a/doc/develop/makefiles.rst b/doc/develop/makefiles.rst
@@ -1430,10 +1430,13 @@ When kbuild executes, the following steps are followed (roughly):
 	A central rule exists to create `$(obj)/%.dtb` from `$(src)/%.dts`;
 	architecture Makefiles do no need to explicitly write out that rule.
 
+        The device tree can now be padded by the specified number of bytes
+        by setting CONFIG_SYS_DTC_PAD_BYTES instead of explicitly setting
+        DTC_FLAGS with the -p option.
+
 	Example::
 
 		targets += $(dtb-y)
-		DTC_FLAGS ?= -p 1024
 
 7.9 Preprocessing linker scripts
 --------------------------------
diff --git a/dts/upstream/src/arm64/Makefile b/dts/upstream/src/arm64/Makefile
@@ -6,5 +6,5 @@ include $(srctree)/scripts/Makefile.dts
 DTC_FLAGS += -a 0x8
 
 ifdef CONFIG_RCAR_64
-DTC_FLAGS += -R 4 -p 0x1000
+	DTC_FLAGS += -R 4
 endif
diff --git a/dts/upstream/src/riscv/Makefile b/dts/upstream/src/riscv/Makefile
@@ -2,5 +2,5 @@
 
 include $(srctree)/scripts/Makefile.dts
 
-DTC_FLAGS += -R 4 -p 0x1000
+DTC_FLAGS += -R 4
 
diff --git a/scripts/Makefile.dts b/scripts/Makefile.dts
@@ -16,6 +16,10 @@ dtb-y += $(subst $(dt_dir)/,,$(dtb-vendor_dts))
 
 endif
 
+ifneq ($(CONFIG_SYS_DTC_PAD_BYTES),0)
+DTC_FLAGS += -p $(CONFIG_SYS_DTC_PAD_BYTES)
+endif
+
 targets += $(dtb-y)
 
 PHONY += dtbs

Original file line number	Diff line number	Diff line change
`@@ -24,4 +24,4 @@ dtb-y += bayleybay.dtb \`
`24`	`24`
`25`	`25`	`include $(srctree)/scripts/Makefile.dts`
`26`	`26`
`27`		`-DTC_FLAGS += -R 4 -p $(if $(CONFIG_EFI_APP),0x8000,0x1000)`
	`27`	`+DTC_FLAGS += -R 4`
Original file line number	Diff line number	Diff line change
`@@ -149,10 +149,8 @@ and with a generic ARMv7 root file system.`
`149`	`149`	`* to the Linux source tree as a .dts file.`
`150`	`150`	`*`
`151`	`151`	`* To support modifications to the device tree`
`152`		`- * in-place in U-Boot, add to Linux's`
`153`		`- * arch/arm/boot/dts/Makefile:`
`154`		`- *`
`155`		`- * DTC_FLAGS ?= -p 4096`
	`152`	`+ * in-place in U-Boot, set the config variable`
	`153`	`+ * CONFIG_SYS_DTC_PAD_BYTES as needed.`
`156`	`154`	`*`
`157`	`155`	`* This will leave some padding in the DTB and`
`158`	`156`	`* thus reserve room for node additions.`