[PATCH] dtc: Add Kconfig option to pad device tree blob

[Anshul Dalal]

On Tue Jun 3, 2025 at 10:36 PM IST, Eric Schikschneit wrote:
> 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 occuring.
>
> Signed-off-by: Eric Schikschneit <eric.schikschneit at novatechautomation.com>
> ---
>  arch/Kconfig                    | 7 +++++++
>  arch/arc/dts/Makefile           | 2 +-
>  arch/m68k/dts/Makefile          | 2 +-
>  arch/microblaze/dts/Makefile    | 2 +-
>  arch/nios2/dts/Makefile         | 2 +-
>  arch/riscv/dts/Makefile         | 2 +-
>  arch/sandbox/dts/Makefile       | 2 +-
>  arch/x86/dts/Makefile           | 2 +-
>  doc/board/broadcom/bcm7xxx.rst  | 6 ++----
>  doc/develop/makefiles.rst       | 5 ++++-
>  dts/upstream/src/arm64/Makefile | 2 +-
>  dts/upstream/src/riscv/Makefile | 2 +-
>  scripts/Makefile.dts            | 4 ++++
>  13 files changed, 26 insertions(+), 14 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index ea33d07c086..bf4c2c80891 100644
> --- 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 4096 if ARC || ARM64 || M68K || MICROBLAZE || NIOS2 \
> +		|| RISCV || SANDBOX || X86
> +	default 32768 if X86 && EFI_APP
> +	default 0
> +
>  config LINKER_LIST_ALIGN
>  	int
>  	default 32 if SANDBOX
> diff --git a/arch/arc/dts/Makefile b/arch/arc/dts/Makefile
> index fe6ad7b849a..87c627c01c8 100644
> --- 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

Thanks for the patch Eric, the changes to padding size look good to me
although I wonder if we should expose the -R flag for configuring the
number of /reserved-memory to user under Kconfig as well.

We have it hardcoded to 4 everywhere now, might be useful to expose it
as CONFIG_SYS_DTC_RSVD or similar for users looking to create compact
DTBs (-R 0) or where more reserved regions need to be added at runtime.

Regards,
Anshul