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

[Tom Rini]

On Fri, May 02, 2025 at 01:38:26PM -0500, 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                 | 11 +++++++++++
>  arch/arc/dts/Makefile        |  6 +++++-
>  arch/arm/dts/Makefile        |  4 ++++
>  arch/m68k/dts/Makefile       |  6 +++++-
>  arch/microblaze/dts/Makefile |  6 +++++-
>  arch/nios2/dts/Makefile      |  6 +++++-
>  arch/powerpc/dts/Makefile    |  4 ++++
>  arch/riscv/dts/Makefile      |  6 +++++-
>  arch/sandbox/dts/Makefile    |  6 +++++-
>  arch/x86/dts/Makefile        |  8 +++++++-
>  10 files changed, 56 insertions(+), 7 deletions(-)

I like the concept and the cleanup. However, I think we should:
- Put the line
DTC_FLAGS += -p $(CONFIG_SYS_DTC_PAD_BYTES)
in scripts/Makefile.dts with the rest of the DTC_LOGIC as:
ifneq $(CONFIG_SYS_DTC_PAD_BYTES,0)
DTC_FLAGS += -p $(CONFIG_SYS_DTC_PAD_BYTES)
endif

Then we can:
> diff --git a/arch/Kconfig b/arch/Kconfig
> index ea33d07c086..a85f751d1e6 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -63,6 +63,17 @@ config SYS_CACHELINE_SIZE
>  	default 64 if RISCV
>  	default 32 if MIPS
>  
> +config SYS_DTC_PAD
> +	bool "Enable DTC padding"
> +	help
> +	  This will allow the final device tree of your specified arch to
> +	  have pad space at the end. This enables the ability for the device
> +	  tree to be modified in place by board files.

Drop this.

> +config SYS_DTC_PAD_BYTES
> +	int "Size in bytes to pad device tree blob"
> +	default 4096 if SYS_DTC_PAD

And change the defaults to match the existing users, roughly:
	default 32768 if X86 && EFI_APP
	default 4096 if ARC || M68K || MICROBLAZE || NIOS2 || RCAR_64 \
		|| RISCV || SANDBOX || (X86 && !EFI_APP)
	default 0

And then we drop the -p lines from all of the other DTC_FLAGS.

Thanks!

-- 
Tom
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