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

[Eric Schikschneit]

Tom,

I think I understand the problems you stated. Obviously, we do not want to cause any regressions,
and I am happy to modify my patch as you suggest. I am unsure what to do about your dts/upstream
Issue though being that portion gets synced from the linux kernel source tree, correct? As far as the
ARM / ARM64 issue I could change the default lines to something similar to:

default  4096 if ARC || ARM64 || M68K.....
default 32768 if X86 && EFI_APP
default 0

That should allow 64-bit ARM platforms to be 4096, but 32-bit ARM platforms to still default to 0.


Eric Schikschneit

Senior Embedded Linux Engineer III  ​



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________________________________
From: Tom Rini
Sent: Monday, June 2, 2025 5:31 PM
To: Eric Schikschneit
Cc: u-boot at lists.denx.de
Subject: Re: [PATCH] dtc: Add Kconfig option to pad device tree blob

On Tue, May 20, 2025 at 01:23:05PM -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>

I was testing merging this and I found a few problems, sorry.

> ---
>  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 ++++-
>  scripts/Makefile.dts           | 4 ++++
>  11 files changed, 24 insertions(+), 12 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index ea33d07c086..a683af3d591 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 || ARM || M68K || MICROBLAZE || NIOS2 \
> +             || RISCV || SANDBOX || X86
> +     default 32768 if X86 && EFI_APP
> +     default 0

I believe this is the logic I suggested and so the problem is on me. For
ARM && ARM64, this is right. But for 32bit ARM, we should be 0 and let
individual boards set this as needed (none today but your board does
need it). This leads to size growth and then failure on a few platforms
is how I noticed this.

The second problem is that the Makefiles in dts/upstream are confusingly
not 100% upstream things and so also need to have -p 0x1000 taken out of
them as they'll get the value this way instead.

--
Tom