Re: [RFC PATCH 05/15] libfdt: Introduce fdt_first_node()

[Luca Ceresoli]

Hi Hervé, David,

I'm trying to review the patches that have no feedback so far.

Being new to the dtc codebase I'm mostly pointing out things that are not
clear from a newcomer point of view. I hope this helps anyway.

On Tue Feb 10, 2026 at 6:33 PM CET, Herve Codina wrote:
> In several places, libfdt assumes that a FDT_BEGIN_NODE tag is present
> at the offset 0 of the structure block.
>
> This assumption is not correct. Indeed, a FDT_NOP can be present at the
> offset 0 and this is a legit case.

I wonder whether this can be proven by showing an example, or the specs, or
whatever use case that makes sense.

> Introduce fdt_first_node() in order to get the offset of the first node
> (first FDT_BEGIN_NODE tag) available in a fdt blob.
>
> Signed-off-by: Herve Codina <herve.codina@xxxxxxxxxxx>
> ---
>  libfdt/fdt.c             | 25 +++++++++++++++++++++++++
>  libfdt/libfdt_internal.h |  1 +
>  2 files changed, 26 insertions(+)
>
> diff --git a/libfdt/fdt.c b/libfdt/fdt.c
> index 56d4dcb..676c7d7 100644
> --- a/libfdt/fdt.c
> +++ b/libfdt/fdt.c
> @@ -252,6 +252,31 @@ int fdt_check_prop_offset_(const void *fdt, int offset)
>  	return offset;
>  }
>

Even though this seems to be quite uncommon in this repository, I think
documenting new functions would be helpful, especially preconditions,
postconditions and parameter values when not obvious.

What about:

  Find the initial node with content (FDT_BEGIN_NODE) in a fdt, skipping
  FDT_NOP [and <other tags> is applicable].

  *return: pointer to the first node into the fdt or e negative error value

> +int fdt_first_node(const void *fdt)
> +{
> +	int nextoffset = 0;
> +	int offset;
> +	uint32_t tag;
> +
> +	do {
> +		offset = nextoffset;
> +		tag = fdt_next_tag(fdt, offset, &nextoffset);
> +		switch (tag) {
> +		case FDT_END_NODE:
> +		case FDT_PROP:
> +			return -FDT_ERR_BADSTRUCTURE;
> +
> +		case FDT_BEGIN_NODE:
> +			return offset;
> +
> +		default:
> +			break;
> +		}
> +	} while (tag != FDT_END);
> +
> +	return (nextoffset < 0) ? nextoffset : -FDT_ERR_NOTFOUND;
> +}
> +

Luca


--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com