Re: [PATCH v3 1/4] libfdt: overlay_merge: Introduce fdt_overlay_merge()

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, May 19, 2025 at 02:40:40PM +0530, Wasim Nazir wrote:
> From: Srivatsa Vaddagiri <vatsa@codeaurora.org>
> 
> fdt_overlay_merge() merges two overlay blobs. It is largely expected to be
> used offline on a build machine to combine two or more overlay blobs into one.
> 
> It is intended to help maintain device-tree overlay code in
> multiple source repositories, but merge their binary forms (overlay blobs)
> into one so that bootloader's task of searching for all relevant overlay blobs
> is simplified.
> 
> Introduce fdt_overlay_merge() which is identical to fdt_overlay_apply().
> Subsequent patches will introduce required changes to merge overlay blobs.
> 
> Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>
> Signed-off-by: Wasim Nazir <quic_wasimn@quicinc.com>
> ---
>  libfdt/fdt_overlay.c | 59 ++++++++++++++++++++++++++++++++++++++++++++
>  libfdt/libfdt.h      | 18 ++++++++++++++
>  2 files changed, 77 insertions(+)
> 
> diff --git a/libfdt/fdt_overlay.c b/libfdt/fdt_overlay.c
> index e6b9eb643958..8690ed55c8f6 100644
> --- a/libfdt/fdt_overlay.c
> +++ b/libfdt/fdt_overlay.c
> @@ -1098,3 +1098,62 @@ err:
> 
>  	return ret;
>  }
> +
> +int fdt_overlay_merge(void *fdt, void *fdto, int *fdto_nospace)

The parameters should be renamed to reflect the new semantics.

The 'fdto_nospace' parameter seems weird.  Why not just return
-FDT_ERR_NOSPACE from the function?

> +{
> +	uint32_t delta = fdt_get_max_phandle(fdt);
> +	int ret;
> +
> +	FDT_RO_PROBE(fdt);
> +	FDT_RO_PROBE(fdto);
> +
> +	*fdto_nospace = 0;
> +
> +	ret = overlay_adjust_local_phandles(fdto, delta);
> +	if (ret) {
> +		if (ret == -FDT_ERR_NOSPACE)
> +			*fdto_nospace = 1;
> +		goto err;
> +	}
> +
> +	ret = overlay_update_local_references(fdto, delta);
> +	if (ret) {
> +		if (ret == -FDT_ERR_NOSPACE)
> +			*fdto_nospace = 1;
> +		goto err;
> +	}
> +
> +	ret = overlay_fixup_phandles(fdt, fdto);
> +	if (ret)
> +		goto err;
> +
> +	ret = overlay_merge(fdt, fdto);
> +	if (ret)
> +		goto err;
> +
> +	ret = overlay_symbol_update(fdt, fdto);
> +	if (ret)
> +		goto err;
> +
> +	/*
> +	 * The overlay has been damaged, erase its magic.
> +	 */
> +	fdt_set_magic(fdto, ~0);
> +
> +	return 0;
> +
> +err:
> +	/*
> +	 * The overlay might have been damaged, erase its magic.
> +	 */
> +	fdt_set_magic(fdto, ~0);
> +
> +	/*
> +	 * The base device tree might have been damaged, erase its

This comment is no longer accurate in the new context.

> +	 * magic.
> +	 */
> +	if (!*fdto_nospace)
> +		fdt_set_magic(fdt, ~0);
> +
> +	return ret;
> +}
> diff --git a/libfdt/libfdt.h b/libfdt/libfdt.h
> index b5e72001d115..664b141d5334 100644
> --- a/libfdt/libfdt.h
> +++ b/libfdt/libfdt.h
> @@ -2313,6 +2313,24 @@ int fdt_overlay_apply(void *fdt, void *fdto);
>  int fdt_overlay_target_offset(const void *fdt, const void *fdto,
>  			      int fragment_offset, char const **pathp);
> 
> +/**
> + * fdt_overlay_merge - Merge two overlays into one
> + * @fdt: pointer to the first device tree overlay blob
> + * @fdto: pointer to the second device tree overlay blob
> + * @fdto_nospace: indicates if FDT_ERR_NOSPACE error code applies to @fdto
> + *
> + * fdt_overlay_merge() will merge second overlay blob into first overlay blob.
> + *
> + * Expect the first device tree to be modified, even if the function
> + * returns an error.
> + *
> + * returns:
> + *	0, on success
> + *	-FDT_ERR_NOSPACE, there's not enough space in first device tree blob
> + *	-FDT_ERR_BADVALUE
> + */
> +int fdt_overlay_merge(void *fdt, void *fdto, int *fdto_nospace);
> +
>  /**********************************************************************/
>  /* Debugging / informational functions                                */
>  /**********************************************************************/

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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