Re: [RFC PATCH 35/77] Add support for FDT_IMPORT_SYM dtb tag

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

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On Mon, Jan 12, 2026 at 03:19:25PM +0100, Herve Codina wrote:
> The FDT_IMPORT_SYM dtb tag is a meta-data tag defining an imported
> symbol. It can be present globally in an addon dtb (i.e. outside nodes
> definition) meaning that this symbol needs to be resolved when the dtb
> is applied.
> 
> The tag is followed by two values and possible alignment paddings:
>   - name (string including \0)
>       The import symbol name. I.e. the name used to reference this
>       imported symbol.
>   - padding:
>       Padding (0x00) added to have the next value aligned on 32bit.
>   - compatible (string including \0)
>       The compatible string that can be used for symbol resolution.
>       This string can be an empty string if it is not relevant.
>   - padding:
>       Padding (0x00) added to have the next value aligned on 32bit.
> 
> Example:
>   FDT_IMPORT_SYM 'foo1' 0x00 0x00 0x00 'bar,foo'
> 
>   This means that 'foo1' is an imported symbol and it is 'bar,foo'
>   compatible.
> 
>   This is what is encoded in the dtb when the related dts has the
>   following imported symbol defined:
>     /import/ foo1: "bar,foo";
> 
> If several symbols are imported, several FDT_IMPORT_SYM are present.
> Each of them defining one imported symbol. For instance, importing
> 'foo1' ("bar,foo" compatible) and 'baz1' ("bar,baz" compatible) leads
> to the following sequence:
>   FDT_IMPORT_SYM 'foo1' 0x00 0x00 0x00 'bar,foo'
>   FDT_IMPORT_SYM 'baz1' 0x00 0x00 0x00 'bar,baz'
> 
> If FDT_IMPORT_SYM tags are present in the dtb, they are present after
> the root node definition (i.e. after the FDT_END_NODE related to the
> first FDT_BEGIN_NODE).
> 
> Add support for this new dtb tag.

Since these are global to the whole tree, would a new block make more
sense?

> 
> Signed-off-by: Herve Codina <herve.codina@bootlin.com>
> ---
>  fdtdump.c    | 11 +++++++++
>  flattree.c   | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++-
>  libfdt/fdt.c | 28 ++++++++++++++++++++++
>  libfdt/fdt.h |  1 +
>  4 files changed, 107 insertions(+), 1 deletion(-)
> 
> diff --git a/fdtdump.c b/fdtdump.c
> index 8baadc4..04e6e38 100644
> --- a/fdtdump.c
> +++ b/fdtdump.c
> @@ -198,6 +198,17 @@ static void dump_blob(void *blob, bool debug)
>  			continue;
>  		}
>  
> +		if (tag == FDT_IMPORT_SYM) {
> +			s = p;
> +			p = PALIGN(p + strlen(s) + 1, 4);
> +			t = p;
> +			p = PALIGN(p + strlen(t) + 1, 4);
> +
> +			printf("%*s// [FDT_IMPORT_SYM] '%s' (%s)\n", depth * shift, "",
> +				s, t);
> +			continue;
> +		}
> +
>  		fprintf(stderr, "%*s ** Unknown tag 0x%08"PRIx32"\n", depth * shift, "", tag);
>  		break;
>  	}
> diff --git a/flattree.c b/flattree.c
> index 47a289f..add02f0 100644
> --- a/flattree.c
> +++ b/flattree.c
> @@ -50,6 +50,7 @@ struct emitter {
>  	void (*ref_phandle)(void *);
>  	void (*export_sym)(void *);
>  	void (*export_sym_ref)(void *);
> +	void (*import_sym)(void *);
>  };
>  
>  static void bin_emit_cell(void *e, cell_t val)
> @@ -119,6 +120,11 @@ static void bin_emit_export_sym_ref(void *e)
>  	bin_emit_cell(e, FDT_EXPORT_SYM_REF);
>  }
>  
> +static void bin_emit_import_sym(void *e)
> +{
> +	bin_emit_cell(e, FDT_IMPORT_SYM);
> +}
> +
>  static struct emitter bin_emitter = {
>  	.cell = bin_emit_cell,
>  	.string = bin_emit_string,
> @@ -131,6 +137,7 @@ static struct emitter bin_emitter = {
>  	.ref_phandle = bin_emit_ref_phandle,
>  	.export_sym = bin_emit_export_sym,
>  	.export_sym_ref = bin_emit_export_sym_ref,
> +	.import_sym = bin_emit_import_sym,
>  };
>  
>  static void emit_label(FILE *f, const char *prefix, const char *label)
> @@ -274,6 +281,17 @@ static void asm_emit_export_sym_ref(void *e)
>  	asm_emit_cell(e, FDT_EXPORT_SYM_REF);
>  }
>  
> +static void asm_emit_import_sym(void *e)
> +{
> +	/*
> +	 * Import symbols are an feature introduced for addons.
> +	 * Addons device-tree blob have to reason to be in the asm format.
> +	 *
> +	 * Need to be implemented if really needed.
> +	 */
> +	die("FDT_IMPORT_SYM not supported in asm output\n");
> +}
> +
>  static struct emitter asm_emitter = {
>  	.cell = asm_emit_cell,
>  	.string = asm_emit_string,
> @@ -286,6 +304,7 @@ static struct emitter asm_emitter = {
>  	.ref_phandle = asm_emit_ref_phandle,
>  	.export_sym = asm_emit_export_sym,
>  	.export_sym = asm_emit_export_sym_ref,
> +	.import_sym = asm_emit_import_sym,
>  };
>  
>  static int stringtable_insert(struct data *d, const char *str)
> @@ -461,6 +480,26 @@ static void make_fdt_header(struct fdt_header *fdt,
>  		fdt->dt_flags = cpu_to_fdt32(dt_flags);
>  }
>  
> +static void flatten_imports(struct symbol *importsymlist, struct emitter *emit,
> +			 void *etarget, struct version_info *vi, uint32_t dt_flags)
> +{
> +	struct symbol *importsym;
> +
> +	if (!(vi->flags & FTF_EXPORT_IMPORT_SYM))
> +		return;
> +
> +	if (!(dt_flags & FDT_FLAG_ADDON) && importsymlist)
> +		die("Only addons can have an import list\n");
> +
> +	for_each_symbol(importsymlist, importsym) {
> +		emit->import_sym(etarget);
> +		emit->string(etarget, importsym->name, 0);
> +		emit->align(etarget, sizeof(cell_t));
> +		emit->string(etarget, importsym->compatible, 0);
> +		emit->align(etarget, sizeof(cell_t));
> +	}
> +}
> +
>  void dt_to_blob(FILE *f, struct dt_info *dti, int version)
>  {
>  	struct version_info *vi = NULL;
> @@ -483,6 +522,7 @@ void dt_to_blob(FILE *f, struct dt_info *dti, int version)
>  	dt_flags |= dti->dtsflags & DTSF_ADDON ? FDT_FLAG_ADDON : 0;
>  
>  	flatten_tree(dti->dt, &bin_emitter, &dtbuf, &strbuf, vi);
> +	flatten_imports(dti->importsymlist, &bin_emitter, &dtbuf, vi, dt_flags);
>  	bin_emit_cell(&dtbuf, FDT_END);
>  
>  	reservebuf = flatten_reserve_list(dti->reservelist, vi);
> @@ -574,6 +614,9 @@ void dt_to_asm(FILE *f, struct dt_info *dti, int version)
>  	const char *symprefix = "dt";
>  	uint32_t dt_flags = 0;
>  
> +	if (dti->importsymlist)
> +		die("Import symbols not supported in asm format\n");
> +
>  	for (i = 0; i < ARRAY_SIZE(version_table); i++) {
>  		if (version_table[i].version == version)
>  			vi = &version_table[i];
> @@ -810,6 +853,16 @@ static struct property *flat_read_property(struct inbuf *dtbuf,
>  	return build_property(name, val, NULL);
>  }
>  
> +static struct symbol *flat_read_importsym(struct inbuf *inb)
> +{
> +	const char *name;
> +	const char *compatible;
> +
> +	name = flat_read_string(inb);
> +	compatible =  flat_read_string(inb);
> +
> +	return build_importsym(name, compatible, NULL);
> +}
>  
>  static struct reserve_info *flat_read_mem_reserve(struct inbuf *inb)
>  {
> @@ -984,6 +1037,8 @@ struct dt_info *dt_from_blob(const char *fname)
>  	int sizeleft;
>  	struct reserve_info *reservelist;
>  	struct node *tree;
> +	struct symbol *importsymlist = NULL;
> +	struct symbol *importsym;
>  	uint32_t val;
>  	int flags = 0;
>  	unsigned int dtsflags = 0;
> @@ -1098,6 +1153,17 @@ struct dt_info *dt_from_blob(const char *fname)
>  	tree = unflatten_tree(&dtbuf, &strbuf, "", flags);
>  
>  	val = flat_read_word(&dtbuf);
> +
> +	if (dtsflags & DTSF_ADDON) {
> +		if (flags & FTF_EXPORT_IMPORT_SYM) {
> +			while (val == FDT_IMPORT_SYM) {
> +				importsym = flat_read_importsym(&dtbuf);
> +				add_symbol(&importsymlist, importsym);
> +				val = flat_read_word(&dtbuf);
> +			}
> +		}
> +	}
> +
>  	if (val != FDT_END)
>  		die("Device tree blob doesn't end with FDT_END\n");
>  
> @@ -1105,5 +1171,5 @@ struct dt_info *dt_from_blob(const char *fname)
>  
>  	fclose(f);
>  
> -	return build_dt_info(DTSF_V1 | dtsflags, reservelist, tree, boot_cpuid_phys, NULL);
> +	return build_dt_info(DTSF_V1 | dtsflags, reservelist, tree, boot_cpuid_phys, importsymlist);
>  }
> diff --git a/libfdt/fdt.c b/libfdt/fdt.c
> index febfa71..c169dd9 100644
> --- a/libfdt/fdt.c
> +++ b/libfdt/fdt.c
> @@ -271,6 +271,23 @@ uint32_t fdt_next_tag_full(const void *fdt, int startoffset, int *nextoffset)
>  			return FDT_END; /* premature end */
>  		break;
>  
> +	case FDT_IMPORT_SYM:
> +		/* Skip name */
> +		do {
> +			p = fdt_offset_ptr(fdt, offset++, 1);
> +		} while (p && (*p != '\0'));
> +		if (!can_assume(VALID_DTB) && !p)
> +			return FDT_END; /* premature end */
> +		offset = FDT_CELLALIGN(offset);
> +
> +		/* Skip compatible */
> +		do {
> +			p = fdt_offset_ptr(fdt, offset++, 1);
> +		} while (p && (*p != '\0'));
> +		if (!can_assume(VALID_DTB) && !p)
> +			return FDT_END; /* premature end */
> +		break;
> +
>  	default:
>  		return FDT_END;
>  	}
> @@ -286,6 +303,7 @@ uint32_t fdt_next_tag(const void *fdt, int startoffset, int *nextoffset)
>  {
>  	uint32_t tag, tmp_tag;
>  	int tmp_offset, tmp_next;
> +	int is_skip_to_end = 0;
>  
>  	/* Retrieve next tag */
>  	tag = fdt_next_tag_full(fdt, startoffset, nextoffset);
> @@ -300,6 +318,9 @@ uint32_t fdt_next_tag(const void *fdt, int startoffset, int *nextoffset)
>  		case FDT_END_NODE:
>  		case FDT_PROP:
>  		case FDT_NOP:
> +			if (is_skip_to_end)
> +				break;
> +
>  		case FDT_END:
>  			/*
>  			 * Next tag is not a meta-data tag -> Ok this next tag
> @@ -320,6 +341,13 @@ uint32_t fdt_next_tag(const void *fdt, int startoffset, int *nextoffset)
>  			 */
>  			break;
>  
> +		case FDT_IMPORT_SYM:
> +			/* Those tags are available at the root level, after the
> +			 * root node -> Skip everything until FDT_END
> +			 */
> +			is_skip_to_end = 1;
> +			break;
> +
>  		default:
>  			break;
>  		}
> diff --git a/libfdt/fdt.h b/libfdt/fdt.h
> index c23723b..b6c23ef 100644
> --- a/libfdt/fdt.h
> +++ b/libfdt/fdt.h
> @@ -67,6 +67,7 @@ struct fdt_property {
>  #define FDT_EXPORT_SYM	0xa		/* export symbol: name, phandle value */
>  #define FDT_EXPORT_SYM_REF 0xb		/* export symbol: name, phandle value (maybe
>  					   unresolved), external label */
> +#define FDT_IMPORT_SYM	0xc		/* import symbol: name, compatible */
>  
>  #define FDT_V1_SIZE	(7*sizeof(fdt32_t))
>  #define FDT_V2_SIZE	(FDT_V1_SIZE + sizeof(fdt32_t))
> -- 
> 2.52.0
> 
> 

-- 
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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