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Message-ID: <aWiH1QRmj8rZY2Nk@zatzit>
Date: Thu, 15 Jan 2026 17:23:17 +1100
From: David Gibson <david@...son.dropbear.id.au>
To: Herve Codina <herve.codina@...tlin.com>
Cc: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk@...nel.org>,
Conor Dooley <conor+dt@...nel.org>,
Ayush Singh <ayush@...gleboard.org>,
Geert Uytterhoeven <geert@...ux-m68k.org>,
devicetree-compiler@...r.kernel.org, devicetree@...r.kernel.org,
linux-kernel@...r.kernel.org, devicetree-spec@...r.kernel.org,
Hui Pu <hui.pu@...ealthcare.com>,
Ian Ray <ian.ray@...ealthcare.com>,
Luca Ceresoli <luca.ceresoli@...tlin.com>,
Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [RFC PATCH 25/77] Add support for FDT_EXPORT_SYM dtb tag
On Mon, Jan 12, 2026 at 03:19:15PM +0100, Herve Codina wrote:
> The FDT_EXPORT_SYM dtb tag is a meta-data tag defining an exported
> symbol. It can be present in a node bloc meaning that a symbol is
> exported at this node level. The node pointed to by this symbol is a
> local node and identified by a phandle value.
>
> The tag is followed by two values and a possible alignment padding:
> - name (string including \0)
> The export symbol name. I.e. the name used to reference this
> exported symbol.
> - padding:
> Padding (0x00) added to have the next value aligned on 32bit.
> - phandle (32bit)
> The phandle value identifying the node referenced by this
> symbol.
I'd suggest putting the phandle before the name, to avoid internal
padding.
>
> Example:
> FDT_EXPORT_SYM 'foo1' 0x00 0x00 0x00 0x00000004
>
> This means that 'foo1' is an exported symbol and the node referenced
> by this symbol is the node with the phandle value equals 4.
>
> This is what is encoded in the dtb when the related dts has the
> following exported symbol defined:
> /export/ foo1: &foo;
> with 'foo' a reference to an existing node where the phandle value is
> 0x0000004.
>
> If several symbols are exported at a given node level, several
> FDT_EXPORT_SYM are present. Each of them defining one symbol.
>
> For instance, exporting 'foo' pointing to phandle 4 and 'bar' pointing
> to phandle 8 leads to the following sequence:
> FDT_EXPORT_SYM 'foo' 0x00 0x00 0x00 0x00000004
> FDT_EXPORT_SYM 'bar' 0x00 0x00 0x00 0x00000008
>
> Add support for this new dtb tag.
>
> Signed-off-by: Herve Codina <herve.codina@...tlin.com>
> ---
> dtc.h | 1 +
> fdtdump.c | 12 +++++++++-
> flattree.c | 47 ++++++++++++++++++++++++++++++++++++++--
> libfdt/fdt.c | 17 +++++++++++++++
> libfdt/fdt.h | 2 ++
> libfdt/libfdt_internal.h | 1 +
> livetree.c | 2 +-
> 7 files changed, 78 insertions(+), 4 deletions(-)
>
> diff --git a/dtc.h b/dtc.h
> index 024e172..22816ba 100644
> --- a/dtc.h
> +++ b/dtc.h
> @@ -289,6 +289,7 @@ struct symbol *build_exportsym(const char *name, const char *ref, cell_t phandle
> struct srcpos *srcpos);
> struct symbol *chain_symbol(struct symbol *first, struct symbol *list);
> struct symbol *reverse_symbol(struct symbol *list);
> +void add_symbol(struct symbol **list, struct symbol *new);
>
> struct node *build_node(struct property *proplist, struct node *children,
> struct symbol *exportsymlist, struct srcpos *srcpos);
> diff --git a/fdtdump.c b/fdtdump.c
> index 9b6f41a..d1af5b6 100644
> --- a/fdtdump.c
> +++ b/fdtdump.c
> @@ -57,7 +57,7 @@ static void dump_blob(void *blob, bool debug)
> const char *p_strings = (const char *)blob + off_str;
> uint32_t version = fdt32_to_cpu(bph->version);
> uint32_t totalsize = fdt32_to_cpu(bph->totalsize);
> - uint32_t tag, offset;
> + uint32_t tag, offset, val32;
> const char *p, *s, *t;
> const char *last_prop_name = NULL;
> int depth, sz, shift;
> @@ -176,6 +176,16 @@ static void dump_blob(void *blob, bool debug)
> continue;
> }
>
> + if (tag == FDT_EXPORT_SYM) {
> + s = p;
> + p = PALIGN(p + strlen(s) + 1, 4);
> + val32 = fdt32_to_cpu(GET_CELL(p));
> +
> + printf("%*s// [FDT_EXPORT_SYM] '%s' -> phandle 0x%08"PRIx32"\n",
> + depth * shift, "", s, val32);
> + continue;
> + }
> +
> fprintf(stderr, "%*s ** Unknown tag 0x%08"PRIx32"\n", depth * shift, "", tag);
> break;
> }
> diff --git a/flattree.c b/flattree.c
> index 36b795d..bd52e81 100644
> --- a/flattree.c
> +++ b/flattree.c
> @@ -15,6 +15,7 @@
> #define FTF_NOPS 0x40
> #define FTF_REF_XXX 0x80
> #define FTF_DTFLAGS 0x100
> +#define FTF_EXPORT_IMPORT_SYM 0x200
>
> static struct version_info {
> int version;
> @@ -34,7 +35,7 @@ static struct version_info {
> FTF_BOOTCPUID|FTF_STRTABSIZE|FTF_STRUCTSIZE|FTF_NOPS},
> {18, 18, FDT_V18_SIZE,
> FTF_BOOTCPUID|FTF_STRTABSIZE|FTF_STRUCTSIZE|FTF_NOPS|FTF_REF_XXX|
> - FTF_DTFLAGS},
> + FTF_DTFLAGS|FTF_EXPORT_IMPORT_SYM},
> };
>
> struct emitter {
> @@ -47,6 +48,7 @@ struct emitter {
> void (*property)(void *, struct label *labels);
> void (*ref_local)(void *);
> void (*ref_phandle)(void *);
> + void (*export_sym)(void *);
> };
>
> static void bin_emit_cell(void *e, cell_t val)
> @@ -106,6 +108,11 @@ static void bin_emit_ref_phandle(void *e)
> bin_emit_cell(e, FDT_REF_PHANDLE);
> }
>
> +static void bin_emit_export_sym(void *e)
> +{
> + bin_emit_cell(e, FDT_EXPORT_SYM);
> +}
> +
> static struct emitter bin_emitter = {
> .cell = bin_emit_cell,
> .string = bin_emit_string,
> @@ -116,6 +123,7 @@ static struct emitter bin_emitter = {
> .property = bin_emit_property,
> .ref_local = bin_emit_ref_local,
> .ref_phandle = bin_emit_ref_phandle,
> + .export_sym = bin_emit_export_sym,
> };
>
> static void emit_label(FILE *f, const char *prefix, const char *label)
> @@ -243,6 +251,14 @@ static void asm_emit_ref_phandle(void *e)
> asm_emit_cell(e, FDT_REF_PHANDLE);
> }
>
> +static void asm_emit_export_sym(void *e)
> +{
> + FILE *f = e;
> +
> + fprintf(f, "\t/* FDT_EXPORT_SYM */\n");
> + asm_emit_cell(e, FDT_EXPORT_SYM);
> +}
> +
> static struct emitter asm_emitter = {
> .cell = asm_emit_cell,
> .string = asm_emit_string,
> @@ -253,6 +269,7 @@ static struct emitter asm_emitter = {
> .property = asm_emit_property,
> .ref_local = asm_emit_ref_local,
> .ref_phandle = asm_emit_ref_phandle,
> + .export_sym = asm_emit_export_sym,
> };
>
> static int stringtable_insert(struct data *d, const char *str)
> @@ -274,6 +291,7 @@ static void flatten_tree(struct node *tree, struct emitter *emit,
> void *etarget, struct data *strbuf,
> struct version_info *vi)
> {
> + struct symbol *exportsym;
> struct property *prop;
> struct node *child;
> bool seen_name_prop = false;
> @@ -342,6 +360,18 @@ static void flatten_tree(struct node *tree, struct emitter *emit,
> emit->align(etarget, sizeof(cell_t));
> }
>
> + if (vi->flags & FTF_EXPORT_IMPORT_SYM) {
> + for_each_symbol(tree->exportsymlist, exportsym) {
> + if (exportsym->is_local) {
> + emit->export_sym(etarget);
> + emit->string(etarget, exportsym->name, 0);
> + emit->align(etarget, sizeof(cell_t));
> + emit->cell(etarget, exportsym->phandle);
> + continue;
> + }
> + }
> + }
> +
> for_each_child(tree, child) {
> flatten_tree(child, emit, etarget, strbuf, vi);
> }
> @@ -820,9 +850,11 @@ static struct node *unflatten_tree(struct inbuf *dtbuf,
> node->name = xstrdup(flatname);
>
> do {
> + struct symbol *exportsym;
> struct property *prop;
> struct node *child;
> struct marker *m;
> + cell_t phandle;
>
> val = flat_read_word(dtbuf);
> switch (val) {
> @@ -876,6 +908,17 @@ static struct node *unflatten_tree(struct inbuf *dtbuf,
> prop->val = data_append_markers(prop->val, m);
> break;
>
> + case FDT_EXPORT_SYM:
> + if (!(flags & FTF_EXPORT_IMPORT_SYM))
> + die("FDT_EXPORT_SYM tag found in flat tree"
> + " version <18\n");
> + str = flat_read_string(dtbuf);
> + phandle = flat_read_word(dtbuf);
> + exportsym = build_exportsym(str, NULL, phandle, NULL);
> + exportsym->is_local = true;
> + add_symbol(&node->exportsymlist, exportsym);
> + break;
> +
> default:
> die("Invalid opcode word %08x in device tree blob\n",
> val);
> @@ -996,7 +1039,7 @@ struct dt_info *dt_from_blob(const char *fname)
> }
>
> if (version >= 18) {
> - flags |= FTF_REF_XXX | FTF_DTFLAGS;
> + flags |= FTF_REF_XXX | FTF_DTFLAGS | FTF_EXPORT_IMPORT_SYM;
> dtsflags |= fdt32_to_cpu(fdt->dt_flags) & FDT_FLAG_ADDON ?
> DTSF_ADDON : 0;
> }
> diff --git a/libfdt/fdt.c b/libfdt/fdt.c
> index 8f3c35d..44d7399 100644
> --- a/libfdt/fdt.c
> +++ b/libfdt/fdt.c
> @@ -232,6 +232,22 @@ uint32_t fdt_next_tag_full(const void *fdt, int startoffset, int *nextoffset)
> return FDT_END; /* premature end */
> break;
>
> + case FDT_EXPORT_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 phandle */
> + tmp32p = fdt_offset_ptr(fdt, offset, sizeof(*tmp32p));
> + if (!can_assume(VALID_DTB) && !tmp32p)
> + return FDT_END; /* premature end */
> + offset += sizeof(fdt32_t);
> + break;
> +
> default:
> return FDT_END;
> }
> @@ -273,6 +289,7 @@ uint32_t fdt_next_tag(const void *fdt, int startoffset, int *nextoffset)
>
> case FDT_REF_LOCAL:
> case FDT_REF_PHANDLE:
> + case FDT_EXPORT_SYM:
> /*
> * Next tag is a meta-data tag present in the middle
> * of the structure -> Skip it and look at next one
> diff --git a/libfdt/fdt.h b/libfdt/fdt.h
> index 94f65e6..e85bc07 100644
> --- a/libfdt/fdt.h
> +++ b/libfdt/fdt.h
> @@ -53,6 +53,7 @@ struct fdt_property {
> #define FDT_MAGIC 0xd00dfeed /* 4: version, 4: total size */
> #define FDT_FLAG_ADDON 0x1
> #define FDT_TAGSIZE sizeof(fdt32_t)
> +#define FDT_CELLSIZE sizeof(fdt32_t)
>
> #define FDT_BEGIN_NODE 0x1 /* Start node: full name */
> #define FDT_END_NODE 0x2 /* End node */
> @@ -63,6 +64,7 @@ struct fdt_property {
> #define FDT_REF_PHANDLE 0x6 /* external phandle reference: offset,
> external label */
> #define FDT_END 0x9
> +#define FDT_EXPORT_SYM 0xa /* export symbol: name, phandle value */
>
> #define FDT_V1_SIZE (7*sizeof(fdt32_t))
> #define FDT_V2_SIZE (FDT_V1_SIZE + sizeof(fdt32_t))
> diff --git a/libfdt/libfdt_internal.h b/libfdt/libfdt_internal.h
> index 0e103ca..d80d218 100644
> --- a/libfdt/libfdt_internal.h
> +++ b/libfdt/libfdt_internal.h
> @@ -9,6 +9,7 @@
>
> #define FDT_ALIGN(x, a) (((x) + (a) - 1) & ~((a) - 1))
> #define FDT_TAGALIGN(x) (FDT_ALIGN((x), FDT_TAGSIZE))
> +#define FDT_CELLALIGN(x) (FDT_ALIGN((x), FDT_CELLSIZE))
>
> int32_t fdt_ro_probe_(const void *fdt);
> #define FDT_RO_PROBE(fdt) \
> diff --git a/livetree.c b/livetree.c
> index 7cf3ee5..7efa1da 100644
> --- a/livetree.c
> +++ b/livetree.c
> @@ -74,7 +74,7 @@ struct symbol *reverse_symbol(struct symbol *list)
> return head;
> }
>
> -static void add_symbol(struct symbol **list, struct symbol *new)
> +void add_symbol(struct symbol **list, struct symbol *new)
> {
> struct symbol **s;
>
> --
> 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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