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Message-ID: <aWhB4fYnXCD2f6Uq@zatzit>
Date: Thu, 15 Jan 2026 12:24:49 +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 11/77] Add support for FDT_REF_PHANDLE dtb tag
On Mon, Jan 12, 2026 at 03:19:01PM +0100, Herve Codina wrote:
> The FDT_REF_PHANDLE dtb tag is similar to the FDT_REF_LOCAL tag except
> that it identifies a reference to an external phandle. The node
> referenced by the phandle is not present in the device-tree blob.
The names FDT_REF_PHANDLE and FDT_REF_LOCAL are perhaps a little
misleading - both are marking a phandle, the difference is in the form
of reference. That's quite difference from the distinction between
the REF_PHANDLE and REF_PATH marker types, where the difference is in
what the reference expands to in the property.
> The FDT_REF_PHANDLE dtb tag is a meta-data tag attached to a property.
>
> It indicates that the property defined before this tag (FDT_PROP) uses a
> phandle value and the node related to this phandle value is not local
> node (i.e. the node is not present in the device-tree blob). This tag
> can be available only in overlay or addon device-tree blobs. The phandle
> value used in the property has to be resolved when the device-tree blob
> is applied on top of a base device-tree.
This is kind of looking ahead, but does that imply that this tag is
only valid in addon dtbs?
> It is followed by two values and a possible alignment padding:
> - offset (32bit):
> Offset in the property data where the phandle is available.
> - label (string including \0):
> The label to use to resolve the phandle value.
I expect it will become apparent later in the series, but it would be
helpful to clarify what the scope of that label is. A single node?
The whole tree? Across a tree and all its possible addons?
> - padding:
> Padding (0x00) added to have the next tag aligned on 32bit.
>
> Example:
> FDT_PROP 0x00000008 xxxxxxxx 0x00 0x01 0x02 0x03 0xff 0xff 0xff 0xff
> FDT_REF_PHANDLE 0x00000004 "foo1" 0x00 0x00 0x00
>
> This means that at the offset 4 of the property data, the value
> (0xffffffff) is an unresolved phandle value and the related node is
> the node referenced by "foo1".
>
> This is what is encoded in the dtb when the related dts has a property
> with the value set to <0x00010203 &foo1> with 'foo1' a reference
> to an non local node.
>
> If several non local phandles are used in the property data, several
> FDT_REF_PHANDLE are present after the FDT_PROP tag. Each of them points
> with its offset value to the position of one phandle.
>
> For instance, if a first property with 8 bytes of data has a
> unresolved phandle value at offset 4 referenced by "foo" and a second
> property with 16 bytes of data has unresolved phandle values at offset 0
> and 8 referenced by "bar" and "baz", the following tags sequence is
> present:
> FDT_PROP 0x00000008 xxxxxxxx <data bytes>
> FDT_REF_PHANDLE 0x00000004 "foo" 0x00 0x00 0x00
> FDT_PROP 0x00000010 xxxxxxxx <data bytes>
> FDT_REF_LOCAL 0x00000000 "bar" 0x00 0x00 0x00
> FDT_REF_LOCAL 0x00000008 "baz" 0x00 0x00 0x00
>
> Add support for this new dtb tag.
>
> Suggested-by: David Gibson <david@...son.dropbear.id.au>
> Link: https://lore.kernel.org/all/aL-2fmYsbexEtpNp@zatzit/
> Signed-off-by: Herve Codina <herve.codina@...tlin.com>
> ---
> dtc.c | 12 ++++++++++++
> fdtdump.c | 10 ++++++++++
> flattree.c | 37 +++++++++++++++++++++++++++++++++++++
> libfdt/fdt.c | 16 ++++++++++++++++
> libfdt/fdt.h | 2 ++
> 5 files changed, 77 insertions(+)
>
> diff --git a/dtc.c b/dtc.c
> index d0b4de3..fe8e8e4 100644
> --- a/dtc.c
> +++ b/dtc.c
> @@ -336,6 +336,18 @@ int main(int argc, char *argv[])
> update_phandles_ref(dti);
> mark_local_phandles(dti);
>
> + /*
> + * With FDT_REF_PHANDLE added in dtbs, we need to identified
> + * if some unresolved phandle references are allowed in the dtb
> + * we have parsed (needed for process_check() to run properly).
> + *
> + * Identify plugin device-trees (overlays) based on specific node
> + * presence.
> + */
> + if (get_subnode(dti->dt, "__fixups__") ||
> + get_subnode(dti->dt, "__local_fixups__"))
> + dti->dtsflags |= DTSF_PLUGIN;
> +
> process_checks(force, dti);
>
> if (auto_label_aliases)
> diff --git a/fdtdump.c b/fdtdump.c
> index dffa9a6..7300280 100644
> --- a/fdtdump.c
> +++ b/fdtdump.c
> @@ -158,6 +158,16 @@ static void dump_blob(void *blob, bool debug)
> continue;
> }
>
> + if (tag == FDT_REF_PHANDLE) {
> + offset = fdt32_to_cpu(GET_CELL(p));
> + s = p;
> + p = PALIGN(p + strlen(s) + 1, 4);
> +
> + printf("%*s// [FDT_REF_PHANDLE] %s[%"PRIu32"], ref = %s\n",
> + depth * shift, "", last_prop_name, offset, s);
> + continue;
> + }
> +
> fprintf(stderr, "%*s ** Unknown tag 0x%08"PRIx32"\n", depth * shift, "", tag);
> break;
> }
> diff --git a/flattree.c b/flattree.c
> index 5c597ad..07f7545 100644
> --- a/flattree.c
> +++ b/flattree.c
> @@ -44,6 +44,7 @@ struct emitter {
> void (*endnode)(void *, struct label *labels);
> void (*property)(void *, struct label *labels);
> void (*ref_local)(void *);
> + void (*ref_phandle)(void *);
> };
>
> static void bin_emit_cell(void *e, cell_t val)
> @@ -98,6 +99,11 @@ static void bin_emit_ref_local(void *e)
> bin_emit_cell(e, FDT_REF_LOCAL);
> }
>
> +static void bin_emit_ref_phandle(void *e)
> +{
> + bin_emit_cell(e, FDT_REF_PHANDLE);
> +}
> +
> static struct emitter bin_emitter = {
> .cell = bin_emit_cell,
> .string = bin_emit_string,
> @@ -107,6 +113,7 @@ static struct emitter bin_emitter = {
> .endnode = bin_emit_endnode,
> .property = bin_emit_property,
> .ref_local = bin_emit_ref_local,
> + .ref_phandle = bin_emit_ref_phandle,
> };
>
> static void emit_label(FILE *f, const char *prefix, const char *label)
> @@ -226,6 +233,14 @@ static void asm_emit_ref_local(void *e)
> asm_emit_cell(e, FDT_REF_LOCAL);
> }
>
> +static void asm_emit_ref_phandle(void *e)
> +{
> + FILE *f = e;
> +
> + fprintf(f, "\t/* FDT_REF_PHANDLE */\n");
> + asm_emit_cell(e, FDT_REF_PHANDLE);
> +}
> +
> static struct emitter asm_emitter = {
> .cell = asm_emit_cell,
> .string = asm_emit_string,
> @@ -235,6 +250,7 @@ static struct emitter asm_emitter = {
> .endnode = asm_emit_endnode,
> .property = asm_emit_property,
> .ref_local = asm_emit_ref_local,
> + .ref_phandle = asm_emit_ref_phandle,
> };
>
> static int stringtable_insert(struct data *d, const char *str)
> @@ -299,6 +315,15 @@ static void flatten_tree(struct node *tree, struct emitter *emit,
> emit->cell(etarget, m->offset);
> continue;
> }
> +
> + if (m->ref[0] == '/')
> + die("Phandle uses a non local reference by path (%s)\n",
> + m->ref);
> +
> + emit->ref_phandle(etarget);
> + emit->cell(etarget, m->offset);
> + emit->string(etarget, m->ref, 0);
> + emit->align(etarget, sizeof(cell_t));
> }
> }
> }
> @@ -767,6 +792,7 @@ static struct node *unflatten_tree(struct inbuf *dtbuf,
> const char *flatname;
> uint32_t val;
> uint32_t offset;
> + const char *str;
>
> node = build_node(NULL, NULL, NULL);
>
> @@ -824,6 +850,17 @@ static struct node *unflatten_tree(struct inbuf *dtbuf,
> prop->val = data_append_markers(prop->val, m);
> break;
>
> + case FDT_REF_PHANDLE:
> + if (!(flags & FTF_REF_XXX))
> + die("REF_PHANDLE tag found in flat tree"
> + " version <18\n");
> +
> + offset = flat_read_word(dtbuf);
> + str = flat_read_string(dtbuf);
> + m = alloc_marker(offset, REF_PHANDLE, xstrdup(str));
> + prop->val = data_append_markers(prop->val, m);
> + break;
> +
> default:
> die("Invalid opcode word %08x in device tree blob\n",
> val);
> diff --git a/libfdt/fdt.c b/libfdt/fdt.c
> index 7268fb6..8f3c35d 100644
> --- a/libfdt/fdt.c
> +++ b/libfdt/fdt.c
> @@ -217,6 +217,21 @@ uint32_t fdt_next_tag_full(const void *fdt, int startoffset, int *nextoffset)
> offset += sizeof(fdt32_t);
> break;
>
> + case FDT_REF_PHANDLE:
> + /* Skip offset value */
> + tmp32p = fdt_offset_ptr(fdt, offset, sizeof(*tmp32p));
> + if (!can_assume(VALID_DTB) && !tmp32p)
> + return FDT_END; /* premature end */
> + offset += sizeof(fdt32_t);
> +
> + /* Skip ref */
> + 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;
> }
> @@ -257,6 +272,7 @@ uint32_t fdt_next_tag(const void *fdt, int startoffset, int *nextoffset)
> return tag;
>
> case FDT_REF_LOCAL:
> + case FDT_REF_PHANDLE:
> /*
> * 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 f8efdf1..530d2e5 100644
> --- a/libfdt/fdt.h
> +++ b/libfdt/fdt.h
> @@ -56,6 +56,8 @@ struct fdt_property {
> size, content */
> #define FDT_NOP 0x4 /* nop */
> #define FDT_REF_LOCAL 0x5 /* local phandle reference: offset */
> +#define FDT_REF_PHANDLE 0x6 /* external phandle reference: offset,
> + external label */
> #define FDT_END 0x9
>
> #define FDT_V1_SIZE (7*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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