linux-kernel - [RFC PATCH 03/77] libfdt: Introduce fdt_next_tag_full() and use it in fdt_next

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Message-ID: <20260112142009.1006236-4-herve.codina@bootlin.com>
Date: Mon, 12 Jan 2026 15:18:53 +0100
From: Herve Codina <herve.codina@...tlin.com>
To: David Gibson <david@...son.dropbear.id.au>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>
Cc: 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>,
	Herve Codina <herve.codina@...tlin.com>
Subject: [RFC PATCH 03/77] libfdt: Introduce fdt_next_tag_full() and use it in fdt_next_tag()

In v18 dtb new tags are added. Prepare libfdt to handle those new tags.

Keep fdt_next_tag() handling only existing tags and introduce
fdt_next_tag_full() to handle new tags.

fdt_next_tag() uses fdt_next_tag_full() but it will filter out new tags
when they are introduced to have those new tags transparent for existing
fdt_next_tag() callers.

Code that will need to handle those new tags will use explicitly
fdt_next_tag_full() to have access to them when they need to.

No new tags have been introduced yet and modifications done here prepare
their introduction.

Signed-off-by: Herve Codina <herve.codina@...tlin.com>
---
 libfdt/fdt.c       | 35 ++++++++++++++++++++++++++++++++++-
 libfdt/libfdt.h    | 18 ++++++++++++++++++
 libfdt/version.lds |  1 +
 3 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/libfdt/fdt.c b/libfdt/fdt.c
index 95f644c..ce051a0 100644
--- a/libfdt/fdt.c
+++ b/libfdt/fdt.c
@@ -159,7 +159,7 @@ const void *fdt_offset_ptr(const void *fdt, int offset, unsigned int len)
 	return fdt_offset_ptr_(fdt, offset);
 }
 
-uint32_t fdt_next_tag(const void *fdt, int startoffset, int *nextoffset)
+uint32_t fdt_next_tag_full(const void *fdt, int startoffset, int *nextoffset)
 {
 	const fdt32_t *tagp, *lenp;
 	uint32_t tag, len, sum;
@@ -220,6 +220,39 @@ uint32_t fdt_next_tag(const void *fdt, int startoffset, int *nextoffset)
 	return tag;
 }
 
+uint32_t fdt_next_tag(const void *fdt, int startoffset, int *nextoffset)
+{
+	uint32_t tag, tmp_tag;
+	int tmp_offset, tmp_next;
+
+	/* Retrieve next tag */
+	tag = fdt_next_tag_full(fdt, startoffset, nextoffset);
+
+	/* Look at next one to see what we need to do */
+	tmp_next = *nextoffset;
+	do {
+		tmp_offset = tmp_next;
+		tmp_tag = fdt_next_tag_full(fdt, tmp_offset, &tmp_next);
+		switch (tmp_tag) {
+		case FDT_BEGIN_NODE:
+		case FDT_END_NODE:
+		case FDT_PROP:
+		case FDT_NOP:
+		case FDT_END:
+			/* Next tag is not new tag introduced in v18 -> Ok */
+			*nextoffset = tmp_offset;
+			return tag;
+
+		default:
+			break;
+		}
+	} while (1);
+
+	/* We shouldn't reach this code */
+	*nextoffset = -FDT_ERR_BADSTRUCTURE;
+	return FDT_END;
+}
+
 int fdt_check_node_offset_(const void *fdt, int offset)
 {
 	if (!can_assume(VALID_INPUT)
diff --git a/libfdt/libfdt.h b/libfdt/libfdt.h
index c5cd35d..d1a9cd5 100644
--- a/libfdt/libfdt.h
+++ b/libfdt/libfdt.h
@@ -154,6 +154,24 @@ static inline void *fdt_offset_ptr_w(void *fdt, int offset, int checklen)
  */
 uint32_t fdt_next_tag(const void *fdt, int offset, int *nextoffset);
 
+/**
+ * fdt_next_tag_full - get next tag in the device tree without any filtering
+ * @fdt:	Pointer to the device tree blob
+ * @offset:	Offset within the blob to start searching
+ * @nextoffset:	Pointer to variable to store the offset of the next tag
+ *
+ * fdt_next_tag_full() returns the tag type of the next tag in the device tree
+ * blob starting from the given @offset. If @nextoffset is non-NULL, it will
+ * be set to the offset immediately following the tag.
+ * fdt_next_tag() can return only a subset of all possible tags performing some
+ * internal filtering. fdt_next_tag_full() doesn't perform this filtering.
+ *
+ * returns:
+ *	the tag type (FDT_BEGIN_NODE, FDT_END_NODE, FDT_PROP, FDT_NOP, FDT_END),
+ *	FDT_END, if offset is out of bounds
+ */
+uint32_t fdt_next_tag_full(const void *fdt, int offset, int *nextoffset);
+
 /*
  * External helpers to access words from a device tree blob. They're built
  * to work even with unaligned pointers on platforms (such as ARMv5) that don't
diff --git a/libfdt/version.lds b/libfdt/version.lds
index cbfef54..7e2dde2 100644
--- a/libfdt/version.lds
+++ b/libfdt/version.lds
@@ -52,6 +52,7 @@ LIBFDT_1.2 {
 		fdt_strerror;
 		fdt_offset_ptr;
 		fdt_next_tag;
+		fdt_next_tag_full;
 		fdt_appendprop;
 		fdt_create_empty_tree;
 		fdt_first_property_offset;
-- 
2.52.0