From 1762568ec52b8b29b5bc3b870adb00f46178b51e Mon Sep 17 00:00:00 2001
From: Michael Brown <mcb30@ipxe.org>
Date: Fri, 30 May 2025 16:37:28 +0100
Subject: [PATCH] [fdt] Provide ability to locate the parent device node

Signed-off-by: Michael Brown <mcb30@ipxe.org>
---
 src/core/fdt.c         | 96 +++++++++++++++++++++++++++++++++++++++++-
 src/include/ipxe/fdt.h |  2 +
 src/tests/fdt_test.c   | 12 ++++++
 3 files changed, 109 insertions(+), 1 deletion(-)

diff --git a/src/core/fdt.c b/src/core/fdt.c
index c8ae4d949..b2ee0133b 100644
--- a/src/core/fdt.c
+++ b/src/core/fdt.c
@@ -181,7 +181,7 @@ static int fdt_next ( struct fdt *fdt, struct fdt_descriptor *desc ) {
  * @ret rc		Return status code
  */
 static int fdt_enter ( struct fdt *fdt, unsigned int offset,
-		struct fdt_descriptor *desc ) {
+		       struct fdt_descriptor *desc ) {
 	int rc;
 
 	/* Find begin node token */
@@ -212,6 +212,100 @@ static int fdt_enter ( struct fdt *fdt, unsigned int offset,
 	}
 }
 
+/**
+ * Find node relative depth
+ *
+ * @v fdt		Device tree
+ * @v offset		Starting node offset
+ * @v target		Target node offset
+ * @ret depth		Depth, or negative error
+ */
+static int fdt_depth ( struct fdt *fdt, unsigned int offset,
+		       unsigned int target ) {
+	struct fdt_descriptor desc;
+	int depth;
+	int rc;
+
+	/* Enter node */
+	if ( ( rc = fdt_enter ( fdt, offset, &desc ) ) != 0 )
+		return rc;
+
+	/* Find target node */
+	for ( depth = 0 ; depth >= 0 ; depth += desc.depth ) {
+
+		/* Describe token */
+		if ( ( rc = fdt_next ( fdt, &desc ) ) != 0 ) {
+			DBGC ( fdt, "FDT +%#04x has malformed node: %s\n",
+			       offset, strerror ( rc ) );
+			return rc;
+		}
+
+		/* Check for target node */
+		if ( desc.offset == target ) {
+			DBGC2 ( fdt, "FDT +%#04x has descendant node +%#04x "
+				"at depth +%d\n", offset, target, depth );
+			return depth;
+		}
+	}
+
+	DBGC ( fdt, "FDT +#%04x has no descendant node +%#04x\n",
+	       offset, target );
+	return -ENOENT;
+}
+
+/**
+ * Find parent node
+ *
+ * @v fdt		Device tree
+ * @v offset		Starting node offset
+ * @v parent		Parent node offset to fill in
+ * @ret rc		Return status code
+ */
+int fdt_parent ( struct fdt *fdt, unsigned int offset, unsigned int *parent ) {
+	struct fdt_descriptor desc;
+	int pdepth;
+	int depth;
+	int rc;
+
+	/* Find depth from root of tree */
+	depth = fdt_depth ( fdt, 0, offset );
+	if ( depth < 0 ) {
+		rc = depth;
+		return rc;
+	}
+	pdepth = ( depth - 1 );
+
+	/* Enter root node */
+	if ( ( rc = fdt_enter ( fdt, 0, &desc ) ) != 0 )
+		return rc;
+	*parent = desc.offset;
+
+	/* Find parent node */
+	for ( depth = 0 ; depth >= 0 ; depth += desc.depth ) {
+
+		/* Describe token */
+		if ( ( rc = fdt_next ( fdt, &desc ) ) != 0 ) {
+			DBGC ( fdt, "FDT +%#04x has malformed node: %s\n",
+			       offset, strerror ( rc ) );
+			return rc;
+		}
+
+		/* Record possible parent node */
+		if ( ( depth == pdepth ) && desc.name && ( ! desc.data ) )
+			*parent = desc.offset;
+
+		/* Check for target node */
+		if ( desc.offset == offset ) {
+			DBGC2 ( fdt, "FDT +%#04x has parent node at +%#04x\n",
+				offset, *parent );
+			return 0;
+		}
+	}
+
+	DBGC ( fdt, "FDT +#%04x has no parent node\n", offset );
+	return -ENOENT;
+}
+
 /**
  * Find child node
  *
diff --git a/src/include/ipxe/fdt.h b/src/include/ipxe/fdt.h
index 461b09632..d5a0d2179 100644
--- a/src/include/ipxe/fdt.h
+++ b/src/include/ipxe/fdt.h
@@ -170,6 +170,8 @@ fdt_reservations ( struct fdt *fdt ) {
 
 extern int fdt_describe ( struct fdt *fdt, unsigned int offset,
 			  struct fdt_descriptor *desc );
+extern int fdt_parent ( struct fdt *fdt, unsigned int offset,
+			unsigned int *parent );
 extern int fdt_path ( struct fdt *fdt, const char *path,
 		      unsigned int *offset );
 extern int fdt_alias ( struct fdt *fdt, const char *name,
diff --git a/src/tests/fdt_test.c b/src/tests/fdt_test.c
index 1dead2126..408f77c6d 100644
--- a/src/tests/fdt_test.c
+++ b/src/tests/fdt_test.c
@@ -263,6 +263,18 @@ static void fdt_test_exec ( void ) {
 	ok ( strcmp ( desc.data, "memory" ) == 0 );
 	ok ( desc.depth == 0 );
 
+	/* Verify parent lookup */
+	ok ( fdt_path ( &fdt, "/soc/ethernet@10090000/ethernet-phy@0",
+			&offset ) == 0 );
+	ok ( fdt_parent ( &fdt, offset, &offset ) == 0 );
+	ok ( fdt_describe ( &fdt, offset, &desc ) == 0 );
+	ok ( strcmp ( desc.name, "ethernet@10090000" ) == 0 );
+	ok ( fdt_parent ( &fdt, offset, &offset ) == 0 );
+	ok ( fdt_describe ( &fdt, offset, &desc ) == 0 );
+	ok ( strcmp ( desc.name, "soc" ) == 0 );
+	ok ( fdt_parent ( &fdt, offset, &offset ) == 0 );
+	ok ( offset == 0 );
+
 	/* Verify device tree creation */
 	image = image_memory ( "test.dtb", sifive_u, sizeof ( sifive_u ) );
 	ok ( image != NULL );
-- 
2.47.2