Re: [PATCH v3 07/15] fdtdump: Handle unknown tags

On Thu, 10 Sep 2026 15:25:06 +1000
David Gibson [off-list ref] wrote:

On Wed, Aug 26, 2026 at 10:31:38AM +0200, Herve Codina wrote:

quoted

The structured tag value definition introduced recently gives the
ability to ignore unknown tags without any error when they are read.

Add support for those structured tags in fdtdump and introduce a command
line option to dump unknown tags that should be ignored.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
---
 fdtdump.c                              |  45 ++++++-
 tests/Makefile.tests                   |   3 +-
 tests/meson.build                      |   2 +
 tests/run_tests.sh                     |  41 +++++++
 tests/treegen.c                        | 162 +++++++++++++++++++++++++
 tests/unknown_tags_can_skip.dtb.expect |  29 +++++
 6 files changed, 278 insertions(+), 4 deletions(-)
 create mode 100644 tests/unknown_tags_can_skip.dtb.expect

diff --git a/fdtdump.c b/fdtdump.c
index 0e7a2659..7a8b2784 100644
--- a/fdtdump.c
+++ b/fdtdump.c

@@ -44,7 +44,7 @@ static const char *tagname(uint32_t tag)
 #define dumpf(fmt, args...) \
 	do { if (debug) printf("// " fmt, ## args); } while (0)
 
-static void dump_blob(void *blob, bool debug)
+static void dump_blob(void *blob, bool debug, int dump_unknown)
 {
 	uintptr_t blob_off = (uintptr_t)blob;
 	struct fdt_header *bph = blob;

@@ -146,20 +146,55 @@ static void dump_blob(void *blob, bool debug)
 			continue;
 		}
 
+		if ((tag & FDT_TAG_STRUCTURED) && (tag & FDT_TAG_SKIP_SAFE)) {
+			sz = 0;
+			switch (tag & FDT_TAG_DATA_MASK) {
+			case FDT_TAG_DATA_NONE:
+				break;
+			case FDT_TAG_DATA_1CELL:
+				sz = FDT_CELLSIZE;
+				break;
+			case FDT_TAG_DATA_2CELLS:
+				sz = 2 * FDT_CELLSIZE;
+				break;
+			case FDT_TAG_DATA_VARLEN:
+				/* Get the length */
+				sz = fdt32_to_cpu(GET_CELL(p));
+				break;
+			}
+
+			if (dump_unknown) {  

I'd do this unconditionally: fdtdump is specifically for low-level and
debug dumping of a tree.  If you want pretty printing, dtc -Odts is
the thing to use.

Ok, will do it unconditionally.

quoted

+				printf("%*s// Unknown tag ignored: 0x%08"PRIx32", data len %d",
+				       depth * shift, "", tag, sz);  

"Unknown" seems redundant.  "ignored" is not really true - you're
dumping its contents, which is all that fdtdump could be asked to do
with it.  So I'd just say:

	// Tag 0x<tag>: <data>

Or maybe "Metadata tag" / "skippable tag" if we go with one of those
terms as I suggested on an earlier patch.

I am fine with 
  // Tag 0x<tag>: <data>

I will use that.

Best regards,
Hervé