linux-kernel - [RFC PATCH 04/15] libfdt: fdt_rw: Introduce fdt_downgrade

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Message-ID: <20260210173349.636766-5-herve.codina@bootlin.com>
Date: Tue, 10 Feb 2026 18:33:32 +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 04/15] libfdt: fdt_rw: Introduce fdt_downgrade_version()

Current code perform a version downgrade at one place only, the end of
fdt_rw_probe_().

In order to offer a finer grain and choose to downgrade or not depending
on the exact writes done, introduce fdt_downgrade_version() to perform
the downgrade operation.

The modification doesn't introduce any functional changes.

Signed-off-by: Herve Codina <herve.codina@...tlin.com>
---
 libfdt/fdt_rw.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/libfdt/fdt_rw.c b/libfdt/fdt_rw.c
index 7475caf..90ea14e 100644
--- a/libfdt/fdt_rw.c
+++ b/libfdt/fdt_rw.c
@@ -22,6 +22,12 @@ static int fdt_blocks_misordered_(const void *fdt,
 		    (fdt_off_dt_strings(fdt) + fdt_size_dt_strings(fdt)));
 }
 
+static void fdt_downgrade_version(void *fdt)
+{
+	if (!can_assume(LATEST) && fdt_version(fdt) > FDT_LAST_SUPPORTED_VERSION)
+		fdt_set_version(fdt, FDT_LAST_SUPPORTED_VERSION);
+}
+
 static int fdt_rw_probe_(void *fdt)
 {
 	if (can_assume(VALID_DTB))
@@ -33,9 +39,8 @@ static int fdt_rw_probe_(void *fdt)
 	if (fdt_blocks_misordered_(fdt, sizeof(struct fdt_reserve_entry),
 				   fdt_size_dt_struct(fdt)))
 		return -FDT_ERR_BADLAYOUT;
-	if (!can_assume(LATEST) && fdt_version(fdt) > 17)
-		fdt_set_version(fdt, 17);
 
+	fdt_downgrade_version(fdt);
 	return 0;
 }
 
-- 
2.52.0