kernel/install.d: store DT files uncompressed for U-Boot

U-Boot's btrfs driver can't decompress inline zstd extents, so small
zstd-compressed .dtbo overlays fail to load (zstd_decompress error 70).
On every kernel-install add, set the kernel rockchip/ DT dir and
/etc/kernel/dtbo to compression=none and rewrite the flipper base .dtb
+ overlays uncompressed (property alone only affects future writes).
Covers build-time and future/runtime kernel installs; clones inherit
via COW.

Also create /etc/kernel/dtbo compression=none in the image build,
before @Minimal_stock is snapshotted, so every profile root carries
the property via COW even ahead of the kernel-install hook.

Author: Yury-MonZon

debian-rk3576-img.yaml

@@ -110,6 +110,17 @@ actions:
       mkdir -p "$IMAGEMNTDIR/var/log/journal"
       chattr +C "$IMAGEMNTDIR/var/log/journal"
 
+  # U-Boot can't read inline-zstd .dtbo overlays, so the DT drop-in dir must be
+  # compression=none for later drop-ins to inherit it. Set before @Minimal_stock so
+  # every profile snapshot carries it via COW.
+  - action: run
+    description: Ship /etc/kernel/dtbo uncompressed for U-Boot overlay reads
+    chroot: false
+    command: |
+      set -eu
+      mkdir -p "$IMAGEMNTDIR/etc/kernel/dtbo"
+      btrfs property set "$IMAGEMNTDIR/etc/kernel/dtbo" compression none
+
   - action: overlay
     description: Copy Linux kernel packages into the target
     source: {{ $kerneldir }}

overlays/configs/kernel/install.d/85-flipper-dtb-uncompress.install

@@ -0,0 +1,48 @@
+#!/bin/sh
+# SPDX-License-Identifier: LGPL-2.1-or-later
+#
+# Store the device-tree files U-Boot reads UNCOMPRESSED. U-Boot's btrfs driver cannot
+# decompress *inline* zstd extents (regular extents are fine), so small zstd-compressed
+# .dtbo overlays fail to load ("zstd_decompress: failed to decompress: 70"). btrfs stores
+# small files inline, so overlays are exactly the affected case.
+#
+# Fix: set the DT dirs to compression=none (new files inherit it) and rewrite the small,
+# inline-prone files (the flipper base .dtb + every .dtbo) so their existing extents are
+# uncompressed too. Runs on every kernel-install add (build-time apt install AND any later
+# install), so new kernels are covered. Idempotent; no-op off btrfs. Ordered before
+# 90-loaderentry.install (which then references these files in devicetree-overlay=).
+set -e
+
+[ "$KERNEL_INSTALL_LAYOUT" = bls ] || exit 0
+[ "$1" = add ] || exit 0
+KERNEL_VERSION="$2"
+command -v btrfs >/dev/null 2>&1 || exit 0
+
+# rewrite a file so its data lands uncompressed (inherits the dir's compression=none);
+# --reflink=never forces a real copy, not an extent-sharing reflink. Best-effort.
+uncompress() {
+	[ -f "$1" ] || return 0
+	if cp --reflink=never -p "$1" "$1.tmp" && mv -f "$1.tmp" "$1"; then return 0; fi
+	rm -f "$1.tmp" 2>/dev/null || :
+	echo "85-flipper-dtb-uncompress: could not rewrite $1 uncompressed" >&2
+}
+
+# kernel-shipped DTBs/overlays (build-kernel-bsp installs them here, board-prefixed)
+D="/usr/lib/linux-image-$KERNEL_VERSION/rockchip"
+if [ -d "$D" ]; then
+	btrfs property set "$D" compression none 2>/dev/null || exit 0   # not btrfs -> nothing to do
+	for f in "$D"/rk3576-flipper-one*.dtb "$D"/*.dtbo; do
+		uncompress "$f"
+	done
+fi
+
+# user overlay drop-in dir (version-independent): create it uncompressed so a later
+# `cp x.dtbo /etc/kernel/dtbo/` inherits compression=none, AND rewrite anything already
+# there in case it was dropped in while the dir was still compressible.
+install -d -m 755 /etc/kernel/dtbo
+btrfs property set /etc/kernel/dtbo compression none 2>/dev/null || :
+for f in /etc/kernel/dtbo/*.dtbo; do
+	uncompress "$f"
+done
+
+exit 0