[PATCH] scripts/make-gpt.sh: reserve 8 MiB at start of GPT

[Alyssa Ross <hi@alyssa.is>]

In some cases (e.g. a product built on Spectrum where updates to both
firmware and OS are delivered by the same vendor), it makes sense to
include firmware in the same image as the OS.  This firmware tends to
live in the start of the image, after the GPT header.  (The exact
location varies by board.)

The 1 MiB reserved by sfdisk by default is not enough for this.  The
conventional value seems to be 8 MiB.  On NixOS, this is configurable
with the sdImage.firmwarePartitionOffset option, but this defaults to
8 MiB and I wasn't able to find any examples in searches on DuckDuckGo
and GitHub of anybody using any other value, so it looks like 8 MiB is
broadly acceptable.

Signed-off-by: Alyssa Ross <hi@alyssa.is>
---
Yuriy, if you'd like to test this patch, here's some documentation
about how to do that. :)

https://spectrum-os.org/doc/testing-patches.html

 scripts/make-gpt.sh | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/scripts/make-gpt.sh b/scripts/make-gpt.sh
index 56076d3..55f38d6 100755
--- a/scripts/make-gpt.sh
+++ b/scripts/make-gpt.sh
@@ -1,12 +1,13 @@
 #!/bin/sh -eu
 #
 # SPDX-FileCopyrightText: 2021-2022 Alyssa Ross <hi@alyssa.is>
+# SPDX-FileCopyrightText: 2022 Unikie
 # SPDX-License-Identifier: EUPL-1.2+
 #
 # usage: make-gpt.sh GPT_PATH PATH:PARTTYPE[:PARTUUID]...
 
 ONE_MiB=1048576
-TWO_MiB=2097152
+NINE_MiB=9437184
 
 # Prints the number of 1MiB blocks required to store the file named
 # $1.  We use 1MiB blocks because that's what sfdisk uses for
@@ -41,8 +42,9 @@ shift
 nl=$'\n'
 table="label: gpt"
 
-# Keep 1MiB free at the start, and 1MiB free at the end.
-gptBytes=$TWO_MiB
+# Keep 8MiB free at the start (to provide space for firmware if
+# required), and 1MiB free at the end.
+gptBytes=$NINE_MiB
 for partition; do
 	sizeMiB="$(sizeMiB "$(partitionPath "$partition")")"
 	table="$table${nl}size=${sizeMiB}MiB,$(awk -f "$scriptsDir/sfdisk-field.awk" -v partition="$partition")"
@@ -52,6 +54,7 @@ done
 rm -f "$out"
 truncate -s "$gptBytes" "$out"
 sfdisk "$out" <<EOF
+first-lba: 16384
 $table
 EOF
 
-- 
2.37.1