u-boot script to allow choosing between which rootfs part to boot (RAUC) - yocto

Question

I've managed to create an image with two rootfs partitions to run on my jetson nano with yocto/poky. I've followed the meta-rauc layer README and rauc user manual, to create the system.conf file and rauc_%.bbappend file and I am able to create bundles successfully.
As I understand, I need some sort of u-boot script:
In order to enable RAUC to switch the correct slot, its system configuration must specify the name of the respective slot from the bootloader’s perspective. You also have to set up an appropriate boot selection logic in the bootloader itself, either by scripting (as for GRUB, U-Boot) or by using dedicated boot selection infrastructure (such as bootchooser in Barebox).
The bootloader must also provide a set of variables the Linux userspace can modify in order to change boot order or priority.
Having this interface ready, RAUC will care for setting the boot logic appropriately. It will, for example, deactivate the slot to update before writing to it and reactivate it after having completed the installation successfully.
Do I make a script somewhere in the yocto layer or build folder or is it a script i need to put on the jetson nano after making the image? - and what would the contents of this script be?
**************************************************EDIT********************************************************
I've made this script:
test -n "${BOOT_ORDER}" || setenv BOOT_ORDER "system0 system1"
test -n "${BOOT_system0_LEFT}" || setenv BOOT_system0_LEFT 3
test -n "${BOOT_system1_LEFT}" || setenv BOOT_system1_LEFT 3
setenv bootargs
for BOOT_SLOT in "${BOOT_ORDER}"; do
if test "x${bootargs}" != "x"; then
# skip remaining slots
elif test "x${BOOT_SLOT}" = "xsystem0"; then
if test ${BOOT_system0_LEFT} -gt 0; then
setexpr BOOT_system0_LEFT ${BOOT_system0_LEFT} - 1
echo "Found valid slot system0, ${BOOT_system0_LEFT} attempts remaining"
setenv distro_bootpart "1"
setenv boot_line "mmc 1:1 any ${scriptaddr} /boot/extlinux/extlinux.conf"
setenv bootargs "${default_bootargs} root=/dev/mmcblk0p1 rauc.slot=system0"
fi
elif test "x${BOOT_SLOT}" = "xsystem1"; then
if test ${BOOT_system1_LEFT} -gt 0; then
setexpr BOOT_system1_LEFT ${BOOT_system1_LEFT} - 1
echo "Found valid slot system1, ${BOOT_system1_LEFT} attempts remaining"
setenv distro_bootpart "13"
setenv boot_line "mmc 1:D any ${scriptaddr} /boot/extlinux/extlinux.conf"
setenv bootargs "${default_bootargs} root=/dev/mmcblk0p13 rauc.slot=system1"
fi
fi
done
if test -n "${bootargs}"; then
saveenv
else
echo "No valid slot found, resetting tries to 3"
setenv BOOT_system0_LEFT 3
setenv BOOT_system1_LEFT 3
saveenv
reset
fi
sysboot ${boot_line}
And I i got this recipe recipes-bsp/u-boot/u-boot-script.bb in my meta-layer:
LICENSE = "GPLv2+"
LIC_FILES_CHKSUM = "file://Licenses/README;md5=30503fd321432fc713238f582193b78e"
S = "${WORKDIR}/git"
PACKAGE_ARCH = "${MACHINE_ARCH}"
DEPENDS = "u-boot-mkimage-native"
inherit deploy
BOOTSCRIPT ??= "${THISDIR}/uboot.sh"
do_mkimage () {
uboot-mkimage -A arm -O linux -T script -C none -a 0 -e 0 \
-n "boot script" -d ${BOOTSCRIPT} ${S}/boot.scr
}
addtask mkimage after do_compile before do_install
do_compile[noexec] = "1"
do_install () {
install -D -m 644 ${S}/boot.scr ${D}/boot.scr
}
do_deploy () {
install -D -m 644 ${D}/boot.scr \
${DEPLOYDIR}/boot.scr-${MACHINE}-${PV}-${PR}
cd ${DEPLOYDIR}
rm -f boot.scr-${MACHINE}
ln -sf boot.scr-${MACHINE}-${PV}-${PR} boot.scr-${MACHINE}
}
addtask deploy after do_install before do_build
FILES_${PN} += "/"
COMPATIBLE_MACHINE = "jetson-nano"
I can see that the script image is getting into work/jetson_nano_poky-linux/u-boot-tegra/2016.07.../git/ folder.
But how do I use it in u-boot? - How do i make sure this script is run automatically every boot?

Answer 1

U boot part of the default boot sequence tries to find a file named boot.src in the first partition from where it has booted.
if this file is found then it will try to run this script.
The commands put in the file can be based on RAUC syntax so that when RAUC gets activated in the user space it can update the same environment variables.
So RAUC handles the boot sequence via the commands put int the script file. RAUC has no way to directly alter the flow of U Boot boot up sequence