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the industry standard "distro boot" boot flow for U-boot has been implemented as default by Xilinx. (see xilinx-wiki.atlassian "Using Distro Boot With Xilinx U-Boot")

The distro boot functionality is implemented as an extension of the existing U-Boot bootcmd functionality in U-Boot. It extends its functionality by redirecting the call to bootcmd to a call to the distro_bootcmd variable.

bootcmd=run distro_bootcmd

Distro_bootcmd and Boot_targets

The distro_bootcmd variable typically contains a sequence of commands that scans a pre-defined list of potential boot targets in search of boot collateral as shown below in an snippet of the printenv

scriptaddr=0x3000000
script_offset_f=fc0000
script_size_f=0x40000

boot_targets=qspi jtag mmc0 mmc1 qspi nand nor usb0 usb1 pxe dhcp
distro_bootcmd=for target in ${boot_targets}; do run bootcmd_${target}; done

bootcmd_qspi=sf probe 0 0 0 && sf read ${scriptaddr} ${script_offset_f} ${script_size_f} && echo QSPI: Trying to boot script at ${scriptaddr} && source ${scriptaddr}; echo QSPI: SCRIPT FAILED: continuing...;
bootcmd_mmc0=devnum=0; run mmc_boot
mmc_boot=if mmc dev ${devnum}; then devtype=mmc; run scan_dev_for_boot_part; fi

The boot_targets list is defined in the platform definition in the U-Boot source tree.

Depending on the specifics of the target platform, the distro boot infrastructure can either use the boot_targets list to explicitly find specific boot collateral (eg. hard-coded loading once a valid boot target is located) or it can be used to locate some kind of decoupled configuration information.

Boot Method Precedence

The order of precedence for searching for boot components is as follows

1. extlinux.conf file (located in /extlinux/ or /boot/extlinux/)
2. boot.scr file + image.ub file
3. boot.scr file + individual files (Image, system.dtb, etc.)

In absence of a valid extlinux.conf file, U-Boot will scan the boot_targets list looking for a file named boot.scr.uimg or boot.scr (in that order) and run any commands located in the script file. If your environment requires a different sequence of commands or behaviour, you can edit the boot.scr file to suit your needs.

sudo apt-get update
sudo apt install u-boot-tools

dd if=boot.scr of=boot.script bs=72 skip=1

# This is a boot script for U-Boot
# Generate boot.scr:
# mkimage -c none -A arm -T script -d boot.script boot.scr
#
# Generate boot.script (for modifications in boot.scr):
# dd if=boot.scr of=boot.script bs=72 skip=1
#
################
imageub_addr=0x10000000
# 
imageub_flash_addr=0x200000
imageub_flash_size=0xD90000

for boot_target in ${boot_targets};
do

	# Boot target is mmc0 or usb0: image.ub on mmc0 or usb0
	if test "${boot_target}" = "${devtype}0"; then
		if test -e ${devtype} 0:${distro_bootpart} /image.ub; then
			fatload ${devtype} 0:${distro_bootpart} ${imageub_addr} image.ub;
			bootm ${imageub_addr};
		fi
	fi
	# Boot target is mmc1 or usb1: image.ub on mmc1 or usb1
	if test "${boot_target}" = "${devtype}1"; then
		if test -e ${devtype} 1:${distro_bootpart} /image.ub; then
			fatload ${devtype} 1:${distro_bootpart} ${imageub_addr} image.ub;
			bootm ${imageub_addr};
		fi
	fi
	# Boot target is qspi: image.ub on qspi; image.ub is included in BOOT.bin
	if test "${boot_target}" = "qspi"; then
		sf probe 0 0 0;
		sf read ${imageub_addr} ${imageub_flash_addr} ${imageub_flash_size};
		bootm ${imageub_addr};
	fi
done

The alias name ${devnum}, used in Xilinx default files, does not work in all variants tested, therefore the ${devtype} boot_targets were split up

mkimage -c none -A arm -T script -d boot.script boot.scr

The following table provides an overview of the possible boot combinations for the devices SD card, USB and QSPI in SD Boot mode. The table is not complete, but gives an impression of the possibilities of the Distro Boot.

Variant 1 (Boot.bin, image.ub and boot.scr on SD card):

1. Copy image.ub, boot.scr and Boot.bin on SD-Card.
2. Set Boot Mode to SD-Boot.
3. Insert SD-Card in SD-Slot and start the board.

Variant 2 (Boot.bin on SD card; image.ub and boot.scr on USB):

1. Copy image.ub on SD-Card.
2. Copy boot.scr and Boot.bin on USB Stick
3. Set Boot Mode to SD-Boot.
4. Insert SD-Card in SD-Slot.
5. Insert USB-Stick in USB-Slot and start the board.

The following table provides an overview of the possible boot combinations for the devices SD card, USB and QSPI in QSPI Boot mode. The table is not complete, but gives an impression of the possibilities of the Distro Boot.

Variant 1 (Boot.bin on QSPI Flash; image.ub and boot.scr on SD card):

1. Connect JTAG and power on carrier with module (boot mode set to SD boot and no SD card is inserted)
2. Open Vivado Project and program QSPI flash
3. Copy boot.scr and image.ub on SD card
4. Set boot mode to QSPI-Boot mode, insert SD card and start the board.

Variant 2 (Boot.bin, image.ub and boot.scr on QSPI Flash):

1. Connect JTAG and power on carrier with module (boot mode set to SD boot and no SD card is inserted)
2. Open Vivado Project and program QSPI flash
3. Set boot mode to QSPI-Boot mode and start the board.

Variant 3 (Boot.bin on QSPI Flash; image.ub and boot.scr on USB):

1. Connect JTAG and power on carrier with module (boot mode set to SD boot and no SD card is inserted)
2. Open Vivado Project and program QSPI flash
3. Copy boot.scr and image.ub on SD card.
4. Set boot mode to QSPI-Boot mode, insert USB and start the board.