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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 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

Boot Method Precedence

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

1. extlinux.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

echofor [TE_BOOT-01] Trenz Boot-file version 1.0 (development);
echo [TE_BOOT-10] Found boot.scr in device: ${target}
echo [TE_BOOT-11] Chosen Bootmode is ${modeboot} from boot_targets=${boot_targets};
echo [TE_BOOT-11] Use RAM Address 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} for linux image.ub;


for boot_target in ${boot_targets			bootm ${imageub_addr};
do		fi
	fi
	#echo [TE_BOOT-11] currently trying to load from ${target};
	#echo [TE_BOOT-12] boot_target = ${boot_target};
	#echo [TE_BOOT-13] devtype = ${devtype}; 
	#echo [TE_BOOT-14] devnum = ${devnum}; 
	#echo [TE_BOOT-15] distro_bootpart = ${distro_bootpart}; 
	#echo [TE_BOOT-16] boot_prefixes = ${boot_prefixes}; 
	#echo [TE_BOOT-17] source =  ${prefix}${script};
	
	# Boot target is mmc0 or usb0: image.ub on mmc0 or usb0# 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}" = "${devtype}0qspi"; then
		if || test -e "${devtypeboot_target} 0:${distro_bootpart} /image.ub" = "qspi0"; then
			echo [TE_BOOT-20] boot_target issf probe 0 0 0;
		sf read ${bootimageub_targetaddr}. Found image.ub on ${devtype}0:${distro_bootpart} ;
			fatload ${devtype} 0:${distro_bootpart} ${imageub_addr} image.ubimageub_flash_addr} ${imageub_flash_size};
			bootm ${imageub_addr};
		fi
done
	fi
	# Boot target is mmc1 or usb1: image.ub on mmc1 or usb1
	if test "${boot_target}" = "${devtype}1"; then
		if test -e ${devtype} 1:${distro_bootpart} /image.ub; then
			echo [TE_BOOT-21] boot_target is ${boot_target}. Found image.ub on ${devtype}1:${distro_bootpart};
			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
		echo [TE_BOOT-30] Try to use image.ub from ${boot_target}, load image.ub from qspi-flash address ${imageub_flash_addr}; 
		echo [TE_BOOT-30] max. image.ub size is ${imageub_flash_size} ;
		sf probe 0 0 0;
		sf read ${imageub_addr} ${imageub_flash_addr} ${imageub_flash_size};
		bootm ${imageub_addr};
	fi
done

	# Boot target is mmc0: image.ub on mmc0
	if test "${boot_target}" = "mmc0"; then
		mmc dev 0
		if test -e ${devtype}mmc 0:${distro_bootpart} /image.ub; then
			#fatloadecho [TE_BOOT-20] boot_target is ${devtypeboot_target} 0. Found image.ub on mmc0:${distro_bootpart} ${imageub_addr_bootpart} image.ub;
			fatload mmc 0:${distro_bootpart} ${imageub_addr} image.ub;
			bootm ${imageub_addr};
		fi
	fi
	# Boot target is mmc1: image.ub on mmc1
	if test "${boot_target}" = "mmc1"; then
		mmc dev 1
		if test -e ${devtype}mmc 1:${distro_bootpart} /image.ub; then
			echo [TE_BOOT-21] boot_target is ${boot_target}. Found /image.ub; then
			#fatload ${devtype} 0 on mmc1:${distro_bootpart} ${imageub_addr} image.ub;
			fatload mmc 1:${distro_bootpart} ${imageub_addr} image.ub;
			bootm ${imageub_addr};
		fi
	fi

		fi
	fi