Exploring the U-Boot boot scripts on i.MX8M Mini EVK

U-Boot scripts are usually contained in the U-Boot environment. This design decision allows scripts even before actual file systems are accessible. So to understand what is going on in the bootsequence, we will always reference the default environment as shown here.

We can use the U-Boot command env default -a to revert to this setup at any time, but be aware that variables injected during production (e.g. ethaddr) will be lost this way. It is always a good thing to dump the environment to a text file before doing any modifications. This way, you will be able to subsequently refer even to those variables.

But without much further ado, let's take a look at the default environment:

U-Boot SPL 2021.04-lf_v2021.04+g263b27e076 (Nov 22 2021 - 01:39:23 +0000)
Can't find PMIC:PCA9450
DDRINFO: start DRAM init
DDRINFO: DRAM rate 3000MTS
DDRINFO:ddrphy calibration done
DDRINFO: ddrmix config done
SEC0:  RNG instantiated
Normal Boot
Trying to boot from MMC1
NOTICE:  BL31: v2.4(release):lf-5.10.72-2.2.0-0-g5782363f9
NOTICE:  BL31: Built : 12:17:17, Nov 18 2021


U-Boot 2021.04-lf_v2021.04+g263b27e076 (Nov 22 2021 - 01:39:23 +0000)

CPU:   i.MX8MMQ rev1.0 at 1200MHz
CPU:   Commercial temperature grade (0C to 95C) at 46C
Reset cause: POR
Model: NXP i.MX8MM EVK board
DRAM:  2 GiB
TCPC:  Vendor ID [0x1fc9], Product ID [0x5110], Addr [I2C1 0x52]
Power supply on USB2
TCPC:  Vendor ID [0x1fc9], Product ID [0x5110], Addr [I2C1 0x50]
MMC:   FSL_SDHC: 1, FSL_SDHC: 2
Loading Environment from MMC... OK
[*]-Video Link 0adv7535_mipi2hdmi adv7535@3d: Can't find cec device id=0x3c
fail to probe panel device adv7535@3d
mxs_video lcdif@32e00000: failed to get any video link display timings
probe video device failed, ret -22

	[0] lcdif@32e00000, video
	[1] mipi_dsi@32e10000, video_bridge
	[2] adv7535@3d, panel
adv7535_mipi2hdmi adv7535@3d: Can't find cec device id=0x3c
fail to probe panel device adv7535@3d
mxs_video lcdif@32e00000: failed to get any video link display timings
probe video device failed, ret -22
In:    serial
Out:   serial
Err:   serial
SEC0:  RNG instantiated

 BuildInfo:
  - ATF 5782363

switch to partitions #0, OK
mmc1 is current device
flash target is MMC:1
Net:   eth0: ethernet@30be0000
Fastboot: Normal
Normal Boot
Hit any key to stop autoboot:  0 
u-boot=> env default -a
## Resetting to default environment
u-boot=> printenv
arch=arm
baudrate=115200
board=imx8mm_evk
board_name=imx8mm_evk
boot_a_script=load ${devtype} ${devnum}:${distro_bootpart} ${scriptaddr} ${prefix}${script}; source ${scriptaddr}
boot_efi_binary=load ${devtype} ${devnum}:${distro_bootpart} ${kernel_addr_r} efi/boot/bootaa64.efi; if fdt addr ${fdt_addr_r}; then bootefi ${kernel_addr_r} ${fdt_addr_r};else bootefi ${kernel_addr_r} ${fdtcontroladdr};fi
boot_efi_bootmgr=if fdt addr ${fdt_addr_r}; then bootefi bootmgr ${fdt_addr_r};else bootefi bootmgr;fi
boot_extlinux=sysboot ${devtype} ${devnum}:${distro_bootpart} any ${scriptaddr} ${prefix}${boot_syslinux_conf}
boot_fit=no
boot_net_usb_start=usb start
boot_prefixes=/ /boot/
boot_script_dhcp=boot.scr.uimg
boot_scripts=boot.scr.uimg boot.scr
boot_syslinux_conf=extlinux/extlinux.conf
boot_targets=usb0 mmc1 mmc2 
bootcmd=run distro_bootcmd;run bsp_bootcmd
bootcmd_mfg=run mfgtool_args;if iminfo ${initrd_addr}; then if test ${tee} = yes; then bootm ${tee_addr} ${initrd_addr} ${fdt_addr}; else booti ${loadaddr} ${initrd_addr} ${fdt_addr}; fi; else echo "Run fastboot ..."; fastboot 0; fi;
bootcmd_mmc1=devnum=1; run mmc_boot
bootcmd_mmc2=devnum=2; run mmc_boot
bootcmd_usb0=devnum=0; run usb_boot
bootdelay=2
bootm_size=0x10000000
bootscript=echo Running bootscript from mmc ...; source
bsp_bootcmd=echo Running BSP bootcmd ...; mmc dev ${mmcdev}; if mmc rescan; then if run loadbootscript; then run bootscript; else if run loadimage; then run mmcboot; else run netboot; fi; fi; fi;
bsp_script=boot.scr
console=ttymxc1,115200
cpu=armv8
distro_bootcmd=for target in ${boot_targets}; do run bootcmd_${target}; done
efi_dtb_prefixes=/ /dtb/ /dtb/current/
emmc_dev=2
ethprime=FEC
fdt_addr=0x43000000
fdt_addr_r=0x43000000
fdt_high=0xffffffffffffffff
fdtfile=imx8mm-evk.dtb
image=Image
initrd_addr=0x43800000
initrd_high=0xffffffffffffffff
jh_clk= 
jh_mmcboot=mw 0x303d0518 0xff; setenv fdtfile ${jh_root_dtb};setenv jh_clk clk_ignore_unused mem=1340MB; if run loadimage; then run mmcboot; else run jh_netboot; fi; 
jh_netboot=mw 0x303d0518 0xff; setenv fdtfile ${jh_root_dtb}; setenv jh_clk clk_ignore_unused mem=1340MB; run netboot; 
jh_root_dtb=imx8mm-evk-root.dtb
kboot=booti 
kernel_addr_r=0x40480000
load_efi_dtb=load ${devtype} ${devnum}:${distro_bootpart} ${fdt_addr_r} ${prefix}${efi_fdtfile}
loadaddr=0x40480000
loadbootscript=fatload mmc ${mmcdev}:${mmcpart} ${loadaddr} ${bsp_script};
loadfdt=fatload mmc ${mmcdev}:${mmcpart} ${fdt_addr_r} ${fdtfile}
loadimage=fatload mmc ${mmcdev}:${mmcpart} ${loadaddr} ${image}
mfgtool_args=setenv bootargs console=${console},${baudrate} rdinit=/linuxrc clk_ignore_unused 
mmc_boot=if mmc dev ${devnum}; then devtype=mmc; run scan_dev_for_boot_part; fi
mmcargs=setenv bootargs ${jh_clk} console=${console} root=${mmcroot}
mmcautodetect=yes
mmcboot=echo Booting from mmc ...; run mmcargs; if test ${boot_fit} = yes || test ${boot_fit} = try; then bootm ${loadaddr}; else if run loadfdt; then booti ${loadaddr} - ${fdt_addr_r}; else echo WARN: Cannot load the DT; fi; fi;
mmcdev=1
mmcpart=1
mmcroot=/dev/mmcblk1p2 rootwait rw
nandfit_part=yes
netargs=setenv bootargs ${jh_clk} console=${console} root=/dev/nfs ip=dhcp nfsroot=${serverip}:${nfsroot},v3,tcp
netboot=echo Booting from net ...; run netargs;  if test ${ip_dyn} = yes; then setenv get_cmd dhcp; else setenv get_cmd tftp; fi; ${get_cmd} ${loadaddr} ${image}; if test ${boot_fit} = yes || test ${boot_fit} = try; then bootm ${loadaddr}; else if ${get_cmd} ${fdt_addr_r} ${fdtfile}; then booti ${loadaddr} - ${fdt_addr_r}; else echo WARN: Cannot load the DT; fi; fi;
scan_dev_for_boot=echo Scanning ${devtype} ${devnum}:${distro_bootpart}...; for prefix in ${boot_prefixes}; do run scan_dev_for_extlinux; run scan_dev_for_scripts; done;run scan_dev_for_efi;
scan_dev_for_boot_part=part list ${devtype} ${devnum} -bootable devplist; env exists devplist || setenv devplist 1; for distro_bootpart in ${devplist}; do if fstype ${devtype} ${devnum}:${distro_bootpart} bootfstype; then run scan_dev_for_boot; fi; done; setenv devplist
scan_dev_for_efi=setenv efi_fdtfile ${fdtfile}; for prefix in ${efi_dtb_prefixes}; do if test -e ${devtype} ${devnum}:${distro_bootpart} ${prefix}${efi_fdtfile}; then run load_efi_dtb; fi;done;run boot_efi_bootmgr;if test -e ${devtype} ${devnum}:${distro_bootpart} efi/boot/bootaa64.efi; then echo Found EFI removable media binary efi/boot/bootaa64.efi; run boot_efi_binary; echo EFI LOAD FAILED: continuing...; fi; setenv efi_fdtfile
scan_dev_for_extlinux=if test -e ${devtype} ${devnum}:${distro_bootpart} ${prefix}${boot_syslinux_conf}; then echo Found ${prefix}${boot_syslinux_conf}; run boot_extlinux; echo SCRIPT FAILED: continuing...; fi
scan_dev_for_scripts=for script in ${boot_scripts}; do if test -e ${devtype} ${devnum}:${distro_bootpart} ${prefix}${script}; then echo Found U-Boot script ${prefix}${script}; run boot_a_script; echo SCRIPT FAILED: continuing...; fi; done
scriptaddr=0x43500000
sd_dev=1
soc=imx8m
splashimage=0x50000000
usb_boot=usb start; if usb dev ${devnum}; then devtype=usb; run scan_dev_for_boot_part; fi
vendor=freescale

Environment size: 5091/16380 bytes
u-boot=>

bootcmd=run distro_bootcmd;run bsp_bootcmd