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Each STM32MP1 System-On-Module comes pre-loaded with U-Boot. U-Boot runs as the primary firmware from the on-module eMMC Flash on each power-on / reset.

U-Boot is probably the most popular firmware monitor for Linux. It is developed and maintained by DENX Software Engineering (www.denx.de). If you need detailed information on any aspects of U-Boot operation, DENX publishes extensive U-Boot user documentation at their web site.

On the STM32MP1 SOM, U-Boot image is stored in the eMMC device. On a power-on / reset, U-Boot is relocated to DDR RAM.

As soon as the module is powered on or reset, the core proceeds to boot the U-Boot firmware printing the following output to the serial console:

U-Boot 2018.09-stm32mp-r2-2.8.1 (Feb 14 2019 - 15:13:06 +0000)

CPU: STM32MP15x.B
Model: Emcraft STM32MP1 SOM Rev 1A, www.emcraft.com
Board: stm32mp1 in trusted mode
       Watchdog enabled
DRAM:  1 GiB
Clocks:
- MPU : 650 MHz
- MCU : 196.608 MHz
- AXI : 266.500 MHz
- PER : 24 MHz
MMC:   STM32 SDMMC2: 0, STM32 SDMMC2: 1
In:    serial
Out:   vidconsole
Err:   vidconsole
Model: Emcraft STM32MP1 SOM Rev 1A, www.emcraft.com
Board: stm32mp1 in trusted mode
Error: 16 bit/pixel mode, but BMP has 24 bit/pixel
Net:   eth0: ethernet@5800a000
Boot over mmc1!
Hit any key to stop autoboot:  0 
STM32MP1-SOM>

If you hit any key on the serial console before the number of seconds defined by the U-Boot bootdelay variable has elapsed, you will enter the U-Boot interactive command monitor. From the command monitor you can run U-Boot commands to examine memory, load an image from Ethernet, boot Linux from a loaded image or perform any other action supported by U-Boot.

U-Boot makes use of the so-called environment variables to define various aspects of the target functionality. On the STM32MP1 SOM, the U-Boot environment is stored in the eMMC Flash and is persistent across power or reset cycles. Parameters defined by the U-boot environment variables include: target IP address, target MAC address, location in RAM where a Linux bootable image will be loaded, and many others.

To manipulate the U-Boot environment the following commands are used:

  • printenv <var> - print the value of the variable var. Without arguments, prints all environment variables:

    u-boot => printenv
bootcmd=run sdboot
bootdelay=3
baudrate=115200
...
u-boot =>

  • setenv <var> <val> - set the variable var to the value val: 

    u-boot => setenv image vlad/imx8m/my.Image
u-boot=>

Running setenv <var> will unset (undefine) a specified U-Boot variable.

  • saveenv - save the up-to-date U-Boot environment, possibly updated using setenv commands, into the Flash. Running saveenv makes sure that any updates you have made to the U-Boot environment are persistent across power cycles and resets.

    u-boot => saveenv
Saving Environment to MMC...
Writing to MMC... done
u-boot =>

The autoboot sequence in U-Boot is controlled by the two environment variables called bootdelay and bootcmd.

The bootdelay variable defines a delay, in seconds, before running the autoboot command defined by bootcmd. During the bootdelay countdown, you can interrupt the autobooting by pressing any key. This will let you enter the U-Boot command line interface.

The bootcmd variable defines a command executed by U-Boot automatically after the bootdelay countdown is over. Typically, this would be run netboot to boot Linux from TFTP during development or run flashboot to boot Linux from the on-module Flash on deployed units.

In deployed configurations, where boot time to the service provided by your embedded device is critical, you will probably want to set bootdelay to 0:

u-boot => set bootdelay 0
u-boot => saveenv

This will make sure that on each power on / reset U-Boot immediately executes the command defined by bootcmd, typically booting Linux from the on-module Flash.

With bootdelay set to 0 the U-Boot countdown is disabled, so there is a question how you enter the U-Boot command monitor, should you need to enter it for some reason. To do so, push the Ctrl-C keys down and don't release them until you have hit the reset button on the baseboard. This will interrupt the U-Boot bootcmd sequence and let you enter the U-Boot command monitor:

U-Boot 2018.09-stm32mp-r2-2.8.0 (Feb 14 2019 - 15:13:06 +0000)
...
Hit any key to stop autoboot:  0
u-boot => <INTERRUPT>

From the command monitor, you would be able to reset bootdelay to 3 or whatever value makes sense to you.

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