4.28. STMicroelectronics STM32MP

The STM32MP is a line of 32-bit ARM SoCs. They reuse peripherals of the STM32 line of microcontrollers and can have a STM32 MCU embedded as co-processor as well.

The boot process of the STM32MP1 SoC is a two step process. The first stage boot loader (FSBL) is loaded by the ROM code into the built-in SYSRAM and executed. The FSBL sets up the SDRAM, install a secure monitor and then the second stage boot loader (SSBL) is loaded into DRAM.

When building barebox, the resulting barebox-${board}.img file has the STM32 header preprended, so it can be loaded directly as SSBL by the ARM TF-A (https://github.com/ARM-software/arm-trusted-firmware). Each entry point has a header-less image ending in *.pblb as well.

Use of barebox as FSBL is not supported.

4.28.1. Building barebox

With multi-image and device trees, it’s expected to have stm32mp_defconfig as sole defconfig for all STM32MP boards:

make ARCH=arm stm32mp_defconfig

The resulting images will be placed under images/:

barebox-stm32mp15xx-dkx.img # both DK1 and DK2
barebox-stm32mp157c-lxa-mc1.img
barebox-stm32mp157c-seeed-odyssey.img

4.28.2. Flashing barebox

An appropriate image for a SD-Card can be generated with following genimage(1) config:

image @STM32MP_BOARD@.img {
    hdimage {
        align = 1M
        gpt = "true"
    }
    partition fsbl1 {
        image = "tf-a-@STM32MP_BOARD@.stm32"
        size = 256K
    }
    partition fsbl2 {
        image = "tf-a-@STM32MP_BOARD@.stm32"
        size = 256K
    }
    partition ssbl {
        image = "barebox-@STM32MP_BOARD@.img"
        size = 1M
    }
    partition barebox-environment {
        image = "/dev/null"
        size = 1M
    }
}

For eMMC, the boot partitions are used as the FSBL partitions and so the user partitions may look like this:

image @STM32MP_BOARD@.img {
    partition ssbl {
        image = "barebox-@STM32MP_BOARD@.img"
        size = 1M
    }
    partition barebox-environment {
        image = "/dev/null"
        size = 1M
    }
}

The fsbl1 and fsbl2 can be flashed by writing to barebox /dev/mmcX.boot0 and /dev/mmcX.boot1 respectively or from a booted operating system.

Additionally, the eMMC’s ext_csd register must be modified to activate the boot acknowledge signal (BOOT_ACK) and to select a boot partition.

Assuming CONFIG_CMD_MMC_EXTCSD is enabled and the board shall boot from /dev/mmc1.boot1:

mmc_extcsd /dev/mmc1 -i 179 -v 0x50

The STM32MP1 BootROM does not support booting from eMMC without fast boot acknowledge.

4.28.3. Boot source selection

The STM32MP BootROM samples three boot pins at reset. Usually BOOT1 is pulled down and BOOT0 and BOOT2 are connected to a 2P DIP switch:

      +-------+
BOOT2 | O O-- |
BOOT0 | N O-- |  <---- SDMMC
      +-------+
      +-------+
BOOT2 | O O-- |
BOOT0 | N --O |  <---- QSPI-NOR Flash
      +-------+
      +-------+
BOOT2 | O --O |
BOOT0 | N --O |  <---- DFU on UART and USB OTG
      +-------+

4.28.4. Boot status indicator

The ROM code on the first Cortex-A7 core pulses the PA13 pad. An error LED on this pad can be used to indicate boot status:

  • Boot Failure: LED lights bright
  • UART/USB Boot: LED blinks fast
  • Debug access: LED lights weak