STM32MP151 Dev Board Buildroot

NOTE: I've moved on from this system and it is no longer maintained. You can find some progress on the errors in my STM32MP135 board.

A Buildroot external tree for a custom PCB based on an MYiR SoM. This project, along with the PCB design below, were an educational experience to learn the Buildroot system, Linux kernel, and a bit of PCB design. It currently boots to Linux 6.3 with a minimal collection of packages, and has a demo of LVGL using the framebuffer.

• PCB Design Files

Current State

The build is very similar to the mainline STM32MP157c-DK2 device tree however there are some minor changes due to the pinout of the SoM, and the RAM configuration. Not all of module-specific pinouts have been implemented yet.

• Uses Linux kernel 6.3
• Uses U-Boot 2023.07
• Tested with Buildroot 2023.05
• Uses with ATF 2.7
• Uses mainline STM32 dtsi where possible, and ARM Trusted Firmware with sp_min for BL2.
• Custom dts and dtsi are included for board specific changes. Not all of the SoM specific pinouts are reflected in the files.
• Includes a file system overlay with some configuration, but boot/extlinux.conf is the only file which is actually required.
• Includes an external package for the ESP8089 WiFi Chipset.

Usage

This should behave like any external buildroot tree. General steps for use:

• Clone this repository, as well as Buildroot.
• In the Buildroot directory, create a config file based on this external tree:

BR2_EXTERNAL=../STM32MP151_Dev_Board_Buildroot make stm32mp151_dev_board_defconfig

• Review or modify the system in make menuconfig.
• Build the image using make.
• NOTE: There is an issue with UBoot the first time you try to build, see below.
• Use dd, Rufus, or your software of choice to image an SD card with the output/images/sdcard.img file.

Issues

There are so many! But some big ones:

• U-Boot does not include the custom DTB file in the Makefile when building for the first time. If you get errors about the dtb not being correctly specified, manually adding the dtb filename to the file buildroot/output/build/uboot-2023.04/arch/arm/dts/Makefile:
  

stm32mp151_dev_board.dtb

• esp8089 does not seem to communicate. I'm not sure if the problem is hardware, hookup, or the driver. I assume there is a design issue but I cannot figure it out.

The Overlay Folder

There are a few config files in the overlay file for either quality of life, or to make things actually work. Most are optional depending, or may even conflict with your desired settings.

• \boot\extlinux.conf - Required to boot linux. U-Boot will look for this.
• \etc\init.d\S40xorg - A dummy file to overwrite a default which is creatred when Xorg compiles. I don't want Xorg to start on boot.
• \etc\modprobe.d\esp.conf - Config file telling the esp8089 driver which GPIO to use for reset. Since the driver doesn't work anyway, this will probably be removed.
• \etc\opkg\distfeeds.conf - Contains OpenWRT package feed. Need to force it to search the correct architecture when using opkg.
• \etc\wpa_supplicant\wpa_cupplicant.conf - A blank wpa_supplicant configuration to fill out.
• \etc\X11\xorg.conf - Sometimes the default are bad, sometimes they aren't.
• \etc\directfbrc - DirectFB config file because it doesn't seem to use FBDev by default, even though it says it does.
• \lib\firmwmare - Firmware files for some wifi cards.
• \root - Just some extra files for testing and playing with.

Trips and Tricks

Some things that caught me out, or helped me out, along the way:

• I had trouble figuring out how to convince ARM Trusted Firmwawre to build a FIP package from within Buildroot. For reference, or if you want, or need, to create one manually you can do so from within the buildroot/output/build/arm-trusted-firmware/build/stm32mp1/debug directory:
  

fiptool create --tos-fw bl32.bin --fw-config fdts/stm32mp151_dev_board-tf-a-fw-config.dtb --hw-config u-boot.dtb --nt-fw u-boot.bin --tos-fw-config fdts/stm32mp151_dev_board-tf-a-bl32.dtb fip.bin

Where the u-boot.bin, and u-boot.dtb files are found in the buildroot/output/build/uboot directory.

• Remember that the FIP package contains the second stage bootloader, and loads the U-Boot DTB file. So if you modify the device tree you will need to manually rebuild U-Boot, and ARM Trusted Firmware, and probably the Linux kernel as well depending on your changes.

1. make linux-rebuild
2. make uboot-rebuild
3. make arm-trusted-firmware-rebuild
4. Finally; make to generate the image.

• This seems to boot on the STM32MP157c-DK2 development kit, however the STM32MP157c-DK2 default build does not boot on this board because the RAM timing seems to be different or something. Of course the pin configuration for peripherals are different, in particular the MMC_DETECT signal is ACTIVE_HIGH on this board, but ACTIVE_LOW on the STM32MP157c-DK2.