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Building the Bootloader (U-boot)

1. Prerequisites 2. Building the FPGA Image 3. Building the Bootloader 4. Building the Filesystem 5. Building the SD Card

Introduction

The main boot loader for the MitySOM-A10S is Das U-Boot. This boot loader is used to configure the pin mux, DDR, program the FPGA, and launch the next stage application/OS. Critical Link started from Intel's port of Das U-Boot and made minor modifications such as reading the MAC address from the on board EEPROM the ethernet MAC.

U-Boot utilizes scripts/macros in order to tailor the boot sequence for the user's needs. The development kit's U-boot is set up to read the core FPGA image (named mitysom_a10s_devkit.rbf) from the FAT partition on the SD card into RAM then load this image into the FPGA. Next, it would load the kernel and device tree into RAM from the ext4 partition and then boot the kernel. These scripts are stored in the U-boot environment, which is created from ubootenv.txt that is located in the FPGA project.

These instructions have been tested with Quartus Pro 18.1

Repository

Description Repository Branch U-Boot Version
U-Boot Source https://git.criticallink.com/git/u-boot-socfpga.git mitysom_a10 v2014.10

Development Kit Pre-built Outputs

Description Link
270 KLE U-boot & environment https://support.criticallink.com/redmine/attachments/download/26781/a10s-p8-uboot-20200929.zip
480 KLE U-boot & environment https://support.criticallink.com/redmine/attachments/download/26784/a10s-p9-uboot-20200929.zip

Quick steps to Building U-Boot

  1. Open a terminal
  2. Enter the following command that will allow you to use the quartus tools from the command line:
    /opt/intelFPGA_pro/18.1/embedded/embedded_command_shell.sh
  3. Change into the directory where project was cloned into (Example: /home/user/mitysom-a10s)
    cd /home/user/mitysom-a10s/mitysom-a10s-devkit-goldenref
  4. Run the following command that will do the following:
    1. Create the device tree u-boot will use (containing HPS pin muxing, clock setup)
    2. Pulls Critical Links U-boot repo
    3. Builds U-boot
    4. Creates bootloader and U-boot environment Binaries
make bootloader ubootenv

Outputs

Description Location
Bootloader Binary <Quartus Project Folder>/software/bootloader/uboot_w_dtb-mkpimage.bin
U-Boot Environment Binary <Quartus Project Folder>/software/bootloader/ubootenv.bin

Customizing U-Boot

Customization through updating the U-boot environment

Most U-Boot customization for a carrier board can be done by adjusting the scripts/macros in the U-boot environment. This includes changing the name of the core FPGA image (RBF), Kernel, or device tree block (DTB). The U-boot environment is stored in the SD Card, 512 Bytes from the beginning of the SD Card. It can also be updated inside of U-boot by using the editenv, setenv, and saveenv commands. Please see the U-Boot documentation in Further Reading for more usages.

The U-Boot environment binary is built through the FPGA project, it's source is the ubootenv.txt

Important U-Boot Environment Variables

The core RBF location is in rbfcoreimage:

rbfcoreimage=mitysom_a10s.core.rbf

The DTB location is in fdtimage:

fdtimage=/boot/mitysom_a10s_devkit.dtb

Customizing through Patching U-Boot Source

The Makefile for creating the Bootloader, which is created by running the Intel's bsp-editor, allows for applying patches to the U-Boot source during the build process. Using this feature is easy to customize U-Boot by creating a patch of the U-Boot source and putting it in <Quartus Project Folder>/software/bootloader.

Example of Adding Addition Command to U-Boot

This example will add the itest command to U-Boot.

Important files:
Description Location
Header for MitySOM-A10S that sets peripheral settings and enabled commands <U-Boot Source>/include/configs/mitysom_a10s_dsc.h
Handles early init of U-Boot specific to the MitySOM-A10S <U-Boot Source>/include//board/cl/common/common_init.c
MitySOM-A10S defconfig <U-Boot Source>/configs/mitysom_a10s_dsc_defconfig

In order to add the itest command #undef CONFIG_CMD_ITEST would need to be removed to <U-Boot Source>/include/configs/mitysom_a10s_dsc.h. Once that line is removed a patch will need to be made. Do this by running the following command from the top level of the U-boot repository that was checked out: 

git diff > enable-itest.patch

Now, copy enable-itest.patch into <Quartus Project Folder>/software/bootloader, and the next U-boot is built that patch will be applied.

Advanced: Manual steps for Building U-Boot

These are the steps to build U-Boot without using the Makefile that is in Critical Link's reference Quartus Project.

  1. Open a terminal
  2. Enter the following command that will allow you to use the quartus tools from the command line:
    /opt/intelFPGA_pro/18.1/embedded/embedded_command_shell.sh
  3. Change into the directory where project was cloned into (Example: /home/user/mitysom-a10s)
    cd /home/user/mitysom-a10s
  4. Run the bsp-editor to generate the Makefile and device tree for U-Boot
    bsp-create-settings \
        --type uboot \
        --bsp-dir software/bootloader \
        --preloader-settings-dir "hps_isw_handoff" \
        --settings software/bootloader/settings.bsp \
        --set uboot.boot_device SDMMC \
        --set uboot.rbf_filename mitysom_a10s.periph.rbf \
        --set uboot.core_rbf_filename "" \
        --set uboot.peripheral_rbf_filename "" \
        --set uboot.model "MitySOM-A10S"
  5. Download the U-Boot source from Critical Link
    git archive --format=tar.gz --prefix=uboot-socfpga/ --remote=git://support.criticallink.com/home/git/u-boot-socfpga.git mitysom_a10 > software/bootloader/uboot-socfpga.tar.gz
  6. Build U-Boot
    make -C software/bootloader TGZ=uboot-socfpga.tar.gz SOCFPGA_BOARD_CONFIG=mitysom_a10s_dsc_defconfig
  7. Build the u-boot environment
    software/bootloader/uboot-socfpga/tools/mkenvimage -s 4096 -o software/bootloader/ubootenv.bin software/bootloader/ubootenv.txt

FAQ

When should I rebuild my bootloader?

Your bootloader should be rebuilt anytime the settings in the HPS block in Platform Designer is changed. This includes but not limited to: changing the HPS pin mux, changing any of the clocks generated by the HPS, enabling/changing and the FPGA bridges (Light Weight HPS-to-FPGA, HPS-to-FPGA, FPGA-to-HPS, FPGA-to-SDRAM). When in doubt rebuilt and reflash U-Boot.

Further Read

1. Prerequisites 2. Building the FPGA Image 3. Building the Bootloader 4. Building the Filesystem 5. Building the SD Card

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