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When the Yocto build is initiated, U-Boot used in Gemstone boards is automatically downloaded, compiled, and its outputs are packaged. Therefore, no additional commands need to be run. If there is a situation that requires recompiling U-Boot (e.g., adding support for booting the operating system from an SSD), the following steps can be followed.
Some operations, such as modifying kernel parameters or adding device-tree overlays, can be done without recompiling U-Boot by updating the /boot/uEnv.txt file.
All the commands below will be executed inside the distrobox. Therefore, you must first run the devbox shell —> task box commands. Additionally, to switch to the Yocto environment, you need to run the m:oeinit t3-gem-o1 command.

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Updating Configuration with Menuconfig

If you want to enable/disable various features (such as enabling the TFTP command) instead of modifying the source code, you can access menuconfig using the following commands.
In this example, the T3-GEM-O1 board will be used for explanation. If you are applying these steps for a different machine, change the machine name accordingly.
bitbake -c menuconfig virtual/bootloader
After making the desired changes, press the ESC key until the following window appears. Then, select the “Yes” button.
At this stage, you can recompile U-Boot, but your changes will only remain on your local machine and will be lost if you clean the build directory. To prevent this, follow the steps below.
bitbake -c diffconfig virtual/bootloader
cp /home/workdir/build/tmp-musl/work/t3_gem_o1-gemstone-linux-musl/u-boot-t3-gem-o1/2025.04+git/fragment.cfg \
    /home/workdir/yocto/meta-gemstone/dynamic-layers/meta-ti/recipes-bsp/u-boot-t3-gem-o1/files/fragment.config
Rename the fragment.config file in the meta-gemstone/dynamic-layers/meta-ti/recipes-bsp/u-boot-t3-gem-o1/files directory to a name that describes your changes without altering the .config extension. Then, add this name to the SRC_URI variable in the meta-gemstone/dynamic-layers/meta-ti/recipes-bsp/u-boot-t3-gem-o1/u-boot-t3-gem-o1_2025.04.bb file. For example, let’s assume we renamed the file to fragment-enable-tftp.config.
SRC_URI:append:t3-gem-o1 = " \
    file://uEnv-desktop.txt \
    file://uEnv-rd.txt \
    file://fragment-mmc-env.config \
    file://fragment-enable-tftp.config \
"

bitbake -c clean virtual/bootloader
bitbake virtual/bootloader
If the above commands complete successfully, the u-boot.img and u-boot-spl.bin files will be generated in the <sdk-directory>/build/t3-gem-o1/deploy-ti/images directory.

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Modifying Source Code

Open the <sdk-directory>/build/tmp-musl/work/t3_gem_o1-gemstone-linux-musl/u-boot-t3-gem-o1/2025.04+git/git folder in your text editor. After making the desired changes, compile U-Boot using the following commands. To avoid losing your changes, you can create your own fork of the T3 Gemstone U-Boot repository and push your changes there. 
bitbake virtual/bootloader
After the compilation is complete, copy the generated files in the <sdk-directory>/build/t3-gem-o1/deploy-ti/images directory to the /boot directory on the SD card. If you have fixed an issue or added a new feature, you can create a pull request to the T3 Gemstone U-Boot repository so the community can benefit from it.