RPi U-Boot

= Overview = Das U-Boot, often abbreviated to just U-Boot, is a bootloader commonly used on embedded systems. U-Boot can be used on the RPi to add flexibility by allowing other boot configurations to be used on top of the single specified file on the SD card.

This page explains how to get U-Boot working on the RPi, and explains how to boot images off both the SD card and over TFTP from the network.

= Prepare a U-Boot image file =

Get the source
Currently, the main U-Boot source tree does not fully support the RPi. Instead, you will need to get the code from Gonzoua's GitHub branch. Note that the code in 'master' (shown by default) is the unmodified U-Boot code - you should select the 'rpi' branch to view the correct code.

You can download the source tarball using this link.

Compile the source
Firstly, you will need to export the cross-compiler prefix in order to target the RPI. Assuming your gcc binary is arm-none-linux-gnueabi-gcc, the command is: CROSS_COMPILE=arm-none-linux-gnueabi- export CROSS_COMPILE

Then, in the U-Boot source directory, run the following to start the build: make rpi_b

The build process should take no more than a few minutes.

Prepare the image file
Follow the instructions on the kernel compilation page to prepare the image file. The file u-boot.bin should be used as the input file.

Once complete, transfer your kernel.img file to the SD card. You may want to rename it to something like uboot.img to distinguish it from an actual kernel image. Ensure that you have the following line in your config.txt file: kernel=uboot.img

Test the image
Boot up the RPi with your new U-Boot image. You should see U-Boot load on the screen, and also on the serial port output (if you have it connected up).

= Preparing a kernel image for use with U-Boot = This assumes you have the kernel source and are familiar with the kernel compilation process. As the kernel source already has the ability to build U-Boot image files, preparing a kernel image is relatively easy. First, you will need to put the U-Boot tool mkimage where it can be found by the kernel make process. This is done by copying it to the cross-compiler bin directory. From the U-Boot source directory, run the following, substituting the location and prefix of your cross compiler accordingly: cp tools/mkimage /bin/arm-none-linux-gnueabi-mkimage

In the kernel source directory, you can now build the U-Boot image: make uImage

The generated boot image file can be found at arch/arm/boot/uImage.

= U-Boot boot options =

Boot from SD card
Booting from the SD card allows multiple kernel images to be used without having to edit the config.txt</tt> file. Place your uImage file(s) on the SD card, then boot up. Press a key to drop to the command line, then enter the following: mmc rescan fatload mmc 0:1 ${loadaddr} <your_uImage_file> bootm

Network boot via TFTP
You will need to have a TFTP server configured. Place your uImage file(s) in the TFTP root directory, and ensure that the file permissions are set accordingly (everyone should have read access - use chmod a+r uImage</tt> if unsure).

To boot using DHCP, ensure that the next-server</tt> and filename</tt> options are set in your DHCP server. Then, boot up the RPi, press a key to drop to the command line, then enter the following: usb start dhcp bootm

To boot using static IP: usb start setenv serverip <tftp_server_ip> setenv ipaddr <a_spare_ip_address> tftpboot uImage bootm

= U-Boot script files = The above instructions can be written into a text file and compiled into a U-Boot script file so that it automatically executes on boot. First, generate the script as follows (substituting your mkimage</tt> with that created earlier: arm-none-linux-gnueabi-mkimage -A arm -O linux -T script -C none -d <your_script_text_file> boot.scr

Place the boot.scr</tt> file on the SD card and it will automatically be used when you power up the RPi.