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U-Boot as a payload to OpenSBI
==============================
[U-Boot](https://www.denx.de/wiki/U-Boot) is an open-source primary boot loader.
It can be used as first and/or second stage boot loader in an embedded
environment. In the context of OpenSBI, U-boot can be specified as a payload to
OpenSBI firmware, becoming the boot stage following OpenSBI firmware
execution.
The current stable upstream code of U-boot does not yet include all patches
necessary to fully support OpenSBI. To use U-Boot as an OpenSBI payload, the
following out-of-tree patch series must be applied to the upstream U-Boot source
code.
HiFive Unleashed support for U-Boot
https://lists.denx.de/pipermail/u-boot/2019-February/358058.html
This patch series enables a single CPU to execute U-Boot. As a result, the next
stage boot code such as Linux kernel can also only execute a single CPU. U-Boot
SMP support for RISC-V can be enabled with the following additional patches.
https://lists.denx.de/pipermail/u-boot/2019-February/358393.html
Building and Generating U-Boot images
=====================================
Please refer to U-Boot build documentation for detailed instructions on how to build U-Boot images.
Once U-Boot images are built, Linux kernel image need to be converted to a format
that U-Boot understands.
```
<uboot-dir>/tools/mkimage -A riscv -O linux -T kernel -C none -a 0x80200000 -e 0x80200000 -n Linux -d \
<linux_build_directory>arch/riscv/boot/Image \
<linux_build_directory>/arch/riscv/boot/uImage
```
Copy the uImage to your tftpboot server path if network boot is required.
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