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This document describes how to use U-Boot on the Broadcom 7445 SoC, as
a third stage bootloader loaded by Broadcom's BOLT bootloader.
BOLT loads U-Boot as a generic ELF binary. Some U-Boot features such
as networking are not yet available but other important features are,
- ext4 file system traversal
- support for loading FIT images
- advanced scripting
- support for FIT-provided DTBs instead of relying on the
BOLT-provided DTB
A customized version of this port has been used in production. The
same approach may work on other BCM7xxx boards, with some
configuration adjustments and memory layout experimentation.
make bcm7445_defconfig
${CROSS_COMPILE}strip u-boot
Flash the u-boot binary into board storage, then invoke it from BOLT.
For example:
BOLT> boot -bsu -elf flash0.u-boot1
This port assumes that I-cache and D-cache are already enabled when
U-Boot is entered.
Flattened Image Tree Support
What follows is an example FIT image source file. Build it with:
mkimage -f image.its image.itb
Booting the resulting image.itb was tested on BOLT v1.20, with the
following kernels:
and with a generic ARMv7 root file system.
/ {
description = "BCM7445 FIT";
images {
kernel@1 {
description = "Linux kernel";
* This kernel image output format can be
* generated with:
* make vmlinux
* ${CROSS_COMPILE}objcopy -O binary -S vmlinux vmlinux.bin
* gzip -9 vmlinux.bin
* For stblinux-3.14, the specific Broadcom
* board type should be configured in the
* kernel, for example CONFIG_BCM7445D0=y.
data = /incbin/("<vmlinux.bin.gz>");
type = "kernel";
arch = "arm";
os = "linux";
compression = "gzip";
load = <0x8000>;
entry = <0x8000>;
hash@1 {
algo = "sha256";
ramdisk@1 {
description = "Initramfs root file system";
data = /incbin/("<initramfs.cpio.gz>");
type = "ramdisk";
arch = "arm";
os = "linux";
compression = "gzip";
* Set the environment variable initrd_high to
* 0xffffffff, and set "load" and "entry" here
* to 0x0 to keep initramfs in-place and to
* accommodate stblinux bmem/CMA reservations.
load = <0x0>;
entry = <0x0>;
hash@1 {
algo = "sha256";
fdt@1 {
description = "Device tree dumped from BOLT";
* This DTB should be similar to the
* BOLT-generated device tree, after BOLT has
* done its runtime modifications to it. For
* example, it can be dumped from within
* U-Boot (at ${fdtcontroladdr}), after BOLT
* has loaded U-Boot. The result can be added
* to the Linux source tree as a .dts file.
* To support modifications to the device tree
* in-place in U-Boot, add to Linux's
* arch/arm/boot/dts/Makefile:
* DTC_FLAGS ?= -p 4096
* This will leave some padding in the DTB and
* thus reserve room for node additions.
* Also, set the environment variable fdt_high
* to 0xffffffff to keep the DTB in-place and
* to accommodate stblinux bmem/CMA
* reservations.
data = /incbin/("<bolt-<version>.dtb");
type = "flat_dt";
arch = "arm";
compression = "none";
hash@1 {
algo = "sha256";
configurations {
default = "conf@bcm7445";
conf@bcm7445 {
description = "BCM7445 configuration";
kernel = "kernel@1";
ramdisk = "ramdisk@1";
fdt = "fdt@1";