Summary
=======

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,
including:

   - 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.

Build
=====

make bcm7445_defconfig
make
${CROSS_COMPILE}strip u-boot

Run
===

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:

https://github.com/Broadcom/stblinux-3.14
https://github.com/Broadcom/stblinux-4.1
https://github.com/Broadcom/stblinux-4.9

and with a generic ARMv7 root file system.

image.its:
/dts-v1/;
/ {
	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";
		};
	};
};