BeagleBone Black Bootloaders

© 2015-18 SysPlay Workshops <workshop@sysplay.in>
All Rights Reserved.
BeagleBone Black Bootloaders

2© 2015-18 SysPlay Workshops <workshop@sysplay.in>
What to Expect?
BBB Memory Organization
Beagle Booting Process
W's of X-Loader
BSP in X-Loader
W's of U-Boot
BSP in U-Boot

DDR
512MB
ROM
Internal
RAM
64KB
SOC
BeagleBone Black
0x800000
00
0x402000
00
0x402F0400 EMMC
4GB
Ext.
MMC

General Booting of BeagleBoard
ROM
Code
Internal
ROM
X-Loader
Internal
SRAM
Internal
ROM
U-Boot
External
DDR
Kernel
External
DDR

BBB Images
ROM
Code
X-Loader
SOC
BeagleBone Black
ROM
Internal
RAM
DDR
u-boot
bbb.dtb
uImage
Ramdisk/initrd
(Ramdisk Boot)

X-Loader

W's of X-Loader
First stage bootloader for Beagle Board
Derived from u-boot – the second stage
bootloader
Named as MLO (Memory Loader) in
filesystem.
Runs in an internal SRAM
Loads the second stage bootloader i.e. U-
Boot

Let's Get Down to Source Code

X-Loader Code Flow
cpu/armv7/start.S
reset()
Disable IRQ & FIQ.
Switch to
supervisor mode
Low Level Initialization
cpu_init_cp15()
Invalidate and
disable
Instruction & data
Cache
Disable MMU
cpu/armv7/lowlevel_i
nit.S
lowlevel_init()
arm/lib/crt0.S
_main()
C Runtime setup
arm/lib/spl.c
board_init_f()
Early Board Setup
Clear BSS and jump
to board_init_r()
Common/spl/spl.c
board_init_r()
Load the u-boot

X-Loader for BBB
Board configuration
include/configs/am335x_evm.h
CPU dependent code
arch/arm/cpu/armv7/*.c
arch/arm/cpu/armv7/lowlevel_init.S
arch/arm/lib/crt0.S
arch/arm/cpu/armv7/am33x/board.c
arch/arm/lib/spl.c
Board dependent code
Board/ti/am335x/board.*
Board independent code
common/spl/spl.c

X-loader Hands on
On which pin is the LED connected?
How is it connected – Active high/Active
low?
Pin muxing/Direction
Registers to manipulate the LED

X-loader Hands on...
Schematic (BBB SRM)
On which pin is the LED connected?
How is it connected - Active high/Active low?
Datasheet (TRM of AM33XX)
Pin muxing / Direction
Registers to manipulate the LED

U-Boot

W's of U-Boot
Universal Bootloader (U-Boot)
An Open Source Bootloader
With minimal changes, can be ported for any
board
GRUB/LILO
Designed with x-86 in mind
Huge in Size
Needs to be changed drastically for porting on
other architecture

U-Boot Source Tree
arch – Architecture dependent Code
board – Board dependent Code
common – Environment & Command Line Code
doc – Documentation
drivers – Device specific Drivers
fs – File System support Code
include – Headers
lib – Compression, Encryption related Code
net – Minimal Network Stack
tools – U-Boot Utilities (mkimage is here)

U-Boot Initialization Details
Bootloader starts its execution from flash /RAM
Hardware Diagnostics, like POST, …
Configuring the CPU speed, MMU setting, etc
Memory setup & initialization
Setting up interfacing ports like serial, VGA, …
Sets up the address of the boot parameters

Let's Get Down to Source Code

U-Boot Code Flow
cpu/armv7/start.S
reset()
Disable IRQ & FIQ.
Switch to
supervisor mode
Low Level Initialization
cpu_init_cp15()
Invalidate and
disable
Instruction & data
Cache
Disable MMU
cpu/armv7/lowlevel_i
nit.S
lowlevel_init()
arm/lib/crt0.S
_main()
C Runtime setup
arm/lib/board.c
board_init_f()
Early Board Setup
Calculate
Addresses (SP,
Dest, GD) for
Relocation
Call the board
initialization
functions
Arch/arm/lib/reloc
ate.S
relocate_code()
General Relocation
arm/lib/crt0.S
_main()
Clear BSS, Setup GD and jump
to board_init_r()
arm/lib/board.c
board_init_r()
Final Board Setup
Board/it/am335x/board.c
board_init()
Board specific device setup
env_relocate()
Setup Environment
common/main.c
main_loop()
Boot the kernel or give out
the u-boot shell

Let's Check
What is the starting point of u-boot?
Where is the address of the Environmental Variables set?
Where is RAM initialized?
Which file is the interface between the architecture dependent code & board
dependent code?
Where is serial initialized?
From where is the kernel invoked? And what are the parameters passed to the
kernel?
Where is default environment defined?
where is the board dependent file for BBB?
Where is the configuration file for BBB?
Where is the architecture number set?
Where is the pin multiplexing done?
From where does the boot delay comes?

U-Boot BSP
Board configuration
include/configs/am335x_evm.h
CPU dependent code
arch/arm/cpu/armv7/*.c
arch/arm/cpu/armv7/lowlevel_init.S
arch/arm/lib/crt0.S
arch/arm/lib/relocate.S
arch/arm/cpu/armv7/am33x/board.c
arch/arm/lib/board.c
Board dependent code
Board/ti/am335x/board.*
Board independent code
common/*
driver, fs, common(cmd, flash, env..)

U-Boot Configuration
Creating a configuration file for the board
Adding a Kconfig file in
'board/<vendor>/<board> with below
info:
Architecture
CPU
Board
Vendor (May be NULL)
SoC (May be NULL)

U-Boot Configuration Output
Configuration files for use in C Sources
include/generated/autoconf.h
spl/include/generated/autoconf.h (For SPL)
include/config.h
include/configs/<board>.h
Configuration files for Makefile
include/config/auto.conf
spl/include/config/auto.conf (For SPL)
include/autoconf.mk

U-Boot Porting
Implies adding a new Board to U-Boot
That entails
Adding board specific code at the right places
Adding the new board directory under board/ with
Makefile
Initialization Code for the Board
Kconfig file
Adding the new board header under include/configs/
with
Configuration for the Board

U-Boot Porting Hands On
Add the configuration file .h in include/configs
Add the Kconfig file at board/<vendor>/<soc>/
Modify the arch/arm/Kconfig to add the menu item for the
board and source the board dependent Kconfig file
Add the board dependent file at board/<vendor>/<soc>/
Modify the path for linker script at
include/configs/<config_name.h>
In the linker script, add the path for built_in.o for the board.
Add the defconfig file in configs folder. Add atleast
CONFIG_ARM and CONFIG_TARGET_<BOARD>

Env in I2C eeprom
Configure for Env is in eeprom
I2C EEPROM Slave Address
Env offset in eeprom
Page write delay
Page write bits

What all have learnt?
Beagle Booting Process
W's of X-Loader
BSP in X-Loader
W's of U-Boot
BSP in U-Boot

Any Queries?

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