BeagleBone Black Booting Process

© 2015-17 SysPlay Workshops <workshop@sysplay.in>
All Rights Reserved.
BeagleBone Black Booting Process

2© 2015-17 SysPlay Workshops <workshop@sysplay.in>
What to Expect?
PC Booting Process
Beagle Booting Process
Booting through SD Card
Partitioning & Creating Root Filesystem
W's of Bootloaders
U-Boot
Serial Booting
Adding the command in U-Boot

PC Booting Process
Power-Up / Reset Operation
BIOS
(POST)
Primary
Bootloader
(446 Bytes)
Primary
Bootloader
(446 Bytes)
0XAA55
Partition Table
(64 Bytes)
GRUB/LILO
(Kernel Loader)
Kernel
Initrd
Init
(User Space
App.)
System
Startup
Stage 1
Bootloader
Stage 2
Bootloader
KernelStage 2
Bootloader
User
Space

BBB Memory Organization
DDR
512MB
ROM
Internal
RAM
128KB
SOC
BeagleBone Black
0x80000000
0x40200000
0x402F0400 EMMC
4GB
Ext.
MMC

General Booting of BeagleBoard
Image under “CC BY-SA 3.0 US” from http://omappedia.org/wiki/Bootloader_Project

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

AM335x Processor Boot Modes
Memory Booting
NAND, NOR, MMC
Peripheral Booting
USB, UART, Ethernet
The Order / Sequence is determined by
The set of GPIO configuration pins called SYSBOOT
pins (5 pins)

(SD Card)
ROM code loads x-loader (MLO)
Performs External DRAM configuration
X-loader loads u-boot
Default location is /media/mmcblkp01/u-boot.bin
U-boot executes default environment/commands
Default location is /mmcblkp01/uEnv.txt
Commands load the Kernel
Default location is /media/mmcblkop2/boot/uImage
Kernel reads root file system

Board Bring Up
Spoil the board
Delete u-boot.img, uImage and user.txt
Get the u-boot.img and user.txt
Change the ipaddr in user.txt
Reboot the board
Press the user button until countdown
This will boot the board with ramdisk
Re-partition the card
Create 3 partitions and filesystem on them
Restore the required images
Get MLO, u-boot.img, uEnv.txt and am335x-boneblack.dtb
Get the RootFs.xz in second partition and untar the same

Recovering the Board
Copy the user.txt and u-boot.img
tftp -g <serverip> -r u-boot_bringup.img -l u-boot.img
tftp -g <serverip> -r user.txt -l user.txt
Change the ip address in user.txt
Delete the kernel - /boot/zImage
Delete the Root Filesystem
Reboot the system

SD Card Partitioning
fdisk /dev/mmcblk0
Create a windows partition of 116MB
Make it bootable
Create two primary Linux partitions of sizes 3GB
and 1GB approx

Creating & Filling the Filesystem
Create the Filesystem of type Fat 32 on first partition
mkfs.vfat -F 32 /dev/mmcblk0p1 -n boot
Create the Filesystem of type ext2 on Second and third partitions
mke2fs -L FirstRootFs /dev/mmcblk0p2
mke2fs -L SecondRootFs /dev/mmcblk0p3
Fill the filesystem with contents
tftp -g 192.168.20.181 -r RootFs.xz
tar -zxvf RootFs.xz
Copy u-boot_bringup.img, user.txt and uEnv.txt in vfat partition
tftp -g <server_ip> -r u-boot_bringup.img -l u-boot.img
tftp -g <server_ip> -r user.txt
tftp -g <server_ip> -r uEnv.txt
tftp -g <server_ip> -r am335x-boneblack.dtb

Board Bring up Hands On
Delete all the contents of External SD card
Get the MLO, u-boot.img, user.txt
Press the boot switch to change the booting sequence
Kernel boots with ramdisk
Re-partition the card and create the filesystem(vfat on first
partition and Ext2 on other two)
Get the MLO, u-boot.img, am335x-boneblack.dtb, uEnv.txt
and user.txt in first partition
Get the Root Filesystem in second partition

What is a Bootloader?
Also, referred to as boot monitor, is a small piece
of software that executes soon after powering up
a system
Simply, a loader (program), which boots up
(starts) the system
A Customized Program started by
Controller's Internal Code in Embedded Systems, Or
External Pre-programmed Code (like BIOS) in
Desktops

Tasks of a Bootloader
Initialization Tasks
Memory Setup & Initialization
System Peripheral Initialization
for the kernel
Actual Task
Load the RAM-based File System, like initrd, initramfs, ...
Load the Kernel with proper arguments
Jump to the start of the Kernel
Additional Tasks
Multiple Kernel Boots

Design of Bootloaders
As Bootloader is started by a fixed code
It needs to be placed at a hard-coded location
Hard-coded locations are not big enough for the
complete code (/ logic / tasks) of the bootloader
Hence, it is typically split into 2 portions
Stage 1 – Small enough to load Stage 2 from our desired
location
Stage 2 – The actual bootloader we want to have

Bootloader Comparisons
On Desktops
Initialization Tasks are done by BIOS
Bootloader is to just Boot the Kernel
On Embedded Systems
All needs to be done by the Bootloader
But in an optimized way
Hence, the 2 bootloaders are
Quite different from each other
Later being more board dependent & constrained

Stage 2 Bootloader Flavours
Prevalent Desktop Bootloaders
LILO
GRUB
SYSLINUX
loadlin
Coreboot (Earlier called LinuxBIOS)
Popular Embedded System Bootloaders
BootLoader Object (BLOB)
Redboot
U-Boot

Check
Name the Stage 1 & Stage 2 bootloaders
Desktops
Embedded Systems

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 Hands-on
Stopping at the U-Boot
Help - “?”
Commands
Booting: bootp, bootm, boot, ...
NOR Flash: erase, cp, protect, …
NAND Flash: nand
Miscellaneous: reset, ...
...
Environment Variables
printenv
setenv
saveenv

Booting through the Serial
Boot ROM can load any binary x-loader image into
the internal SRAM of 109K
For Serial Boot,
A simple ID is written out of the serial port
If host responds within that short window of time
The ROM reads from serial port
Transfers the data to the internal SRAM
Control is passed to the start of the SRAM.

BBB Serial Booting
ROM
Code
X-loader
SOC
ROM
Internal RAM
DDR
u-boot
bbb.dtb
uImage
Ramdisk/initrd
(Ramdisk Boot)
X-Loader
u-boot

U-Boot Compiling
Preparing the Makefile
Setup CROSS_COMPILE for cross compilation
Or, invoke make with these options
Configuring for a particular board
make <board>_config
Compiling for the configured board
make (Output would be u-boot.img)
Cleaning up
make clean

Adding the Command in U-Boot
Create the file cmd_<file>.c
Fill the Macro 'U_BOOT_CMD()'
name: the name of the command
maxargs: the maximum number of arguments this function takes
command: func pointer (*cmd)(struct cmd_tbl_s *, int, int, char *[])
usage: Short Description
help: Long description
Add the entry into common/Makefile
COBJS-$(CONFIG_CMD_<NAME>) += cmd_<file>.o
Include the macro CONFIG_CMD_<NAME> in board.h file

What all have learnt?
PC Booting Process
Booting through SD Card
Partitioning & Creating Root Filesystem
W's of Bootloaders
U-Boot
Serial Booting
Adding the command in U-Boot

Any Queries?

BeagleBone Black Booting Process

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