Bootloaders

© 2010-15 SysPlay Workshops <workshop@sysplay.in>
All Rights Reserved.
Bootloaders

2© 2010-15 SysPlay Workshops <workshop@sysplay.in>
What to Expect?
W's of Bootloaders
Specifics of a Bootloader
With U-Boot in consideration
U-Boot Hands-On
U-Boot Source Code
U-Boot Porting

What is a Bootloader?
Simply, a loader (program), which boots up
(starts) the system
A Customized Program started by
Controller's Internal Code in Embedded Systems, Or
Default Program Counter settings 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
Multiple-way 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

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

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

U-Boot Specifics

U-Boot Initialization Details
Bootloader starts its execution from flash / *PROM
Hardware Diagnostics, like POST, …
Configuring the CPU speed, MMU setting, etc
Memory Initialization
Determining on-board memory size
Turning on the caches
Clearing memory
Optionally, Relocate to RAM, and start execution from
there
Setting up interfacing ports like serial, VGA, …

U-Boot's Argument Passing
Three-Party Communication
Kernel hard-codes the offset of argument structure
Developer attaches the load & start addresses
In a U-Boot recognizable Header
To the kernel image
We shall do when building kernel image
U-Boot
Reads the U-Boot Header wrapped kernel image
Loads the kernel at the read load address
Fills the kernel argument structure at the hard-coded offset
Arguments are obtained from its environment
Jumps to the kernel start address to start executing

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

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 Compiling
Preparing the Makefile
Setup (ARCH,) 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.bin)
Cleaning up
make clean

U-Boot Porting
Implies adding a new Board to U-Boot
That entails
Adding <board>_config for make
Adding board specific code at the right places

Adding <board>_config
Adding an entry in the Makefile with
Architecture
CPU
Board
Vendor (May be NULL)
SoC (May be NULL)
Adding the new board directory under board/ with
Makefile
Initialization Code for the Board
Configuration Makefile
Adding the new board header under include/configs/ with
Configuration for the Board
Note: If a new architecture is added, a U-Boot architecture porting would be needed - though, that is uncommon

Adding Board specific Code
Highly depends on the Board
CPU Architecture
Clock Interfaces
Console Interfaces
LCD Interfaces
Network Interfaces
Peripherals
…
Should be added in the appropriate Folders
With appropriate file names
And following the U-Boot Coding Guidelines
Existing Code may serve as the example for the same

What all have learnt?
W's of Bootloaders
Specifics of a U-Boot
U-Boot Initialization Sequence
U-Boot's argument passing to Kernel
U-Boot Hands-On
U-Boot Source Code
Understanding the Source Structure
Configuring for a Board
Compiling the u-boot image
U-Boot Porting for a New Board

Any Queries?

Bootloaders

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Bootloaders