Done as part of OMAP Tutorial Hour http://omapedia.org/wiki/Tutorial_Hour_Topics#Aug_25th_at_8:30_am_CST_.28Central_Standard_Time.29

1. Linux without a boot loader – the
OMAP way!
OR
Why I stopped being scared of X-
loader and started using Linux
Nishanth Menon
25 August 2010

2. Introduction
 Why do we need a bootloader?
 Too many bootloaders, yet, not one that works
for me..
 Heresy: What if Linux is used as a bootloader?
 A single s/w package- Reduce maintenance.
 New features – easier to borrow from Linux
 It'd be kinda fun
 Builds on ELC2010 SFO Greg Ungerer, Magnus
Damm 's work
2/

3. Agenda 1/2
 Concepts of OMAP GP device boot load process
 Why x-loader?
 Content of a Linux image without bootloader
 The tools
 Tagger
 Gpsign
 Demo
3/

4. Agenda 2/2
 What next? Using Linux as a bootloader
 Problems with the Approach
 Summary
 Terminology
 References
4/

5. Concepts of OMAP boot load process
X-loader U-Boot Kernel
I/O device
(Nand/MMC)
U-Boot Kernel
ROM Code “XIP” Memory mapped
device(NOR)
ROM
ROM
Peripheral
2nd U-Boot Kernel
(USB/UART)
SDRAM NOR SRAM
SDRAM NOR SRAM Scope of this presentation
5/

6. Why X-loader?
 SRAM is tiny: 64K(OMAP3 GP), 32K(OMAP4)
 “Traditional Bootloaders” are bloated!– e.g.
 U-boot: beagleboard build is 196K!!
 Barebox: first beagleboard version of
barebox was 36k, it is around 40+k now!
 There is never going to be a solution as long as
we need to keep adding new features!
6/

7. What I love about X-loader
 “shared code” with TI u-boot
 Does the stuff (well.. mostly)
 Simple code (usually!)
 TINY.. well.. mostly so, until we started adding
FAT, MMC, USB...
 TI mostly has some variant for every OMAP
7/

8. What I hate about X-loader
 It does way more than what it needs to do the job
- “load and execute u-boot”
 There is NO mainline x-loader code
 Toooo many variants of x-loader.. There is a
broken up community of x-loader – each centered
around some silicon variant
 I am gonna stop now..
8/

9. Do I REALLY need x-loader?
NO
There are alternatives: Psuedocode:
Switch cpsr to svc
Disable watchdog
 Barebox? Enable uart
Copy image from x to sdram
 my_own_stupid_bootloader.s Give control to image
 Linux? But it is 3 megs big!!
 Soln: Load it to SDRAM directly
9/

10. Basic things for Linux to boot on
OMAP?
 Basic h/w configuration:
 Make the SDRAM ready – clocks
configured
 Very basic mux – we expect the kernel to
take care of it's own needs..
 One serial port configured
 Setup ATAGs as required for any Linux port of
ARM to boot
 Load the kernel up in SDRAM
Ref: http://www.arm.linux.org.uk/developer/booting.php
10 /

11. Linux image without bootloader
Load address, size
gpsign
gpsign
Boot Configuration Header
Header
Disable watchdog
Disable watchdog
Configure one uart
Configure one uart
Sty Setup
Setup tagger
tagger
r0 ==00
r0
r1 ==mach ID
r1 mach ID
r2 ==ATAG address
r2 ATAG address
bzImage
piggy
make bzImage
make bzImage
zImage
11 /

12. How does tagger work?
Sty
tagger
tagger
ATAG
bzImage
Sty template
Config file
Config file
bzImage
bzImage -tags
-tags
- -machid Marker:
machid Size
Marker:
MACHID
12 /

13. Sty in the works
1 – the
setup Disable
WDT
Sty
Cache, int
disable
ATAG
2 – hand Setup
over to UART
piggy
piggy Setup
Uart mux
4- give 3-
control to zImage
uncompres
kernel Relocate atag to
s the real
First 16k memory
image
Uncompressed kernel
13 /

14. GP Sign – Boot Header
Load address
Size of image
Clock
configuration
gpsign
gpsign
DDR
configuration
GPMC CS0
configuration
Config file
Config file MMC
image
image - -clock config
clock config configuration
- -ddr config
ddr config
Configuration
Header
image
14 /

15. How does this GP image boot?
Load address OMAP
Size of image
2 OMAP
Figures out RomCode
Clock
where to
configuration
load
DDR
configuration
1 3- DPLLs
Configures Loads
GPMC CS0 clocks image
configuration 4 – gives
control
MMC
configuration
Configuration
Header SDRAM
SDRAM
image
There is more that happens in HS/EMU device..
15 /

16. Demo
16 /

17. Next steps 1/2: optimize
 Choose a sane kernel configuration for the
environment you'd like to work on
 If your board has ethernet, enable ethernet, use
nfs as rootfs
 If you board has usb, you may want to try out
usbnet
 Reduce statically built kernel drivers and have
only the kernel modules you need (if you dont
care for multimedia in your bootloader, why
have dspbridge, audio drivers enabled?)
17 /

18. Next steps 2/2: power features
 Write your own custom shell scripts if your “users”
are interested in “u-boot like” environment, -e.g.
setenv/saveenv pair could be:
setenv() { x=$1;shift;export $x="$*";}
saveenv() { export>/etc/exported_vars;}
and the /etc/profile file uses that file..
 Use kexec! With kexec you get bootm equivalent.
 You can wrap kexec inside a bootm as well.
 Remember you now have the power to do what you
can imagine without thinking how to port it..
18 /

19. Ok, so what's the catch? (1/2)
 Deployment issues
 Single bootimage requirement for multiple
board variants and (clocks sdrc, machid are
all coded in),
 Training new users and re-training old users?
 NAND technology and badblocks → with large
uImage, any bad block can make the image no
longer usable.. but we can debate that at a later
date ;)
19 /

20. Ok, so what's the catch? (2/2)
 “MLO file” MMC with FAT partition on
OMAP3 has limitations of 128k sized image(so
not kernel or u-boot at least :(), but it will
become usable with OMAP4 -are there options?
Probably (see references)
 So now we need to maintain and test kernel in
two defconfig configurations? - yeah maybe..
20 /

21. Summary
 Linux without a bootloader provides a lot of
flexibilities
 There are limitations as well – wont solve everyone's
problems.
 Could be suited in a single platform environment
 Not really hardcore tested solution beyond “lab
environment”
21 /

22. Terminology
 XIP – Execute in Place
 U-Boot – Das U-Boot – the Universal Boot
loader
 X-loader – TI variant of U-Boot. A stripped
down version
 Barebox – Somewhere between Linux and U-
boot – can also be used as a boot loader
 Busybox – a filesystem with tiny versions of
standard unix utils
 OMAP-Uboot-utils – a bunch of utilities to do
the grunt workof this presentation
22 /

23. References
 ARM linux kernel boot requirements:
http://www.arm.linux.org.uk/developer/booting.php
 ELC2010 work (SFO)
 Greg Ungerer: Linux without a bootloader
http://elinux.org/images/7/7b/LinuxWithoutABootLoader.pdf
 Magnus Damm: Linux as a bootloader
http://elinux.org/images/2/2f/ELC-2010-Damm-Kexec.pdf
 Grant Likely: Device tree support
http://elinux.org/images/b/b6/ARM_Device_Tree_Status_Report.pdf
 Ghorai Sukumar: eMMC boot
http://omapedia.org/wiki/E-MMC_boot#You_can_boot_omap3630_without_x-loader
 Kexec: http://www.ibm.com/developerworks/linux/library/l-kexec.html
 Omap u-boot utils: http://code.google.com/p/omap-u-boot-utils/
23 /

24. Backup
 Reference code:
http://github.com/nmenon/uomapfs
branch: ldemo
 You get kexec, linux-omap omap-uboot-fs and a
bunch of reference configuration files for
SDP3630 which can be easily customized and
improved for development platforms.
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