Make a simple coreboot

On my Ubuntu 21.03 host, I follow the coreboot doc to build a simple ROM: https://doc.coreboot.org/tutorial/part1.html

This is the steps I take and a few screenshot below. This coreboot’s payload is a ELF coreinfo application.

sudo apt-get install -y bison build-essential curl flex git gnat libncurses5-dev m4 zlib1g-dev
git clone https://review.coreboot.org/coreboot
cd coreboot
make crossgcc-i386 CPUS=$(nproc)
make -C payloads/coreinfo olddefconfig
make -C payloads/coreinfo
make menuconfig
   select 'Mainboard' menu
   Beside 'Mainboard vendor' should be '(Emulation)'
   Beside 'Mainboard model' should be 'QEMU x86 i440fx/piix4'
   select < Exit >

   select 'Payload' menu
   select 'Add a Payload'
   choose 'An Elf executable payload'
   select 'Payload path and filename'
   enter 'payloads/coreinfo/build/coreinfo.elf'
   select < Exit >
   select < Exit >
   select < Yes >
make

The goal is to boot this coreboot.rom (256KB) on Simics and it looks like this

Intel Simics

Download from Intel, https://www.intel.com/content/www/us/en/download/645996/30403/simics-simulator-public-release-preview.html

or

https://software.intel.com/content/www/us/en/develop/articles/simics-simulator.html

Extract and install according to the installation guide, https://downloadmirror.intel.com/30403/eng/installation%20and%20getting%20started%20guide.pdf

Once done, create a script based on the “qsp-client-core.simics”

decl {

 params from "%simics%/targets/qsp-x86/qsp-client-core.simics"
  default enable_efi = FALSE
  default bios_image = "/home/james/Works/coreboot/coreboot/build/coreboot.rom"
}

run-command-file "%simics%/targets/qsp-x86/qsp-client-core.simics"

Here is what printed on the serial console

then it reaches,

Debugging coreboot

To debug, Simics need to load the debug symbol, which located in the “build/cbfs/fallback/”

The very first elf file x86 run is bootblock (0xffff_fff0) -> romstage -> ramstage -> postcar

On Simics CLI or on the script we created earlier, add

enable-debugger
add-symbol-file "/home/james/Works/coreboot/coreboot/build/cbfs/fallback/bootblock.debug"

Now we are ready to do single stepping the code

Conditional Probability

Flipping a fair coin has a probability of 0.5 landing a tail, and 0.5*0.5=0.25 for landing tail twice in a row. There are no relationship between each coin flip.

It is more interesting when we can improve a probability from observing from others.

This is a short tour about conditional probability in a few diagrams and a simple example attached.

Notation

p(A) refers to the probability of A

p(A|B) refers to the probability of A, given B already happens.

p(A*B|B) refers to the probability of both (A and B), given B already happens

p(A*B)/p(B) refers to the probability of both (A and B), scale respect to the probability of B happens

p(A|B)
= p(A*B|B)
= p(A*B)/p(B)

Example

Consider the following table and diagram:

S = 10, refers to the sampling space (10 dots)
p(A) = 6/10 = 0.6
p(B) = 3/10 = 0.3
p(A*B) = 2/10 = 0.2

p(A|B) = p(A*B|B) = p(A*B) / p(B) => 0.2 / 0.3 = 0.667
p(B|A) = p(B*A|A) = p(A*B) / p(A) => 0.2 / 0.6 = 0.333

The conditional probability shows us that, both probability improves by 11% after adding new condition to restrict sampling space accordingly.

p(A) = 0.6
p(A|B) = 0.667 <-- 11% improvement

p(B) = 0.3
p(B|A) = 0.333 <-- 11% improvement

Bayes’ Theorem

It is an interpretation of conditional probability that: Continuously updating hypothesis by its test evidences

Notation

p(A|B) refers to the probability of A (hypothesis or belief) , given B already happens (observation)

p(B|A) refers to the probability of B (observation), when A happens

p(A) refers to probability of A, prior to observation.

p(B) refers to probability of B, observation happens in both correctly and incorrectly (false positive).

p(A|B) = p(A*B) / p(B)
p(B|A) = p(B*A) / p(A) 
=> p(B|A)*p(A) = p(B*A)

where p(A*B) = p(B*A),

p(A|B) = p(B|A)*p(A) / p(B)

Example

Consider the following table and diagram:

S = 10, refers to the sampling space (10 dots)
p(A) = 6/10 = 0.6
p(B) = 3/10 = 0.3
p(A*B) = 2/10 = 0.2

p(A|B) = p(B|A) * p(A)/p(B) =>  0.333 * 0.6/0.3 = 0.666

Using STM32F429I-DISCO (gen 0)

Background

I must have own this STM32F429I-DISCO for over 6 years, comparing DISCO with the newer DISC1 (32F429IDISCOVERY – Discovery kit with STM32F429ZI MCU * New order code STM32F429I-DISC1 (replaces STM32F429I-DISCO) – STMicroelectronics), the main different is the debugger that does not support VCP. Hence, the very first thing needed to sort out is a USB-UART adaptor cable.

This cable needs to connects with USART1 via the P1 header

P1 Header
- RX (yellow) to PA9
- TX (red) to PA10
- GND (black) to one of the GND available on the board
The UART is configured at 115200.

Detail documentation for the MCU, https://www.st.com/resource/en/reference_manual/rm0090-stm32f405415-stm32f407417-stm32f427437-and-stm32f429439-advanced-armbased-32bit-mcus-stmicroelectronics.pdf

The board has 2MB Flash (0x0800_0000) and 8MB Ram (0x9000_0000), there is no mmc interface nor ethernet port available. Therefore the goal is to create XIP Linux to boot from this 2MB flash memory.

Building uBoot

This goal is to reduce uBoot to under 128Kbytes, so it fits to Sector 0 to 4.

Buildroot

XIP Kernel

CROSS_COMPILE=arm-none-eabi- ARCH=arm make menuconfig
CROSS_COMPILE=arm-none-eabi- ARCH=arm make -j 8
mkimage -A arm -O linux -T kernel -C none -a 0x08020000 -e 0x08020040 -d arch/arm/boot/xipImage uxipImage

Programming flash

Using the official STM32 Programmer is fairly easy

.\STM32_Programmer_CLI.exe --connect port=SWD -d C:\st\u-boot.bin 0x08000000

Openocd is the opensource alternative, openocd.org/getting-openocd/

.\openocd.exe -f board/stm32f4discovery.cfg `
-c "program C:\\stm32f4\\u-boot.bin 0x08000000 verify" `
-c "program C:\\stm32f4\\uxipImage_0x08020000.bin 0x08020000 verify" `
-c "program C:\\stm32f4\\stm32f429-disco.dtb 0x081e0000 verify" `
-c "reset run" -c "shutdown"

Reference

Here are some links I found it useful

1. https://youtu.be/p5LxCXbhSCo
2. https://youtu.be/A21I6NrLszE
3. Running Linux 4.9 on Cortex-M4 STM32F4 (29I-DISC1) – Stack Overflow
4. 32F429IDISCOVERY – Discovery kit with STM32F429ZI MCU * New order code STM32F429I-DISC1 (replaces STM32F429I-DISCO) – STMicroelectronics
5. OpenOCD User’s Guide: Top
6. microcontroller – Trying to program an STM32 device with ST-LINK_CLI.exe – Electrical Engineering Stack Exchange
7. arm – Why Device Tree Structure (DTS) file is needed both in bootloader and kernel source code? – Stack Overflow
8. embedded – How to override U-Boot “bootcmd” variable – Stack Overflow
9. [PATCH] ARM: add dtbImage.<dt> and dtbuImage.<dt> rules – 摩斯电码 – 博客园 (cnblogs.com)
10. embedded linux – What is different between u-boot.bin and u-boot.img – Stack Overflow
11. memory – Linux kernel cannot execute any binary (error -12) – Unix & Linux Stack Exchange
12. LinuxRamdiskRoot < DULG < DENX

What we need

1. nasm
2. qemu-system-x86 (for testing before trying it out on real hardware)

Print a character

Create a “print.asm” file,

ORG 0x7c00
BITS 16

main:
        mov ah, 0eh
        mov al, 'z'
        mov bx, 0
        int 0x10
        jmp $

times 510-($ - $$) db 0
dw 0xAA55

Create a “Makefile”,

all:
        nasm ./print.asm -o ./print.bin -f bin

.PHONY disasm:
        ndisasm print.bin

.PHONY run:
        qemu-system-x86_64 -hda ./print.bin

Notes:

Once downloaded

This machine has Ubuntu 20.04 LTS. Once we have upgrade using,

sudo apt update
sudo apt upgrade

grep "menuentry '" /boot/grub/grub.cfg

sudo vi /etc/default/grub

sudo grub-mkconfig -o /boot/grub/grub.cfg

or

sudo update-grub

Reboot and Clean up

Not clean enough

dpkg --list | egrep -i --color 'linux-image|linux-headers|linux-modules' | awk '{ print $2 }'

sudo apt purge linux-headers-5.7.1 linux-image-5.7.1 linux-image-5.7.1-dbg

Script

dpkg --list | egrep -i --color 'linux-image|linux-headers|linux-modules' | awk '{ print $2 }' > list.txt
vi list.txt

on vi, we can replace the newline to space, and adding “apt purge” on the front

:%s/\n/ /g
<ESC>
a
apt purge
<ESC>
:wq

sudo sh list.txt

What is needed

1. VcXsrv Windows X Server – Browse /vcxsrv at SourceForge.net
2. WSL 2 (I am using Alpine)

Setting up

Here are the steps to install VcXsrv

• 

Basically it creates a x11config.xlaunch file like this,

Try it out

Creating a startup file “.profile” at home directory ~/

~/.profile

===

export DISPLAY=$(awk '/nameserver / {print $2; exit}' /etc/resolv.conf 2>/dev/null):0
export LIBGL_ALWAYS_INDIRECT=1

Reference

Alpine On WSL2 – Qiita