Porting of U-boot on Raspberrypi-3

Introduction

The U-Boot utility is a multi-platform, open-source, universal boot-loader with comprehensive support for loading and managing boot images, such as the Linux kernel. This article is a quick start up guide on porting U-Boot for Raspberry-PI board using SD card.

Booting process of RPi3

Stage 1

To reduce cost, the Raspberry Pi (Model A & B) omits any on-board non-volatile memory used to store the boot loaders, Linux Kernels and file systems as seen in more traditional embedded systems. Rather, a SD/MMC card slot is provided for this purpose. (The Raspberry PI Compute Module has 4GB eMMC Flash on-board).

stage 1 boot is in the on-chip ROM. Loads Stage 2 in the L2 cache. The Raspberry Pi’s Broadcom BCM2835 system on a chip (SoC) powers up with its ARM1176JZF-S 700 MHz processor held in reset. The VideoCore IV GPU core is responsible for booting the system. It loads the first stage boot oader from a ROM embedded within the SoC. The first stage bootloader is designed to load the second stage bootloader (bootcode.bin) from a FAT32 or FAT16 file system on the SD Card.

Stage 2

The second stage bootloader – bootcode.bin – is executed on the VideoCore GPU and loads the third stage bootloader – start.elf. (Historically, yet another bootloader called loader.bin was loaded at this stage, but has since been phased out)

Stage 3

The third stage bootloader – start.elf – is where all the action happens. It starts by reading config.txt, a text file containing configuration parameters for both the VideoCore (Video/HDMI modes, memory, console frame buffers etc) and loading of the Linux Kernel (load addresses, device tree, uart/console baud rates etc).

Stage 4

We will use config.txt to tell start.elf to load the kernel.img image. kernel.img, is the first! thing that runs on the ARM processor.We will use u-boot as kernel.img so our config.txt will look as:

            kernel = u-boot.bin

After completing above steps Booting is completed and RPI board is ready to take a commands from user.

Steps To Partitioning SD card

STEP 1:

Insert the formated  microSD Card into your computer and observe which device it registers as by typing command:

            $ sudo fdisk -l

Note:- it will show something like below location.

          /dev/sdc

STEP 2:   Begin the partitioning by typing

           $  sudo fdisk /dev/sdX

(Note:As my device showing as /dev/sdc, I’ll refer /dev/sdc instead of /dev/sdX)

  $ sudo fdisk /dev/sdc

STEP 3:  Execute the below character to list out all the partition’s.

            p

STEP 4:   Execute the below character to delete the previous partitions.

            d

STEP 5:   Execute the below character sequentially to create new partitions

i) To create a new partition

                         n

ii) Partition it would be in primary

                        p

iii) No. Of Partitions

                        1

iv) For default starting value of primary partition

                        [enter]

v) For default size of the primary partition

                        [enter]

STEP 6:   Execute the below character to activate / boot the primary partition

            a

i) To activate the on which partition boot is activated

                         1

STEP 7:   Execute the below character to changing the partition type to FAT32

            t

i) Select the partition number

                        1

ii) For creating FAT32 file system

                        c

STEP 8:   Execute the below character to write all information in to the new partition

            w

STEP 9: After this put following command to format the partition on command line

            $ mkfs.vfat -F 32 -n boot /dev/sdc1

 

Steps To building U-boot image for Rpi-3

Prerequisite:

Cross Compiler           : arm-linux-gnueabi-

GCC                            : GCC Version should be greter than gcc 6.0

Ubuntu                        : Ubuntu 14.04 or more

STEP 1: Install cross compiler and export environment variables:

Run below command to get a ARM based linaro cross compiler.

$ sudo apt-get install gcc-arm-linux-gnueabi

STEP 2:  Download the U-Boot source from the below link

Get the source code by cloning the U-Boot git repository :

$   git clone – -depth 1 – -branch v2017.11 git://git.denx.de/u-boot.git v2017.11

or download the tar file :

 

$   wget ftp://ftp.denx.de/pub/u-boot/u-boot-2017.11.tar.bz2

STEP 3: Compile U-Boot

After downloading you get folder named V2017.11 .Give following commands to compile u-boot.

$   sudo make -C v2017.11/ CROSS_COMPILE=aarch64-linux-gnu- rpi_3_defconfig

$   sudo make -C v2017.11/ CROSS_COMPILE=aarch64-linux-gnu-

After downloading the U-Boot source. it will create a folder, name as V2017.11. After executing all above steps do ‘ ls‘ command,  you can see below images in your folder

u-boot.bin,u-boot.lds,u-boot.map,u-boot.srec

Filename	Description
System.map	The symbol map
u-boot	U-Boot in ELF binary format
u-boot.bin	U-Boot raw binary image that can be written to the boot storage device
u-boot.srec	U-Boot image in Motorola’s S-Record format

Steps to copying u-boot into SD card

STEP 1: Use below link to download bootcode.bin and start.elf according to rpi supported images and copy in to your SDcard.

https://github.com/raspberrypi/firmware/tree/master/boot

STEP 2: Insert the SD card into card reader and connect the USB of card reader to CPU.

STEP 3: You will see a window on monitor, after connecting USB to the CPU

STEP 4: Copy the below images into SD card location using below commands from command line.

u-boot.bin ,  bootcode.bin, start.elf, and config.txt

STEP 5: Give following commands.

$ mount

By putting above command we will get sd card location on which bootable partition is mounted.

$ cp u-boot.bin  (location of SDcard)

eg: In this case location of SD card is-  media/abc/FCF1-DD00/

then follow below commands replacing  with your sdcard location.

# cp bootcode.bin  media/abc/FCF1-DD00/

# cp start.elf  /media/abc/FCF1-DD00/

# vim  /media/abc/FCF1-DD00/config.txt

write config.txt as below:

---

# Serial console output!

enable_uart=1

# 64bit-mode

arm_control=0x200

# Use U-Boot

kernel=u-boot.bin

dtparam=i2c_arm=on

dtparam=spi=on

---

Interfacing with minicom

Connections of RPI:-

1. Insert the SD card containing card reader into RPI board memory card slot.
2. Connect the RPI Tx and Rx pins to RS-232 serial converter.
3. Connect the RS-232 USB to CPU.

RPI GPIO pin number	Rpi pin Description	USB-to-TTL pins
8	Tx	Tx
10	Rx	Rx
6	GND	GND

Steps to use minicom to check the output:-

STEP 1: To open the minicom first install minicom using below command

$ sudo apt-get install minicom

STEP 2: put command dmesg  So that you will get exact address of connecting device(USB-to-TTL)

$ dmesg

STEP 3: Use below command to open the minicom to see the output.

$ sudo minicom  -s

STEP 4: you will get a configuration menu after opening the minicom.

STEP 5: set Serial port setup as below

Serial Device	/dev/ttyUSB0
Bps/Par/Bits	115200 8N1
Hardware Flow Control	No
Software Flow Control	No

STEP 6: Give supply to Rpi3 board,” U-boot>” prompt should be come on minicom  screen.

References

[1]   https://github.com/raspberrypi/firmware/tree/master/boot

[2]   http://elinux.org/RPi_U-Boot