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BBB Linux Boot Sequence - Step 3: U-boot

BBB Linux Boot Sequence

software-components-necessary-for-linux-booting

boot-sequence-up-to-uboot-running

U-boot

Initialize some of the peripherals like I2C, NAND Flash, ETHERNET, UART, USB, MMC, etc, because it supports loading kernel from all these peripherals.
While in U-boot, the target can communicate with the host over UART, USB, etc.
Load the Linux kernel image from various boot sources to the DDR Memory of the board.
Boot sources could be USB, eMMC, SD card, Ethernet, serial port, NAND Flash, etc.
Passing of boot arguments to the kernel.
The file uEnv.txt need to be provided. It contains environment variables that determines the behavior of U-boot.
- You can overwrite these environment variables to alter the behavior of the U-boot.
- Use this file to set the environment variables which drives the U-boot according to your needs.
U-boot always looks for uImage, which is nothing but zImage + U-boot image header.
U-boot gets information from the U-boot image header. (Basically, the total size of the structure image_header is 64 bytes.)

uimage

u-boot-linux-image-header

Demonstration

Download the root file system Angstrom-systemd-image-eglibc-ipk-v2012.12-beagleboard.rootfs to be used in our demo. (Inside the boot/ folder, uImage-3.8.10 is the kernel image we will be using.)

Copy all the folders in side the root file system into the ROOTFS section of your SD card.


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.../Angstrom-systemd-image-eglibc-ipk-v2012.12-beagleboard.rootfs
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$ sudo cp -r * /media/klee/ROOTFS/

And, sync:


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$ sync

Now, you need to tell U-boot where to locate the Linux kernel image using uEnv.txt. Copy this file into BOOT section. It is in under boot/ folder. (It does not come filled out of the box. You need to fill it in.)


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console=ttyUSB0,115200n8
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ipaddr=192.168.7.2
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serverip=192.168.7.1
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loadaddr=0x82000000
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fdtaddr=0x88000000
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loadfromsd=load mmc 0:2 ${loadaddr} /boot/uImage;load mmc 0:2 ${fdtaddr} /boot/am335x-boneblack.dtb
7
linuxbootargs=setenv bootargs console=${console} root=/dev/mmcblk0p2 rw 
8
uenvcmd=setenv autoload no; run loadfromsd; run linuxbootargs; bootm ${loadaddr} - ${fdtaddr}

L4: Load address of DDR memory (Telling the U-boot to keep the Linux kernel at this address)
L5: .dtb file address
L8: bootm is where U-boot hands over the control to the Linux kernel.

Unmount the SD card from your system:


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umount /media/klee/BOOT
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umount /media/klee/ROOTFS

Insert your SD card into BBB and open minicom.

Boot from SD card, then you'll see all the boot messages followed by the login screen from Angstrom.


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...
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mmc0 is current device                               
3
SD/MMC found on device 0
4
...
5
Checking for: /uEnv.txt ...                                                     
6
370 bytes read in 4 ms (89.8 KiB/s)                                             
7
gpio: pin 55 (gpio 55) value is 1                                               
8
Loaded environment from /uEnv.txt                                               
9
Importing environment from mmc ...                                              
10
Checking if uenvcmd is set ...                                                  
11
gpio: pin 56 (gpio 56) value is 1                                               
12
Running uenvcmd ...                                                             
13
4002080 bytes read in 291 ms (13.1 MiB/s)                                       
14
23334 bytes read in 18 ms (1.2 MiB/s)                                           
15
## Booting kernel from Legacy Image at 82000000 ...                             
16
   Image Name:   Angstrom/3.8.10/beaglebone                                     
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   Created:      2013-04-29  19:56:00 UTC                                       
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   Image Type:   ARM Linux Kernel Image (uncompressed)                          
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   Data Size:    4002016 Bytes = 3.8 MiB                                        
20
   Load Address: 80008000                                                       
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   Entry Point:  80008000                                                       
22
   Verifying Checksum ... OK                                                    
23
## Flattened Device Tree blob at 88000000                                       
24
   Booting using the fdt blob at 0x88000000                                     
25
   Loading Kernel Image ... OK                                                  
26
   Loading Device Tree to 8fff7000, end 8ffffb25 ... OK                         
27
                                                                                
28
Starting kernel ...
29

30
Uncompressing Linux... done, booting the kernel.                                
31
[    0.000000] Booting Linux on physical CPU 0x0                                
32
[    0.000000] Initializing cgroup subsys cpu 
33
...

L2: U-boot has detected SD card.
L8: Read uEnv.txt
L13: Read Linux kernel.
L14: Read .dtb file.
L15: U-boot prints information extracted from the U-boot image header.
L22: If the image is corrupted, checksum will not match and the boot will fail.
L28: The very last message printed by the U-boot. From this point on, control is handed over to the Linux bootstrap loader.
L30: Uncompressing of the kernel is handled by misc.c which is part of the Linux bootstrap loader.

Decoding U-boot Header of uImage Manually

Reading U-boot header information of the uImage manually by using U-boot commands

Load the uImage from Memory device (SD card / eMMC) into DDR memory of the board.
- At power on, keep pressing "spacebar" to enter U-boot prompt. (minicom)
  Ctrl + A $\to$ o $\to$ Serial port setup $\to$ Hardware Flow Control : No. If Hardware flow control enabled, then minicom will not be able to send data to BBB via serial cable. Minicom will take log messages from BBB. (This took me 5 hours to figure out!)
- To load a file from FAT based file system into memory, use fatload.
  To load a file from any file system: load
  Example:
```
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fatload usb 0:1 0x82000000 uImage
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fatload mmc 0:1 0x88000000 initramfs
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load mmc 0:1 0x88000000 uImage 
```
  $\to$ $\micro$ SD card
  $\to$ eMMC
- Use help to use help functionality. e.g., help load
- Use load to load uImage from the memory device into DDR memory of the board. First 64 bytes starting from 0x82000000 is the U-boot Image Header.
```
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==> load mmc 0:2 0x82000000 /boot/uImage
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4002080 bytes read in 291 ms (13.1 MiB/s)
```
  - 2 in 0:2 because our uImage is in the boot/ folder in the 2nd partition of the SD card.
  - 0x82000000 is DDR address.
  This loads uImage from the memory device into DDR memory of the board. First 64 bytes starting from 0x82000000 is the U-boot Image Header.

Use the memory dump command of U-boot to dump header information.

Use md to display memory:
```
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==> md 0x82000000 4
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82000000: 56190527 62153da1 50d07e51 e0103d00    '..V.=.bQ~.P.=..
```
Display 4 objects starting from the specified memory address.
First object is ih_magic, the magic number. Since this is in little-endian, the actual magic number is 0x27051956.
Fourth object is ih_size, the image data size. Since this is in little-endian, the actual size is 0x3d10e0, which is 4002016 in decimal, together with 64 byte makes 4002080 that we got earlier with load command.
And so on...

Use imi command to dump image information:


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=> imi 0x82000000
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## Checking Image at 82000000 ...
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   Legacy image found
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   Image Name:   Angstrom/3.8.10/beaglebone
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   Created:      2013-04-29  19:56:00 UTC
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   Image Type:   ARM Linux Kernel Image (uncompressed)
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   Data Size:    4002016 Bytes = 3.8 MiB
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   Load Address: 80008000
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   Entry Point:  80008000
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   Verifying Checksum ... OK

There are many more U-boot command you can use. Consult the U-boot documentation!

References

Nayak, K. (2022). Embedded Linux Step by Step Using Beaglebone Black [Video file]. Retrieved from https://www.udemy.com/course/embedded-linux-step-by-step-using-beaglebone/