Bootstrapping process


Provide information as to what is involved in the boot-strap process which gets the board from the power-off state into the linux kernel running.


Reference information about the board initialization and bootstrap process can be found in:

The Bootstrap process involves multiple stages:

  • ROM Bootloader
    • This bootloader does some platform initialization, determines the boot device, loads the secondary bootloader from the boot device, and starts the secondary bootloader.
  • Secondary Bootloader
    • The secondary bootloader does further initialization of the board (like the peripherals), loads the third-stage bootloader (usually U-boot) and transfers control to the third-stage bootloader.
  • Third-stage bootloader (U-Boot)
    • The final stage of the bootstrap process is usually U-boot. This is a fairly sophisticated program which can be scripted to perform a variety of tasks. These tasks can do further initialization of the board (e.g. load the fpga), load the application image and then transfer control to the application.

Most applications will probably be able to use the secondary bootloader and u-boot as provided in the development kit. Most customizations can be done within the scripting within u-boot. This scripting is performed by defining environment variables and then running the variables as commands. The attached pdf file (Uboot_Flowchart.pdf) contains a flowchart of the u-boot environment as provided in the default image for the development kit. This demonstrates how several environment variables are stitched together to create the bootstrap script. Some environment variables are commands to be executed and others are data to be used by the commands.

Many customizations can be made by changing the environment variables which are used as data.

More complicated customizations can be made by changing the environment variables which contain commands.

In some cases, changes to the u-boot program are needed. Instructions for building u-boot are provided at: Building U-boot

If you need to debug U-boot, see: Debugging U-boot

There may even be a case where the secondary bootloader needs to be changed. The secondary bootloader is built at the same time that u-boot is built.

Stopping in U-boot

When the board is booting up, there is a window in U-boot during which if no key is hit, then the application is loaded and started. If a key is hit, then U-boot stops and issues the u-boot prompt. This lets you look at or make changes to the environment variables if you wish. When you are finished, you can simply type boot and then u-boot will run the "bootcmd" environment variable. The length of the window is the value of the "bootdelay" environment variable in seconds and is 2 by default. You can change the value of the bootdelay to make the window longer if you wish.

To look at the environment variables, use printenv.

After making changes to the environment variables, use saveenv to save the changes.

Use "help" to see what other commands are available. Additional information about the commands can be found on the internet.

The Bootloader Files

In the development kit, the bootloader files are in the first partition on the SD card and can be accessed from Linux, U-boot, or anything that can access the SD card. The application files are in the second partition of the SD card.

First Partition

  • From U-boot
=> ls mmc 0:1
  2192012   fpga_7a15t.bin
  1241804   fpga_7a25t.bin
  2192012   fpga_7a35t.bin
  2192012   fpga_7a50t.bin
   150619   MLO
   648176   u-boot.img
            System Volume Information/

6 file(s), 1 dir(s)

A description of each of these files:

Name Description
MLO Secondary boot loader
System Volume Information A directory of Boot device volume information
fpga_7a15t.bin fpga image for Xilinx XC7A15T FPGA
fpga_7a25t.bin fpga image for Xilinx XC7A25T FPGA
fpga_7a35t.bin fpga image for Xilinx XC7A35T FPGA
fpga_7a50t.bin fpga image for Xilinx XC7A50T FPGA
u-boot.img u-boot program
  • From Linux
root@mitysom-am57x:~# ls -l /run/media/mmcblk0p1/
-rwxrwx---    1 root     disk        150619 Sep 10  2021 MLO
drwxrwx---    2 root     disk          2048 Nov  8 11:21 System Volume Information
-rwxrwx---    1 root     disk       2192012 Sep 10  2021 fpga_7a15t.bin
-rwxrwx---    1 root     disk       1241804 Sep 10  2021 fpga_7a25t.bin
-rwxrwx---    1 root     disk       2192012 Sep 10  2021 fpga_7a35t.bin
-rwxrwx---    1 root     disk       2192012 Sep 10  2021 fpga_7a50t.bin
-rwxrwx---    1 root     disk        648176 Sep 10  2021 u-boot.img

If you rebuild u-boot or the secondary bootloader, you can copy the files to the SD-card using whatever method you are most comfortable with (save the originals just in case) and then try the new programs. If you need to go back to the original files, you can remove the SD-card, restore the original files, insert the SD-card back in, and then reboot.

Second Partition

  • From U-boot
=> ls mmc 0:2
<DIR>       4096 .
<DIR>       4096 ..
<DIR>      16384 lost+found
<DIR>       4096 run
<SYM>         19 linuxrc
<DIR>       4096 opt
<DIR>       4096 boot
<DIR>       4096 dev
<DIR>       4096 proc
<DIR>       4096 bin
<DIR>       4096 mnt
<DIR>       4096 include
<DIR>       4096 sbin
<DIR>       4096 srv
<DIR>       4096 etc
<DIR>       4096 media
<DIR>       4096 usr
<DIR>       4096 var
<DIR>       4096 lib
<DIR>       4096 tmp
<DIR>       4096 sys
<DIR>       4096 www
<DIR>       4096 home

(The linux images and device tree are in the boot directory)

=> ls mmc 0:2 boot
<DIR>       4096 .
<DIR>       4096 ..
<SYM>         26 uImage
         4457024 uImage-4.19.94-g9ce96ed7a5
           90014 am57xx-mitysom-devkit.dtb
<SYM>         26 zImage
         4456960 zImage-4.19.94-g9ce96ed7a5
        16404196 vmlinux-4.19.94-g9ce96ed7a5
           92744 am57xx-mitysom-devkit-fpga.dtb

  • From Linux
root@mitysom-am57x:~# ls -l /
drwxr-xr-x    2 root     root          4096 Sep 10  2021 bin
drwxr-xr-x    2 root     root          4096 Dec  8 19:01 boot
drwxr-xr-x   11 root     root         14240 Dec  9 18:21 dev
drwxr-xr-x   62 root     root          4096 Dec  7 21:15 etc
drwxr-xr-x    3 root     root          4096 Sep  9  2021 home
drwxr-xr-x    2 root     root          4096 Sep  9  2021 include
drwxr-xr-x   10 root     root          4096 Sep 10  2021 lib
lrwxrwxrwx    1 root     root            19 Sep 10  2021 linuxrc -> /bin/busybox.nosuid
drwx------    2 root     root         16384 Sep 10  2021 lost+found
drwxr-xr-x    3 root     root          4096 Sep  9  2021 media
drwxr-xr-x    3 root     root          4096 Sep 10  2021 mnt
drwxr-xr-x    5 root     root          4096 Sep 10  2021 opt
dr-xr-xr-x  103 root     root             0 Jan  1  1970 proc
drwxr-xr-x   13 root     root           500 Dec  9 19:11 run
drwxr-xr-x    3 root     root          4096 Sep 10  2021 sbin
drwxr-xr-x    3 root     root          4096 Sep 10  2021 srv
dr-xr-xr-x   12 root     root             0 Jan  1  1970 sys
drwxrwxrwt   13 root     root           300 Dec  9 19:11 tmp
drwxr-xr-x   11 root     root          4096 Sep 10  2021 usr
drwxr-xr-x    9 root     root          4096 Sep  9  2021 var
drwxr-xr-x    3 root     root          4096 Sep 10  2021 www
root@mitysom-am57x:~# ls -l /boot
-rw-r--r--    1 root     root         92744 Sep  9  2021 am57xx-mitysom-devkit-fpga.dtb
-rw-r--r--    1 root     root         90014 Sep  9  2021 am57xx-mitysom-devkit.dtb
lrwxrwxrwx    1 root     root            26 Sep  9  2021 uImage -> uImage-4.19.94-g9ce96ed7a5
-rw-r--r--    1 root     root       4457024 Sep  9  2021 uImage-4.19.94-g9ce96ed7a5
-rw-r--r--    1 root     root      16404196 Sep  9  2021 vmlinux-4.19.94-g9ce96ed7a5
lrwxrwxrwx    1 root     root            26 Sep  9  2021 zImage -> zImage-4.19.94-g9ce96ed7a5
-rw-r--r--    1 root     root       4456960 Sep  9  2021 zImage-4.19.94-g9ce96ed7a5


An overview of the bootstrap process has been given with links to references for more information. U-boot has been indicated as the spot for most customizations in either the environment variables used as data or in the variables used as commands.

The location of the secondary and third-stage bootloaders on the SD-card has been indicated with access from both Linux and from U-boot demonstrated.

A flowchart of the default u-boot script has been attached. Uboot_Flowchart.pdf

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