How to modify content of the existing rootfs?

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The below steps describe how to modify the content of the existing rootfs.

Steps are explained using LX2160ARDB board, however, the steps are applicable to all Layerscape devices and boards.

Extract and modify contents of cpio.gz archive
Generate .itb image
Set up Ethernet connection between TFTP server and Layerscape board
Boot the Linux kernel using new .itb image

Step1: Extract and modify contents of cpio.gz archive

Create a temporary directory for extracting the contents of the cpio.gz archive image. For example: mkdir temp_folder.
Extract the contents of the cpio.gz archive in the temporary folder.
For example: gunzip -c rootfs_lsdk2012_yocto_tiny_arm64.cpio.gz | sh -c 'cd temp_folder/&& cpio -i'

The temporary folder lists the filesystem as follows:

bin boot dev etc home init lib media mnt proc run sbin sys tmp usr var

Make changes to the filesystem in the temporary folder. For example: copy a 'HelloWorld' file in the filesystem using the following command: cp <path>/HelloWorld .
Repack the filesystem into a new cpio.gz archive. For example: use the following command: sh -c 'cd temp_folder/ && find . | cpio -H newc -o' | gzip -9 > new_rootfs_lsdk2012_yocto_tiny_arm64.cpio.gz

Step2: Generate .itb image

Change the path for new rootfs (new_rootfs_lsdk2012_yocto_tiny_arm64.cpio.gz) in linux_arm64_LS.its using gedit editor.

For example: 
Change directory to flexbuild_lsdk<version>/configs/linux. 
gedit linux_arm64_LS.its 
Update path as follows: 
data = /incbin/("../../packages/rfs/initrd/new_rootfs_lsdk2012_yocto_tiny_arm64.cpio.gz");

Generate .itb image using the following command:
For example: flex-builder -i mkitb -r yocto:tiny

This generates lsdk2012_yocto_tiny_LS_arm64.itb image.
Copy the .itb image to the TFTP server.

Step 3 - Set up Ethernet connection between TFTP server and Layerscape board

Set up Ethernet connection between the board (for example, LX2160ARDB) and host machine on which you have configured the TFTP server.

Boot the board to U-Boot prompt. U-Boot prints a list of enabled Ethernet interfaces. For example, LX2160ARDB U-Boot prints following Ethernet interfaces.
DPMAC2@xlaui4, DPMAC3@xgmii, DPMAC4@xgmii, DPMAC5@25g-aui, DPMAC6@25g-aui, DPMAC17@rgmii-id, DPMAC18@rgmii-id
Set server IP address to the IP address of the host machine on which you have configured the TFTP server.
=> setenv serverip <ipaddress1>
Set ethact and ethprime as the ethernet interface connected to the TFTP server.

See LX2160ARDB Ethernet Port Mapping for the mapping of Ethernet port names appearing on the chassis front panel with the port names in U-Boot and Linux.

=> setenv ethprime <name of interface connected to TFTP server>

For example:

=> setenv ethprime DPMAC3@xgmii

=> setenv ethact <name of interface connected to TFTP server>

For example:

=> setenv ethact DPMAC3@xgmii
Set IP address of the board. You can set a static IP address or, if the board can connect to a dhcp server, you can use the dhcp command.
Static IP address assignment:
=> setenv ipaddr <ipaddress2>
=> setenv netmask <subnet mask>

Dynamic IP address assignment:
=> dhcp
Save the settings.
=> saveenv
Check the connection between the board and the TFTP server.
=> ping $serverip

Using DPMAC3@xgmii device

host 192.168.2.1 is alive

Step4: Boot the Linux kernel using new .itb image

Load the .itb image from TFTP server to DDR memory of the board.
=> tftp 0xa0000000 <itb_file_name>

For example: => tftp 0xa0000000 lsdk2012_yocto_tiny_LS_arm64.itb
Boot the kernel with .itb image as follows:
=> bootm 0xa0000000#<board_name>

For example: => bootm 0xa0000000#lx2160ardb

Let the board boots to Tiny Linux.

List the filesystem.

NXP LSDK tiny 2012 (based on Yocto)
TinyLinux login: root
root@TinyLinux:~# ls
root@TinyLinux:~# cd /
root@TinyLinux:/# ls
HelloWorld boot etc init media new_rootfs_lsdk2012_yocto_tiny_arm64.cpio.gz root sbin tmp var
bin dev home lib mnt proc run sys usr
root@TinyLinux:/#

You will observe the HelloWorld file available in the filesystem.