Issue
Background
I have an embedded Android device device that uses the U-Boot bootloader. The system also uses the Busybox suite.
I have access to the U-Boot shell through a UART port and can dump, modify, and upload a new filesystem but I have no terminal access to the OS. One limitation I have with U-Boot is that the saveenv command has been disabled however, the setenv command seems to work for the current boot session.
The bootargs U-Boot variable is currently set to storagemedia=nand (there is no console= setting).
So far I have tried using echo on various serial ports and modifying the inittab file to setup a TTY shell for the UART port.
My attempts to setup a TTY shell
::respawn:/sbin/getty -L serial 115200 -n -l /bin/autologin
::respawn:/sbin/getty -L serial0 115200 -n -l /bin/autologin
::respawn:/sbin/getty -L s0 115200 -n -l /bin/autologin
::respawn:/sbin/getty -L console 115200 -n -l /bin/autologin
The original Line in the inittab file (doesn't work)
# [THE ORIGINAL LINE]
ttyFIQ0::respawn:/sbin/getty -L ttyFIQ0 0 vt100 #
Regardless of what I do, the last few lines I receive from the UART port are:
## Booting Android Image at 0x62ce3248 ...
Kernel load addr 0x62ce3a48 size 4400 KiB
## Flattened Device Tree blob at 61f00000
Booting using the fdt blob at 0x61f00000
XIP Kernel Image ... OK
CACHE: Misaligned operation at range [62ce3a48, 6312f888]
Loading Device Tree to 60ff1000, end 60fff36c ... OK
Adding bank: 0x60000000 - 0x61000000 (size: 0x01000000)
Adding bank: 0x61200000 - 0x70000000 (size: 0x0ee00000)
Starting kernel ...
After the Starting kernel ... message, I receive nothing indicating that a TTY shell is running even though, the OS boots and runs perfectly. If I type anything into the terminal after the Starting kernel ... message has been received, the OS freezes.
Question
How do I setup a TTY shell for the Operating System that will use the UART port so that I can finally interact with the embedded device OS through a terminal?
Solution
The reason I couldn't establish a shell connection on the UART0 port was because the UART0 port was being used by the fiq-debugger which prevented me from starting a getty program on the UART0 port through device ttyFIQ0. After looking around the .dtb file (in the Flattened Device Tree blob at 61f00000), I noticed that serial devices (named something like serial@20060000) had their status set to status = disabled.
I modified the .dtb file to enable all of the serial ports (set their status to status = okay. Then, I also had to modify the serial-id property of the fiq-debugger in the .dtb file to be a value other than 0x00 since 0x00 refers to UART0 which is the only accessible UART port on circuit board I was using.
Finally, I modified the inittab file on the device to run a getty program on port ttyS0 (this device was originally disabled in the .dtb file). With that final change, the RK3128 allowed a serial terminal on UART0.
Answered By - Fleshy Answer Checked By - Marilyn (WPSolving Volunteer)