No login prompt on serial console


Dear all,

I am unable to get a terminal on the serial port (pins 6-10). Following the solution in [1], I activated the /dev/tty2 overlay:

Retrieving file: /hw_intfc.conf
reading /hw_intfc.conf
1152 bytes read in 2 ms (562.5 KiB/s)
dtoverlay number: 0, name:/overlays/console-on-ttyS2.dtbo 

Using anything but /dev/ttyS2 in the kernel command line results in output stopping after “Starting kernel …”, so I assume that ttyS2 is the right value, even if the kernel log shows only ttyS0.

# dmesg | grep tty
[    0.000000] Kernel command line: console=ttyS2,1500000n8 quiet
[    0.326184] Registered FIQ tty driver
[    0.350383] console [ttyFIQ0] enabled
[    0.350577] Registered fiq debugger ttyFIQ0
[    3.459996] ff180000.serial: ttyS0 at MMIO 0xff180000 (irq = 35, base_baud = 1500000) is a 16550A

Finally, /etc/inittab contains a line for a terminal on /dev/ttyS2, but a console is never opened.

I am running out of ideas, so would be grateful for pointers in the right direction.

Some background information: I managed to integrate the Radxa kernel, u-boot and arm firmware into a Buildroot project. The root file system is not derived from Debian, but Buildroot generated. Init system is handled by BusyBox.

Kind regards,

[1] After installing a new kernel boot fails


I had the same problem with the arm64 build, console output stopped at the same place.

What does your “append:” line look like in extlinux/extlinux.conf in the boot (4th; EFI) partition?

I mounted that partition on another linux system (VirtualBox guest) and changed:

append: earlyprintk console=ttyFIQ0,1500000n8 rw root=PARTUUID=b921b045-1d rootfstype=ext4 init=/sbin/init rootwait


append: earlyprintk console=ttyS2,1500000n8 rw root=PARTUUID=b921b045-1d rootfstype=ext4 init=/sbin/init rootwait

Saved the file, booted, and all worked fine (no need to change the dtoverlay file with this modification.)


Hi Jim, my kernel output is essentially OK; using the overlay it continues even after kernel startup. I just cannot get a login prompt after /sbin/init finishes (sorry for being unclear).

My current extlinux.conf says [1]:

append console=ttyS2,1500000n8 quiet

and /etc/inittab:

# Put a getty on the serial port
ttyS2::respawn:/sbin/getty -L  ttyS2 0 vt100 # GENERIC_SERIAL

But I will definitely try your suggestion to to use ttyS2 without the overlay, it sounds promising.


[1] Kernel uses initramfs as rootfs


I tried some variations of overlays, kernel parameters and serial lines, with the following results:

  • ttyS2 must be enabled, either via overlay or directly in the dtb
  • Kernel parameter has to be /dev/ttyS2, I was not able to get other settings to work
  • Within the RockPi4b, only ttyFIQ0 and ttyS0 are connected
  • Output to /dev/ttyFIQ0 shows up on the other end of the serial connection
  • I can start an interactive shell from the RockPi4b to the connected host by executing [1]
	sh --login < /dev/ttyFIQ0 > /dev/ttyFIQ0
  • Using getty does not open a terminal over any of the serial devices and only a getty for /dev/tty0 shows up in ps (waiting for a login). All other invocations do not spawn and terminate without message
	su daemon -c "getty -L 0 ttyS0"

So it is probably a problem with getty from BusyBox. I will see if I can find anything within this context

OK, Problem found.

My build process overwrites inittab with Buildroot configuration values. I now have the serial port configured in Buildroot .config and finally there is a login prompt on the serial console.

# Put a getty on the serial port
ttyFIQ0::respawn:/sbin/getty -L  ttyFIQ0 0 vt100 # GENERIC_SERIAL

[1] logged in via ssh over network