Disabling Serial Console

For varying privacy and security reasons, some people like to completely silence the serial console output. This needs to be done in the following places:

• Bootloader (serial console)
• Linux Kernel (serial console)
• Linux OS (console login)

Please NOTE: There will be no way to access the board besides over the network after making these changes.

Please make changes at your own risk

Recovery would be through re-programming the board via the Gateworks JTAG Programmer.

Disable U-Boot bootloader serial console

In order to disable serial console for the U-Boot bootloader you must re-build the bootloader with the feature disabled.

• Laguna bootloader
  • see laguna/bootloader for building
  • alter config

(include/configs/cavium_cns3000.h and/or include/configs/cavium_cns3000_spi.h):

• remove '#define CONFIG_CNS3000_SERIAL'
• set CONFIG_BOOTDELAY to 0 to save a couple of seconds on bootup

• Ventana bootloader
  • see ventana/bootloader for building
  • alter config (include/configs/gw_ventana.h):
    • remove '#define CONFIG_DM_SERIAL'
    • set CONFIG_BOOTDELAY to 0 to save a couple of seconds on bootup

Disable Kernel serial console

In order to disable serial console for the Linux Kernel you can either recompile it with the support disabled, or you can alter the bootargs the bootloader passes to the kernel to disable it at runtime.

Kernel configuration:

• Remove the following from the kernel config (.config)
  • CONFIG_SERIAL_CORE_CONSOLE
  • CONFIG_SERIAL_EARLYCON
  • CONFIG_SERIAL_IMX_CONSOLE (Ventana / imx6)
  • CONFIG_SERIAL_8250_CONSOLE (Laguna / cns3xxx)

Kernel bootargs:

• Remove the 'console' bootargs

Example bootloader configuration to remove the console bootarg:

• Laguna - Alter bootargs:

  printenv bootargs
  bootargs=console=ttyS0,115200 root=/dev/mtdblock3 rootfstype=squashfs,jffs2 noinitrd init=/etc/preinit
  setenv bootargs 'root=/dev/mtdblock3 rootfstype=squashfs,jffs2 noinitrd init=/etc/preinit'
  saveenv
  

• Ventana - unset console:

  setenv console
  saveenv
  

Disable Linux OS console login

In order to disable console login in a Linux OS you must disable the service that performs this function. While this is Linux distribution specific it usually involves the 'init' process launching the 'getty' or 'login' application with a parameter of a specific serial device or tty1 (which is the kernel console tty)

OpenWrt

modern OpenWrt uses it's own procd as a replacement for the standard Linux init. While it is a complete re-implementation it still follows the standard of processing an /etc/inittab file for configuring what processes are spawned from init.

To kill off login from the kernel serial console as well as any other serial devices, remove or comment with a leading '#' any lines that invoke /bin/ash --login from /etc/inittab such as:

::askconsole:/bin/ash --login
tty1::askfirst:/bin/ash --login

Yocto (and other Linux distros using the standard sysvinit)

Many Linus distros use the sysvinit package which processes /etc/inittab for commands to spawn a getty on various serial devices or tty's.

To kill off login from the kernel serial console as well as any other serial devices, remove or comment with a leading '#' any lines that invoke the getty process from /etc/inittab. For example, remove the following:

mxc1:12345:respawn:/sbin/getty -L 115200 ttymxc1
1:12345:respawn:/sbin/getty 38400 tty1

Ubuntu

Ubuntu doesn't by default enable serial console or serial port login capability however if you have added this it may have been done by adding an /etc/init/serial.conf that you can alter or remove.