The kgdbcon feature allows you to see printk() messages inside gdb while gdb is connected to the kernel. Kdb does not make use of the kgdbcon feature.
Kgdb supports using the gdb serial protocol to send console messages to the debugger when the debugger is connected and running. There are two ways to activate this feature.
Activate with the kernel command line option:
Use sysfs before configuring an I/O driver
echo 1 > /sys/module/kgdb/parameters/kgdb_use_con
NOTE: If you do this after you configure the kgdb I/O driver, the setting will not take effect until the next point the I/O is reconfigured.
IMPORTANT NOTE: You cannot use kgdboc + kgdbcon on a tty that is an
active system console. An example of incorrect usage is
console=ttyS0,115200 kgdboc=ttyS0 kgdbcon
It is possible to use this option with kgdboc on a tty that is not a system console.