serial console to get kernel boot messages

I want to find a way to be able to modify the extlinux.conf in case my new kernel refuses to boot.
One way is to use the serial console available on J21 connector, but unfortunately it is only available when my TX1 is fully started, when I hit the reboot command, I’m disconnected from this console.

How can I do it?

Thanks

Unless your U-Boot is custom the serial console on J21 should allow dropping in to the U-Boot command line, or picking among entries in extlinux.conf. Having a fall back entry of the original boot configuration is definitely the safe way to do things, and then add test entries for selection via serial console. Is there something custom about your U-Boot itself? If not, then what is your serial terminal program?

You may just start your serial console on host (with gtkterm, putty, minicom…) before powering the jetson. You should be able to see early boot and get interactive mode with u-boot for selecting an extlinux.conf entry.
You may also use poweroff on Jetson for having time to restart console on host and then press poweron on Jetson, but this is not normal…when I have serial console open on host, I always see booting traces from Jetson if I reboot. Do you have flow control enabled (HW or SW) in host ?

thanks guys for the answers, my bad. I was connecting to the wrong UART, the one offered by the USB OTG port. I had both connections active. It’s working now.
Do you have a link of a good post explaining how to recompile and install the kernel?

Thanks

Youi may have a look to http://www.jetsonhacks.com/2017/03/25/build-kernel-and-modules-nvidia-jetson-tx2/.