Enabling Bootloader Serial Debug Console and Early printk in 28.1

How do I enable the bootloader serial console and the early serial console in Linux in 28.1? The standard ways of adding “console=ttyS0,115200n8 earlycon=uart8250,mmio32,0x03100000” to the extlinux.conf file are not working. I do not get any cboot/tboot/uboot during boot and I also cannot get anything on the serial console until getty starts and gives me a shell.

Serial console in U-Boot is already enabled. Something which may catch you off guard though is that CTS/RTS flow control does not work on a TX2 serial console…you have to use software flow control (you can have all the wires connected, just don’t enable hardware flow control). Are you using software or hardware flow control?

For reference, this is the default kernel command line argument after all substitutions:

root=/dev/mmcblk0p1 rw rootwait <b>console=ttyS0,115200n8</b> console=tty0 OS=l4t fbcon=map:0 net.ifnames=0 memtype=0 video=tegrafb no_console_suspend=1 <b>earlycon=uart8250,mmio32,0x03100000</b> nvdumper_reserved=0x2772e0000 gpt tegraid= tegra_keep_boot_clocks maxcpus=6 androidboot.serialno=0334916010131 bl_prof_dataptr=0x10000@0x277240000 sdhci_tegra.en_boot_part_access=1 root=/dev/mmcblk0p1 rw rootwait rootfstype=ext4