Using UART0 on J21

I’m trying to communicate with a GPS module from UART0 via the J21 pins on a Jetson TX1. I know the GPS module works, as I’ve used it from a TK1.

I’ve removed console=ttyS0,115200n8 from the APPEND line in /boot/extlinux/extlinux.conf to disable the serial console on UART0. I still do not see anything on /dev/ttyS0, though. Is there another step involved in enabling UART0?

$ dmesg | grep tty
[ 0.000000] Kernel command line: fbcon=map:0 console=tty0 androidboot.modem=none androidboot.serialno=P2180A00P00940c003fd tegraid= ddr_die=2048M@2048M ddr_die=2048M@4096M section=256M memtype=0 vpr_resize usb_port_owner_info=0 lane_owner_info=0 emc_max_dvfs=0 touch_id=0@63 video=tegrafb no_console_suspend=1 debug_uartport=lsport,0 earlyprintk=uart8250-32bit,0x70006000 maxcpus=4 usbcore.old_scheme_first=1 lp0_vec=0x1000@0xff2bf000 nvdumper_reserved=0xff23f000 core_edp_mv=1125 core_edp_ma=4000 gpt android.kerneltype=normal androidboot.touch_vendor_id=0 androidboot.touch_panel_id=63 androidboot.touch_feature=0 androidboot.bootreason=pmc:software_reset,pmic:0x0 root=/dev/mmcblk0p1 rw rootwait
[ 0.000000] console [tty0] enabled, bootconsole disabled
[ 2.357077] 70006000.serial: ttyS0 at MMIO 0x70006000 (irq = 68) is a Tegra
[ 2.358476] 70006040.serial: ttyTHS1 at MMIO 0x70006040 (irq = 69) is a SERIAL_TEGRA
[ 2.358963] 70006200.serial: ttyTHS2 at MMIO 0x70006200 (irq = 78) is a SERIAL_TEGRA
[ 2.359556] 70006300.serial: ttyTHS3 at MMIO 0x70006300 (irq = 122) is a SERIAL_TEGRA
[ 8.877175] usb 1-3.3: pl2303 converter now attached to ttyUSB0


I haven’t tried to disable it, but ttyS0 is also set up in U-Boot since serial console from APPEND only applies after U-Boot has handed over to the kernel. You might be getting an interaction from this prior environment…if the module connected to ttyS0 has any interaction from the U-Boot console output, then you can bet it will at minimum interfere with boot (typing anything in on serial console during U-Boot stage will interrupt U-Boot…depending on your module the reverse may also be true).