[Q] Nexus 9 - UART topic

I'm trying to access my Nexus 9 using UART hidden in headset jack. I have USB -> UART adapter and I have prepared suitable adapter using this instruction (I have to paste it like this due to forum security rules: accuvant.com/blog/building-a-nexus-4-uart-debug-cable).

To make anything working I had to disable FIQ in kernel config and add console=ttyS0,115200n8 to cmd line. Now I'm able to see all the logs from bootloader and from kernel but when I'm trying to write anything in terminal I get only:

[ 901.015785] [HS_BOARD] (headset_get_debug) AUD_DEBUG_EN=1
[ 901.183302] [HS_BOARD] (headset_get_debug) AUD_DEBUG_EN=1
[ 901.359637] [HS_BOARD] (headset_get_debug) AUD_DEBUG_EN=1
[ 901.525347] [HS_BOARD] (headset_get_debug) AUD_DEBUG_EN=1
[ 901.823791] [HS_BOARD] (headset_get_debug) AUD_DEBUG_EN=1
[ 901.991483] [HS_BOARD] (headset_get_debug) AUD_DEBUG_EN=1
[ 902.135675] [HS_BOARD] (headset_get_debug) AUD_DEBUG_EN=1
[ 902.303744] [HS_BOARD] (headset_get_debug) AUD_DEBUG_EN=1
[ 902.654816] [HS_BOARD] (headset_get_debug) AUD_DEBUG_EN=1

logs from kernel but nothing more (one more for one pressed key). I have tried to add androidboot.console=ttyS0 to cmdline but it also didn't help.

Does anyone know what could be wrong?