Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

[0x41]

Hi everyone,

I'm developing a star-topology network using DECT NR+ on nRF9161 DK nodes,
running modem firmware mfw-nr+_nrf91x1_2.0.0 (nRF Connect SDK 3.3.0).

My issue is a modem crash with error 0x4 on the FT side when it receives
a PDU of ~3200 bytes sent by the PT device via
nrf_modem_dect_dlc_data_tx(). After investigating the error code, I
increased the modem's shared memory using
CONFIG_NRF_MODEM_LIB_SHMEM_RX_SIZE, but the crash still occurs at this
payload size.

On the PT device side, the transmission itself doesn't seem to fail,
since the dlc_data_tx callback is called with status 0.

For clarification: the FT and PT nodes start exchanging data after
association. I haven't yet tried to reproduce the crash using the sample
application from [Hands on with DECT NR+ API release v2.0], and I also
wasn't able to capture a modem trace of the fault using the Modem
library.

Modem error: 0x4
PC: 0x561bc
SDK: 3.3.0

Sorry in advance if I've left out any details.

Kind regards,
Alessandro.

[Syed Maysum Abbas Zaidi]

Hi, 
I would like to update that our team managed to send 4KB with the modified ncs dect nr+ stack by changing DECT_MTU to 4096. Can you try changing it and see if it helps. Thanks

[0x41]

Hi,

I've changed DECT_MTU as suggested. I have same crash, with non-socket based communication.

nrf_modem: Modem has crashed, reason 0x4, PC: 0x561bc

Kind Regards,
Alessandro

[Syed Maysum Abbas Zaidi]

Hi,

Apologies for not mentioning it before, our internal 4KB test that succeeded was using the socket-based path with DECT_MTU set to 4096. The direct DLC API (nrf_modem_dect_dlc_data_tx()) is currently not optimized for large SDUs and the modem memory configuration does not support it well. This has been noted internally, however we cannot confirm if or when this will be optimized.

In the meantime, we would suggest switching to the socket-based approach or continuing with your current workaround of reducing the PDU size. Thanks.