[PPP] Delay on PPP reception

[Coueron]

Hello,

I work on a project using PPP driver on netXduo 6.4.3 and I observe that sometime the received messages have delay.

After investigation, I understand that:

• In the _nx_ppp_thread_entry function, when a NX_PPP_EVENT_PACKET_RECEIVE event occurs, the application read the data in ppp_ptr -> nx_ppp_serial_buffer until a 0x7e character or buffer empty, then process the data.
• The _nx_ppp_byte_receive function set event NX_PPP_EVENT_PACKET_RECEIVE flag when a character 0x7e is received or data length in ppp_ptr -> nx_ppp_serial_buffer is higher than NX_PPP_SERIAL_BUFFER_ALERT_THRESHOLD

But if the _nx_ppp_byte_receive function is called with a priority higher than PPP thread (per example, an uart interrupt), more than one PPP frame may have been added in ppp_ptr -> nx_ppp_serial_buffer before _nx_ppp_thread_entry catch the NX_PPP_EVENT_PACKET_RECEIVE event. As a result, only the first PPP frame is processed, the others stay in the buffer until next _nx_ppp_byte_receive call (if we meet the condition ppp_ptr -> nx_ppp_serial_buffer_byte_count >= NX_PPP_SERIAL_BUFFER_ALERT_THRESHOLD)

In my project, I use an USART with DMA and idle detection, and sometime I receive 4 or 5 consecutive PPP frames then nothing during several seconds. Theses frames stay in the PPP reception buffer and lead to TCP deconnection.

This workaround seems handle this issue: in _nx_ppp_thread_entry function, when we catch NX_PPP_EVENT_PACKET_RECEIVE event, I add an iteration on the _nx_ppp_receive_packet_get / _nx_ppp_receive_packet_process sequence

Regards

[EdouardMALOT]

Congratulation !

[fdesbiens]

Thank you for filing this issue, @Coueron. I will ask a team member to look into it. Thank you for your patience. We did not notice theis issue due to our focus on our recent 202503 release.

[EdouardMALOT]

Thank you Florian @Coueron for reporting this bug !

On a PPP / 4G link I experienced the following symptoms:

• when a packet was lost, we started to see retries
• these retries piled up more and more (because some packets were stuck in the netxduo / PPP stack).

Once the patch was applied the problem disappeared. :white_check_mark: Note: I also use a DMA for PPP frame reception and the modem sometimes concatenate several frames.

@fdesbiens : it seems essential to include this fix in upcoming releases.