ble_nus_data_send() limited to five packets if executed from nus_data_handler() interrupt handler

Question

SDK: v15
SoftDevice: S132
Chip: nRF52832 on BL652 Module

I have a command->response type system. The Central (Android phone) sends a command, and the Peripheral (nRF52 device) responds.

The command sent by central is interpreted by nus_data_handler() handler, and calls another function that needs to send back packets with ble_nus_data_send().

The issue is that I can only send five packets from my function, even with VERY long delays (1 sec) between each send. After five I get error NRF_ERROR_RESOURCES for each later attempt. The one second delay is obviously much longer than the packet interval.

I searched the forum extensively and implemented a flag to track when BLE_GATTS_EVT_HVN_TX_COMPLETE is raised, but I noticed that this event does not happen after every packet is sent. Also, for the packets after the initial five, BLE_GATTS_EVT_HVN_TX_COMPLETE event will not be raised until the function is finished.

I think this has something to do with interrupt priorities but I don't know how to look into that. Can ble_evt_handler() priority be greater than nus_data_handler()?

<info> app: UART_SERVICE_SUBCOMMAND_SEND_ALLEVENTS
<info> app: event_id: 1
<info> app: Sending data for event: 1
<info> app: Sent data for ble. Success.
<info> app: event_id: 2
<info> app: Sending data for event: 2
<info> app: Sent data for ble. Success.
<info> app: event_id: 3
<info> app: Sending data for event: 3
<info> app: Sent data for ble. Success.
<info> app: event_id: 4
<info> app: Sending data for event: 4
<info> app: Sent data for ble. Success.
<info> app: event_id: 5
<info> app: Sending data for event: 5
<info> app: Sent data for ble. Success.
<info> app: event_id: 6
<info> app: Sending data for event: 6
<info> app: Sent data for ble. Err code: 19
<info> app: event_id: 7
<info> app: Sending data for event: 7
<info> app: Sent data for ble. Err code: 19

// This is the last event it tried to send (no retry in this code)

// After the last transfer attempt (#7), only then did the TX_COMPLETE event get raised

<info> app: BLE_GATTS_EVT_HVN_TX_COMPLETE event
<info> app: Connected
<info> app: BLE_GATTS_EVT_HVN_TX_COMPLETE event
<info> app: Connected
<info> app: BLE_GATTS_EVT_HVN_TX_COMPLETE event

Edvin · Answer

This implementation will skip some packets, because if err_code = ble_nus_string_send() does not return NRF_SUCCESS, then you have still incremented i, but the packet was not successfully queued, so we have to roll back "i" to try to send this packet as the next packet whenever there is space in the queue (the TX_COMPLETE event). 
 try: 
 err_code = ble_nus_string_send();
if (err_code == NRF_ERROR_RESOURCES)
{
 m_space_in_buffer = false;
 i -= len;
 return;
} 
 
 BR, 
 Edvin

ble_nus_data_send() limited to five packets if executed from nus_data_handler() interrupt handler

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