How long does notification data need to be valid?

RE: How long does notification data need to be valid?

Andy — Thu, 08 Mar 2018 20:26:58 GMT

Thanks for your help Edvin! :)

RE: How long does notification data need to be valid?

Edvin — Wed, 07 Mar 2018 11:41:31 GMT

Hello,

I checked with the SoftDevice team, and they confirmed that the data is stored in a buffer in the SoftDevice. This means that once the sd_ble_gatts_hvx() call returns NRF SUCCESS you can do whatever you want with the data in the hvx_params variable/pointer. It is the data that is stored in the SoftDevice, not the pointer.

Best regards,

Edvin

RE: How long does notification data need to be valid?

Andy — Tue, 06 Mar 2018 15:58:23 GMT

Hi Edvin, thanks for your answer! Good to know that the hvx_params struct gets buffered. I actually just realized that I asked the wrong question: I really didn't mean to ask about the hvx_params struct itself, but rather the data pointed by it. How long do I need to keep it alive?

If the softdevice only stores the pointer, then ok, I can let the hvx_params struct get out of scope. I want to know how long do I need the data pointed by hvx_params.p_data to be kept alive.

I see in the example you mentioned that the data buffer is declared static in the UART event handler, and gets overwritten every time you get new data. I'm wondering whether the data itself (the one in the UART event handler data buffer) gets buffered by the SoftDevice, or just the pointer to it. Right now I don't know whether my code works because the SoftDevice buffers the data, or because the data gets transmitted over BLE faster than I can currently overwrite the data in that buffer.

I hope I made myself a bit clearer now.

RE: How long does notification data need to be valid?

Edvin — Tue, 06 Mar 2018 15:19:12 GMT

Hello,

I have not seen the source code for this part of the softdevice, but as you can see in e.g. the ble_app_uart example, which uses the ble_nus_string_send() function to send text strings, the ble_gatts_hvx_params_t hvx_params is created within this function, and will not be kept once the ble_nus_string_send() function returns. You will notice that if you try to send several times, it can store up several messages in the sending buffer, but it will eventually return NRF_ERROR_RESOURCES, which indicates that the buffer is full. Then you will have to wait for the BLE_GATTS_EVT_HVN_TX_COMPLETE to be able to queue up more strings in the sending buffer.

So to answer your question, yes, the softdevice copies the data from the hvx_params, so you do not have to keep the data after sd_ble_gatts_hvx has returned NRF_SUCCESS. If it returns NRF_ERROR_RESOURCES, you would want to keep the data for the next BLE_GATTS_EVT_HVN_TX_COMPLETE event, so that you can queue it up in the softdevice again.

Even if the packet is lost over the air, the softdevice will retransmit it, and does not need the data from the application to do that.

Best regards,

Edvin