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Passing data from PDM callback to USB audio device

Hi,

I'm trying to pass audio data from PDM callback function to a USB device and I'm kind of stuck.

Background:
I'm using the nRF52840-DK board and 2 MP34DT05-A microphones. Since there is no PDM example in the SDK I took the liberty of writing the code from scratch based on the non-legacy PDM driver. It is very similar to I2S in design so didnt take much to make it work. To verify that it works I used fstorage library to save the samples into memory nd then Segger J-Flash to dump contents to file on a windows computer and the WaveSurfer to make sure it is actual audio data. (basically up to date code and applications described here)

IT WORKED

Here is a snippet of the relevant code in my program:

static void nrfx_pdm_event_handler(nrfx_pdm_evt_t const *const p_evt) {
	/*NRF_LOG_INFO("nrfx_pdm_event_handler(buffer_released:%d buffer_requested:%d error:%d)",
			p_evt->buffer_released == NULL ? 0 : 1,
			p_evt->buffer_requested ? 1 : 0,
			p_evt->error);*/

	// we have an error ?
	if (p_evt->error != NRFX_PDM_NO_ERROR) {
		NRF_LOG_INFO("PDM error event: %d", p_evt->error);
		return;
	}

	// got a buffer with data ?
	if (p_evt->buffer_released != NULL) {
		// we have buffer ready
		audio_buffer_txed = 0;

		// do we have space to write ?
		if (flash_addr + NRFX_PDM_BUFFER_SIZE*2 < FSTORAGE_END_ADDR) {
			if (fstorage_write_done /*nrf_fstorage_is_busy(&fstorage_instance)*/) {
				// write to flash
				fstorage_write_done = 0;
				ret_code_t rc = nrf_fstorage_write(
					&fstorage_instance,     /* The instance to use. */
					flash_addr,             /* The address in flash where to store the data. */
					p_evt->buffer_released, /* A pointer to the data. */
					NRFX_PDM_BUFFER_SIZE*2, /* Lenght of the data, in bytes. */
					NULL                    /* Optional parameter, backend-dependent. */
				);

				if (rc != NRF_SUCCESS)
					NRF_LOG_ERROR("nrf_fstorage_write() = %d", rc);
			} else NRF_LOG_ERROR("fstorage busy -> dumping buffer");
		}
	}

	// expected to set a new buffer ?
	if (p_evt->buffer_requested) {
		NRF_LOG_INFO("PDM buffer requested");

		// update buffer index
		pdm_buffer_i = pdm_buffer_i == 0 ? 1 : 0;

		// set new buffer
		nrfx_pdm_buffer_set(pdm_buffer[pdm_buffer_i], NRFX_PDM_BUFFER_SIZE);
	}
}

int main(void)
{
    // .
    // .
    // .
    
    nrfx_pdm_config_t config = NRFX_PDM_DEFAULT_CONFIG(ARDUINO_SCL_PIN, ARDUINO_SDA_PIN);
    ret = nrfx_pdm_init(&config, nrfx_pdm_event_handler);
    APP_ERROR_CHECK(ret);
    
    // .
    // .
    // .
}

Now, the problem I'm facing is how to pass that data to a USB audio device.

I started with the USB audio device example in the SDK and stripped all the headphones related code since it is irrelevant.

When I looked at the code I could see that the function app_usbd_audio_class_tx_start() that actually passes data to the USB device is called inside the SOF handler of the headphones.
I needed to find a new home for it ..
My naive first attempt was to place it directly inside the PDM handler (where fstorage_write_done() is right now). To cut a long story short - it didnt work, it returns with code 8 (invalid state). So I went to the documentation and it says there that writing can only occur during an SOF event. (and it has a small example here that I tried to use).

I came up with this code (which doesnt work .. same old return code 8 - invalid state):

static void usbd_user_ev_handler(app_usbd_event_type_t event)
{
    switch (event)
    {
        case APP_USBD_EVT_DRV_SOF:
	    // make sure we're in proper state (configured)
            if (APP_USBD_STATE_Configured != app_usbd_core_state_get()) break;

            // are we in the middle of a transfer ?
            if (!usb_tx_done) break;

            // do we have a buffer ready for transfer ?
            if (audio_buffer_txed) break;

        	// transfer the off-line buffer to the USB device now
            usb_tx_done = 0;
            ret_code_t ret = app_usbd_audio_class_tx_start(&m_app_audio_microphone.base,
        			pdm_buffer[pdm_buffer_i == 0 ? 1 : 0], NRFX_PDM_BUFFER_SIZE);

            // haven't started transferring -> wont reach TX done
            if (ret != NRF_SUCCESS) {
            	usb_tx_done = 1;
            	NRF_LOG_INFO("APP_USBD_EVT_DRV_SOF -> app_usbd_audio_class_tx_start = %d", ret);
            }

            // this buffer is being transferred
            else audio_buffer_txed = 1;

            break;
            
        // .
        // .
        // .
    }
    
    // .
    // .
    // .
}

My question is - how can I pass the data from the PDM callback to the USB device reliably with no errors ?

Remark #1:
In the code snippets I attached the "// ." signify mode code there that I didnt bother to attach.

Remark #2:
I didnt change anything in the definition of the USB microphone which might be a problem.
Especially the following line:

#define MIC_INTERFACES_CONFIG() APP_USBD_AUDIO_CONFIG_IN(2, 3)

When I connect the board to a Linux machine and look at dmesg output I get a warning/error that max interface should be 1. I guess the 2 and 3 values here are wrong.

Remark #3:
I read in the documentation that the isochroneous USB device can have a maximum buffer size of 1000 bytes, so I use a double buffer of 500 int16_t for the PDM:

// buffers for the PDM source
#define NRFX_PDM_BUFFER_SIZE 500
static volatile int8_t pdm_buffer_i = 0;
static int16_t pdm_buffer[2][NRFX_PDM_BUFFER_SIZE];

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