stm32 - HAL_UARTEx_RxEventCallback() circular DMA: What address is the data?

Question

I'm using the HAL with an STM32F3xx, implementing UART receive with circular DMA. The data should continually be received into the huart->pRxBuffPtr buffer, overwriting old data as new data arrives, and the HAL_UARTEx_RxEventCallback() function gets called regularly to copy out the data before it gets overwritten. The HAL_UARTEx_RxEventCallback() function receives a size parameter and of course a pointer huart, but no direct indication of where in the huart->pRxBuffPtr the freshly arrived data was DMA'd to.

How do I know whereabouts in huart->pRxBuffPtr the freshly arrived data starts?

Answer 1

Thank you to Tom V for the hint. For future generations, here is the solution code - a function which returns true and gets the next available byte, otherwise returns false:

bool uart_receiveByte(uint8_t *pData) {
  static size_t dmaTail = 0u;

  bool isByteReceived = false;

  // dmaHead is the next position in the buffer the DMA will write to.
  // dmaTail is the next position in the buffer to be read from.
  const size_t dmaHead = huart->RxXferSize - huart->hdmarx->Instance->CNDTR;

  if (dmaTail != dmaHead) {
    isByteReceived = true;
    *pData = pRxDmaBuffer[dmaTail];
    if (++dmaTail >= huart->RxXferSize) {
      dmaTail = 0u;
    }
  }

  return isByteReceived;
}