Added an additional SPI API function dmaSendAsync which will start the a DMA send of a buffer and return immediately. The next time dmaSendAsync is called it waits if the previous transfer is not complete. Note the buffer is not copied, so ddouble buffering is needed to use this function

STM32F1/libraries/SPI/src/SPI.cpp

@@ -469,6 +469,48 @@ uint8 SPIClass::dmaSend(void * transmitBuf, uint16 length, bool minc)
 	return b;
 }
 
+
+uint8 SPIClass::dmaSendAsync(void * transmitBuf, uint16 length, bool minc)
+{
+	static bool isRunning=false;
+	uint8 b = 0;	
+
+	if (isRunning)
+	{
+
+		uint32_t m = millis();
+		while ((dma_get_isr_bits(_currentSetting->spiDmaDev, _currentSetting->spiTxDmaChannel) & DMA_ISR_TCIF1)==0) {//Avoid interrupts and just loop waiting for the flag to be set.
+			//delayMicroseconds(10);
+			if ((millis() - m) > DMA_TIMEOUT) { b = 2; break; }
+		}		
+		
+		while (spi_is_tx_empty(_currentSetting->spi_d) == 0); // "5. Wait until TXE=1 ..."
+		while (spi_is_busy(_currentSetting->spi_d) != 0); // "... and then wait until BSY=0 before disabling the SPI." 
+		spi_tx_dma_disable(_currentSetting->spi_d);
+		dma_disable(_currentSetting->spiDmaDev, _currentSetting->spiTxDmaChannel);
+		isRunning=false;
+	}
+
+	
+	if (length == 0) return 0;
+	uint32 flags = ( (DMA_MINC_MODE*minc) | DMA_FROM_MEM | DMA_TRNS_CMPLT);
+
+    dma_init(_currentSetting->spiDmaDev);
+	// TX
+	dma_xfer_size dma_bit_size = (_currentSetting->dataSize==DATA_SIZE_16BIT) ? DMA_SIZE_16BITS : DMA_SIZE_8BITS;
+    dma_setup_transfer(_currentSetting->spiDmaDev, _currentSetting->spiTxDmaChannel, &_currentSetting->spi_d->regs->DR, dma_bit_size,
+                       transmitBuf, dma_bit_size, flags);// Transmit buffer DMA 
+	dma_set_num_transfers(_currentSetting->spiDmaDev, _currentSetting->spiTxDmaChannel, length);
+	dma_clear_isr_bits(_currentSetting->spiDmaDev, _currentSetting->spiTxDmaChannel);
+	dma_enable(_currentSetting->spiDmaDev, _currentSetting->spiTxDmaChannel);// enable transmit
+	spi_tx_dma_enable(_currentSetting->spi_d);	
+
+	isRunning=true;
+
+	return b;
+}
+
+
 void SPIClass::attachInterrupt(void) {
 	// Should be enableInterrupt()
 }

STM32F1/libraries/SPI/src/SPI.h

@@ -291,7 +291,7 @@ public:
      * @param length Number of bytes in buffer to transmit.
      */
 	uint8 dmaSend(void * transmitBuf, uint16 length, bool minc = 1);
-
+	uint8 dmaSendAsync(void * transmitBuf, uint16 length, bool minc = 1);
     /*
      * Pin accessors
      */