Add DMAChannel begin()

teensy3/DMAChannel.cpp

 // so C-only code can reserve DMA channels
 uint16_t dma_channel_allocated_mask = 0;
 
-void DMAChannel::init(void)
+void DMAChannel::begin(void)
 {
 	uint32_t ch = 0;
+
 	__disable_irq();
+	if (TCD && ch < DMA_NUM_CHANNELS
+	  && (dma_channel_allocated_mask & (1 << ch))
+	  && (uint32_t)TCD == (0x40009000 + ch * 32)) {
+		// DMA channel already allocated
+		__enable_irq();
+		return;
+	}
 	while (1) {
 		if (!(dma_channel_allocated_mask & (1 << ch))) {
 			dma_channel_allocated_mask |= (1 << ch);

teensy3/DMAChannel.h

 // https://github.com/pixelmatix/SmartMatrix
 // https://github.com/crteensy/DmaSpi
 
-
 #ifdef __cplusplus
 
+#define DMACHANNEL_HAS_BEGIN
 
 class DMABaseClass {
 public:
 	/*************************************************/
 
 	DMAChannel() {
-		init();
+		begin();
 	}
 	DMAChannel(const DMAChannel &c) {
 		TCD = c.TCD;
 		channel = c.channel;
 	}
 	DMAChannel(const DMASetting &c) {
-		init();
+		begin();
 		copy_tcd(TCD, c.TCD);
 	}
 	DMAChannel & operator = (const DMAChannel &rhs) {
 	~DMAChannel() {
 		release();
 	}
+	void begin(void);
 private:
-	void init(void);
 	void release(void);
 
 public: