I have an odd display which uses a modified ST7789 driver to support SPI9 but to update the display is quite slow. I am using an Teensy 3.6 and running the example DMASpi (https://github.com/crteensy/DmaSpi) and would like to convert the bit banging code to DMA transfer.

The ST7789 SPI 9 librry for the driver is here:
https://github.com/ananevilya/Arduino-ST7789-Library

This is the part of the code which bit bangs the data into SPI, the rest of the code is driver code and commands




Code:
inline void Arduino_ST7789::spiwrite(uint8_t c) 
{

		digitalWrite(_sclk, LOW);
		if(_DCbit) digitalWrite(_sid, HIGH);
		else        digitalWrite(_sid, LOW);
		digitalWrite(_sclk, HIGH);
		
		// Fast SPI bitbang swiped from LPD8806 library
		for(uint8_t bit = 0x80; bit; bit >>= 1)
		 {
		digitalWrite(_sclk, LOW);
		if(c & bit) 
			digitalWrite(_sid, HIGH);
		else        
			digitalWrite(_sid, LOW);
		digitalWrite(_sclk, HIGH);
		  
	    }
}
I trying to work out if I can somehow replace the above code with DMA transfer, rather than the block data transfer into the bit transfer I want to add it into the DMA "src" buffer. I have tried the code but it does not work and just wondering if it will be possible.

Code:
  DMASPI0.begin();
  DMASPI0.start();

  Serial.println("Start DMA transfer");
  DmaSpi::Transfer trx(nullptr, 0, nullptr);

  trx = DmaSpi::Transfer(src, DMASIZE, dest);
  clrDest((uint8_t*)dest);
  DMASPI0.registerTransfer(trx);
  while(trx.busy())
  {
  }
  Serial.println("Finished DMA transfer");