Do not use HAL_SPI_Transmit() for high performance (DMA may not be as necessary)

[RhSilicon]

Hi,

I'm using an STM32F407VGT6 at 168MHz, SPI3 clocked at 21MHz (the SPI1, which does up to 42MHz is in use elsewhere).

I did some tests with the SPI ILI9341 display 2.4" of 320x240 pixels, and to fill the screen completely using HAL_SPI_Transmit() takes 191ms.

But if you only use what is necessary (bare metal?), the time is 58ms.

Is that 330% performance improvement? (191/58=3.293103448)

(I tried to change the 8-bit mode to 16-bit in flight, but I haven't been able to do it yet, I don't know if this time of 58ms will go down)

#define I_don_t_want_to_lose_performance_using_HAL 1

void ILI9341_FillRect(uint16_t x, uint16_t y, uint16_t w, uint16_t h,
		uint16_t color) {
	// clipping
	if ((x >= ILI9341_WIDTH) || (y >= ILI9341_HEIGHT))
		return;
	if ((x + w - 1) >= ILI9341_WIDTH)
		w = ILI9341_WIDTH - x;
	if ((y + h - 1) >= ILI9341_HEIGHT)
		h = ILI9341_HEIGHT - y;

	ILI9341_Select();
	ILI9341_SetAddressWindow(x, y, x + w - 1, y + h - 1);

	uint8_t data[] = { color >> 8, color & 0xFF };

	HAL_GPIO_WritePin(ILI9341_DC_GPIO_Port, ILI9341_DC_Pin, GPIO_PIN_SET);

	for (y = h; y > 0; y--) {
		for (x = w; x > 0; x--) {

#if I_don_t_want_to_lose_performance_using_HAL != 1
			HAL_SPI_Transmit(&ILI9341_SPI_PORT, data, sizeof(data),
			HAL_MAX_DELAY);
#else
			*((__IO uint8_t*) &ILI9341_SPI_PORT.Instance->DR) = data[0];

			while(!__HAL_SPI_GET_FLAG(&ILI9341_SPI_PORT, SPI_FLAG_TXE));

			*((__IO uint8_t*) &ILI9341_SPI_PORT.Instance->DR) = data[1];

			while(!__HAL_SPI_GET_FLAG(&ILI9341_SPI_PORT, SPI_FLAG_TXE));
#endif
		}
	}

	ILI9341_Unselect();
}

It might be very interesting to review all the STM32 libraries that use SPI, such as the display ones.

 Original full library here

[TDK]

I guess you're right. I really though it could be 32-bits on the F4, but maybe I've been using the H7 too much lately. I guess you could chain the second transfer off of the TC trigger without too much trouble or delay.

[TDK]

What you wrote will probably work okay if the SPI clock rate is high, but just because TXE=1 the transmission may not be complete. You should wait for BSY=0 after the final transfer before you pull CS high.