DMA

One more trick to gain a little bit of performance is to use Direct Memory Access to write data out to the LCD. We'll make use of the SPI DMA support in the esp-hal crate.

As usual, let's start by importing the necessary types.

use esp_hal::{dma::{DmaRxBuf, DmaTxBuf}, dma_buffers};

Next, we need to declare the DMA buffers:

let (rx_buffer, rx_descriptors, tx_buffer, tx_descriptors) = dma_buffers!(10 * 1024);
let dma_rx_buf = DmaRxBuf::new(rx_descriptors, rx_buffer).expect("failed to create DMA RX buffer");
let dma_tx_buf = DmaTxBuf::new(tx_descriptors, tx_buffer).expect("failed to create DMA TX buffer");

Finally, update the SPI builder with calls to with_dma() and with_buffers() and you're done!

let spi = Spi::new(
    peripherals.SPI2,
    spi::master::Config::default()
        .with_frequency(Rate::from_mhz(80))
        .with_mode(spi::Mode::_0),
)
.expect("could not create SPI instance")
.with_sck(peripherals.GPIO1)
.with_mosi(peripherals.GPIO2)
.with_dma(peripherals.DMA_CH0)         // New!
.with_buffers(dma_rx_buf, dma_tx_buf); // New!

Exercise

Can you measure the time difference between regular and DMA-enabled implementations?