See https://medium.com/@ly.lee/optimising-pinetimes-display-driver-with-rust-and-mynewt-3ba269ea2f5c

dc-pin.rs

//  If this is a Command Byte, set DC Pin to low, else set DC Pin to high.
hal::hal_gpio_write(
    SPI_DC_PIN,
    if is_command { 0 }
    else { 1 }
);