Optimize ST7796 driver: Use bulk SPI transfers and blocking DMA for higher framerate

lib/st7796/st7796.c

@@ -58,6 +58,7 @@
 
 #include "hardware/gpio.h"
 #include "hardware/spi.h"
+#include "hardware/dma.h"
 #include "hardware/pwm.h"
 #include "pico/binary_info.h"
 #include "pico/stdlib.h"
@@ -123,6 +124,10 @@ static uint8_t current_brightness = 100;  // Current brightness level (0-100)
 static uint pwm_slice;      // PWM slice number for backlight control
 static uint pwm_channel;    // PWM channel number for backlight control
 
+// DMA Channel for fast transfers
+static int dma_tx_channel = -1;
+static dma_channel_config dma_tx_config;
+
 /**
  * @brief Activate chip select (pull CS LOW)
  * 
@@ -337,6 +342,12 @@ void st7796_init(const struct st7796_config *c, uint16_t w, uint16_t h) {
     // only use a portion of it. Adjust these if your display is misaligned.
     x_offset = 0;
     y_offset = 0;
+    
+    // Initialize DMA for SPI
+    dma_tx_channel = dma_claim_unused_channel(true);
+    dma_tx_config = dma_channel_get_default_config(dma_tx_channel);
+    channel_config_set_transfer_data_size(&dma_tx_config, DMA_SIZE_8); // 8-bit transfers
+    channel_config_set_dreq(&dma_tx_config, spi_get_dreq(config->spi, true)); // Sync with SPI TX
 
     // Initialize SPI at maximum stable speed for ST7796
     // Datasheet says max 15.15 MHz, but modern displays work much faster
@@ -551,6 +562,39 @@ void st7796_write(const uint16_t *data, size_t len) {
     cs_deselect();
 }
 
+/**
+ * @brief Write raw buffer data directly to display
+ */
+void st7796_write_raw(const uint8_t *data, size_t len) {
+    dc_data();
+    cs_select();
+    
+    // DMA Implementation (Blocking)
+    // 1. Configure DMA channel to write from buffer to SPI TX register
+    dma_channel_configure(
+        dma_tx_channel,
+        &dma_tx_config,
+        &spi_get_hw(config->spi)->dr, // Write to SPI TX register
+        data,                         // Read from buffer
+        len,                          // Element count (bytes)
+        true                          // Start immediately
+    );
+
+    // 2. Wait for DMA to finish
+    dma_channel_wait_for_finish_blocking(dma_tx_channel);
+    
+    // 3. Wait for SPI Transfer to complete (DMA only fills the FIFO)
+    //    We need to wait for the FIFO to drain and the bus to be idle before raising CS
+    while (spi_is_busy(config->spi)) {
+        // tight_loop_contents() is empty on some platforms, 
+        // using a simple volatile read ensures the compiler doesn't optimize this away
+        // efficiently.
+        __asm volatile ("nop");
+    }
+    
+    cs_deselect();
+}
+
 /**
  * @brief Draw single pixel at specific coordinates
  * 

lib/st7796/st7796.h

@@ -233,6 +233,21 @@ void st7796_set_cursor(uint16_t x, uint16_t y);
  */
 void st7796_write(const uint16_t *data, size_t len);
 
+/**
+ * @brief Write raw buffer data directly to display
+ * 
+ * Writes a buffer of bytes directly to the display without any
+ * conversion or byte swapping. This function assume the data is
+ * already in the correct format (Big-Endian RGB565) for the display.
+ * 
+ * This is significantly faster for large block transfers than
+ * st7796_write() as it uses a single SPI transaction.
+ * 
+ * @param data Pointer to raw byte buffer
+ * @param len Number of bytes to send
+ */
+void st7796_write_raw(const uint8_t *data, size_t len);
+
 /**
  * @brief Draw a single pixel at specified coordinates
  *