Make it possible to derive from DVDisplay if you want to do fancy things with the frame header

drivers/dv_display/dv_display.cpp

@@ -333,7 +333,7 @@ namespace pimoroni {
     read(point_to_address_palette(p), l, data);
   }
 
-  void DVDisplay::write_header()
+  void DVDisplay::write_header_preamble()
   {
     uint32_t buf[8];
     uint32_t full_width = width * h_repeat;
@@ -346,7 +346,13 @@ namespace pimoroni {
     buf[6] = 0x00000001;
     ram.write(0, buf, 7 * 4);
     ram.wait_for_finish_blocking();
+  }
 
+  void DVDisplay::write_header()
+  {
+    write_header_preamble();
+
+    uint32_t buf[8];
     uint addr = 4 * 7;
     uint line_type = 0x80000000u + ((uint)mode << 28);
     printf("Write header, line type %08x\n", line_type);

drivers/dv_display/dv_display.hpp

@@ -28,7 +28,7 @@ namespace pimoroni {
     //--------------------------------------------------
     // Variables
     //--------------------------------------------------
-  private:
+  protected:
     // Ram accessed through the APS6404 driver
     APS6404 ram;
 
@@ -158,11 +158,18 @@ namespace pimoroni {
       // The supplied buffer must be at least 128 bytes long
       void get_edid(uint8_t* edid);
 
-    private:
+    protected:
       uint8_t palette[PALETTE_SIZE * 3] alignas(4);
       bool rewrite_header = false;
 
-      static constexpr int PIXEL_BUFFER_LEN_IN_WORDS = 16;
+      virtual void write_palette();
+      virtual void write_header();
+
+      void write_header_preamble();
+      void i2c_modify_bit(uint8_t reg, uint bit, bool enable);
+
+    private:
+      static constexpr int PIXEL_BUFFER_LEN_IN_WORDS = 32;
       uint32_t pixel_buffer[PIXEL_BUFFER_LEN_IN_WORDS];
       Point pixel_buffer_location;
       int32_t pixel_buffer_x;
@@ -173,11 +180,6 @@ namespace pimoroni {
       void read(uint32_t address, size_t len, uint8_t *data);
       void write(uint32_t address, size_t len, const RGB888 colour);
 
-      void write_palette();
-      void write_header();
-
-      void i2c_modify_bit(uint8_t reg, uint bit, bool enable);
-
       uint32_t point_to_address(const Point &p) {
         return base_address + ((p.y * (uint32_t)width * 3) + p.x) * 2;
       }