gbadisplay: add simple driver for the GameBoy Advance display

gbadisplay/gbadisplay.go

@@ -0,0 +1,85 @@
+// Package gbadisplay implements a simple driver for the GameBoy Advance
+// display.
+package gbadisplay
+
+import (
+	"device/gba"
+	"errors"
+	"image/color"
+	"runtime/volatile"
+	"unsafe"
+
+	"tinygo.org/x/drivers/pixel"
+)
+
+// Image buffer type used by the GameBoy Advance.
+type Image = pixel.Image[pixel.RGB555]
+
+const (
+	displayWidth  = 240
+	displayHeight = 160
+)
+
+var (
+	errOutOfBounds = errors.New("rectangle coordinates outside display area")
+)
+
+type Device struct{}
+
+// New returns a new GameBoy Advance display object.
+func New() Device {
+	return Device{}
+}
+
+var displayFrameBuffer = (*[160 * 240]volatile.Register16)(unsafe.Pointer(uintptr(gba.MEM_VRAM)))
+
+type Config struct {
+	// TODO: add more display modes here.
+}
+
+// Configure the display as a regular 15bpp framebuffer.
+func (d Device) Configure(config Config) {
+	// Use video mode 3 (in BG2, a 16bpp bitmap in VRAM) and Enable BG2.
+	gba.DISP.DISPCNT.Set(gba.DISPCNT_BGMODE_3<<gba.DISPCNT_BGMODE_Pos |
+		gba.DISPCNT_SCREENDISPLAY_BG2_ENABLE<<gba.DISPCNT_SCREENDISPLAY_BG2_Pos)
+}
+
+// Size returns the fixed size of this display.
+func (d Device) Size() (x, y int16) {
+	return displayWidth, displayHeight
+}
+
+// Display is a no-op: the display framebuffer is modified directly.
+func (d Device) Display() error {
+	// Nothing to do here.
+	return nil
+}
+
+// SetPixel changes the pixel at (x, y) to the given color.
+func (d Device) SetPixel(x, y int16, c color.RGBA) {
+	if x < 0 || y < 0 || x >= displayWidth || y > displayHeight {
+		// Out of bounds, so ignore.
+		return
+	}
+	val := pixel.NewColor[pixel.RGB555](c.R, c.G, c.B)
+	displayFrameBuffer[(int(y))*240+int(x)].Set(uint16(val))
+}
+
+// DrawBitmap updates the rectangle at (x, y) to the image stored in buf.
+func (d Device) DrawBitmap(x, y int16, buf Image) error {
+	width, height := buf.Size()
+	if x < 0 || y < 0 || int(x)+width > displayWidth || int(y)+height > displayHeight {
+		return errOutOfBounds
+	}
+
+	// TODO: try to do a 4-byte memcpy if possible. That should significantly
+	// speed up the copying of this image.
+	for bufY := 0; bufY < int(height); bufY++ {
+		for bufX := 0; bufX < int(width); bufX++ {
+			val := buf.Get(bufX, bufY)
+			displayFrameBuffer[(int(y)+bufY)*240+int(x)+bufX].Set(uint16(val))
+		}
+	}
+
+	return nil
+}

pixel/pixel.go

@@ -16,7 +16,7 @@ import (
 // particular display. Each pixel is at least 1 byte in size.
 // The color format is sRGB (or close to it) in all cases.
 type Color interface {
-	RGB888 | RGB565BE | RGB444BE
+	RGB888 | RGB565BE | RGB555 | RGB444BE
 
 	BaseColor
 }
@@ -46,6 +46,8 @@ func NewColor[T Color](r, g, b uint8) T {
 		return any(NewRGB888(r, g, b)).(T)
 	case RGB565BE:
 		return any(NewRGB565BE(r, g, b)).(T)
+	case RGB555:
+		return any(NewRGB555(r, g, b)).(T)
 	case RGB444BE:
 		return any(NewRGB444BE(r, g, b)).(T)
 	default:
@@ -140,6 +142,35 @@ func (c RGB565BE) RGBA() color.RGBA {
 	return color
 }
 
+// Color format used on the GameBoy Advance among others.
+//
+// Colors are stored as native endian values, with bits 0bbbbbgg_gggrrrrr (red
+// is least significant, blue is most significant).
+type RGB555 uint16
+
+func NewRGB555(r, g, b uint8) RGB555 {
+	return RGB555(r)>>3 | (RGB555(g)>>3)<<5 | (RGB555(b)>>3)<<10
+}
+
+func (c RGB555) BitsPerPixel() int {
+	// 15 bits per pixel, but there are 16 bits when stored
+	return 16
+}
+
+func (c RGB555) RGBA() color.RGBA {
+	color := color.RGBA{
+		R: uint8(c>>10) << 3,
+		G: uint8(c>>5) << 3,
+		B: uint8(c) << 3,
+		A: 255,
+	}
+	// Correct color rounding, so that 0xff roundtrips back to 0xff.
+	color.R |= color.R >> 5
+	color.G |= color.G >> 5
+	color.B |= color.B >> 5
+	return color
+}
+
 // Color format that is supported by the ST7789 for example.
 // It may be a bit faster to use than RGB565BE on very slow SPI buses.
 //