st7789: avoid heap allocations after the driver is created

Heap allocations can result in significant and unpredictable slowdowns.
For example, they can take over 10ms on the Gopher Badge, which results
in visible tearing (when tearing is otherwise avoided).

st7789/st7789.go

@@ -44,6 +44,7 @@ type Device struct {
 	batchLength     int32
 	isBGR           bool
 	vSyncLines      int16
+	buf             [6]byte
 }
 
 // Config is the configuration for the display
@@ -262,10 +263,12 @@ func (d *Device) SetPixel(x int16, y int16, c color.RGBA) {
 func (d *Device) setWindow(x, y, w, h int16) {
 	x += d.columnOffset
 	y += d.rowOffset
-	d.Tx([]uint8{CASET}, true)
-	d.Tx([]uint8{uint8(x >> 8), uint8(x), uint8((x + w - 1) >> 8), uint8(x + w - 1)}, false)
-	d.Tx([]uint8{RASET}, true)
-	d.Tx([]uint8{uint8(y >> 8), uint8(y), uint8((y + h - 1) >> 8), uint8(y + h - 1)}, false)
+	d.Command(CASET)
+	copy(d.buf[:4], []uint8{uint8(x >> 8), uint8(x), uint8((x + w - 1) >> 8), uint8(x + w - 1)})
+	d.Tx(d.buf[:4], false)
+	d.Command(RASET)
+	copy(d.buf[:4], []uint8{uint8(y >> 8), uint8(y), uint8((y + h - 1) >> 8), uint8(y + h - 1)})
+	d.Tx(d.buf[:4], false)
 	d.Command(RAMWR)
 }
 
@@ -404,12 +407,14 @@ func (d *Device) SetRotation(rotation Rotation) {
 
 // Command sends a command to the display.
 func (d *Device) Command(command uint8) {
-	d.Tx([]byte{command}, true)
+	d.buf[0] = command
+	d.Tx(d.buf[:1], true)
 }
 
 // Data sends data to the display.
 func (d *Device) Data(data uint8) {
-	d.Tx([]byte{data}, false)
+	d.buf[0] = data
+	d.Tx(d.buf[:1], false)
 }
 
 // Tx sends data to the display
@@ -470,17 +475,19 @@ func (d *Device) IsBGR(bgr bool) {
 // SetScrollArea sets an area to scroll with fixed top and bottom parts of the display.
 func (d *Device) SetScrollArea(topFixedArea, bottomFixedArea int16) {
 	d.Command(VSCRDEF)
-	d.Tx([]uint8{
+	copy(d.buf[:6], []uint8{
 		uint8(topFixedArea >> 8), uint8(topFixedArea),
 		uint8(d.height - topFixedArea - bottomFixedArea>>8), uint8(d.height - topFixedArea - bottomFixedArea),
-		uint8(bottomFixedArea >> 8), uint8(bottomFixedArea)},
-		false)
+		uint8(bottomFixedArea >> 8), uint8(bottomFixedArea)})
+	d.Tx(d.buf[:6], false)
 }
 
 // SetScroll sets the vertical scroll address of the display.
 func (d *Device) SetScroll(line int16) {
 	d.Command(VSCRSADD)
-	d.Tx([]uint8{uint8(line >> 8), uint8(line)}, false)
+	d.buf[0] = uint8(line >> 8)
+	d.buf[1] = uint8(line)
+	d.Tx(d.buf[:2], false)
 }
 
 // StopScroll returns the display to its normal state.