Drive the panel through drm and page flip between four scanout buffers

main.go

@@ -602,14 +602,12 @@ func main() {
 
 const (
 	reportInterval = time.Second
-	// Redraw fast so the panel feels live, but measure rates over a much longer
-	// window than a frame, since a frame's worth of a bursty sender is noise.
-	displayInterval = 16 * time.Millisecond
-	// What a bucket covers. Deliberately not displayInterval: how often the
-	// counters are read is a property of the measurement, and tying it to the
-	// refresh would let a slow or blocked draw stretch the window it reports.
-	// Short enough that a step lands promptly, long enough that a bucket holds
-	// tens of thousands of frames at line rate and is not itself noise.
+	// What a bucket covers. Nothing to do with how often the panel is redrawn:
+	// how often the counters are read is a property of the measurement, and
+	// letting the refresh set it would let a slow or blocked draw stretch the
+	// window it reports. Short enough that a step lands promptly, long enough
+	// that a bucket holds tens of thousands of frames at line rate and is not
+	// itself noise.
 	sampleInterval = 16 * time.Millisecond
 	// Both how far back the shown errors reach and how many buckets the median
 	// runs over, so a step in the rate lands half this late.
@@ -778,8 +776,6 @@ func run(aName, bName, sizesArg string,
 	close(startTx)
 	tick := time.NewTicker(reportInterval)
 	defer tick.Stop()
-	frame := time.NewTicker(displayInterval)
-	defer frame.Stop()
 
 	views := make([]view, len(dirs))
 	rows := make([]view, len(dirs))
@@ -793,7 +789,8 @@ func run(aName, bName, sizesArg string,
 			return nil
 		case <-space:
 			start = resetAll(dirs, stats)
-		case now := <-frame.C:
+		case <-disp.fb.flips:
+			now := time.Now()
 			px, py, down := touch.get()
 			x, y := disp.fb.fromPanel(px, py)
 			if disp.holdReset(x, y, down, now) {
@@ -806,8 +803,10 @@ func run(aName, bName, sizesArg string,
 			if m, ok := cfg.cableMetres(views); ok {
 				cable = fmt.Sprintf("%.1f m", m)
 			}
-			disp.render(totalView(views), now.Sub(start),
-				target*float64(len(dirs)), cable)
+			if err := disp.render(totalView(views), now.Sub(start),
+				target*float64(len(dirs)), cable); err != nil {
+				return err
+			}
 		case now := <-tick.C:
 			elapsed := now.Sub(start)
 			// Length needs both directions, so every row is sampled before any of