[ion/display] Add timeout in waitForVBlank

ion/include/ion/display.h

@@ -21,7 +21,7 @@ void pushRect(KDRect r, const KDColor * pixels);
 void pushRectUniform(KDRect r, KDColor c);
 void pullRect(KDRect r, KDColor * pixels);
 
-void waitForVBlank();
+bool waitForVBlank();
 
 constexpr int Width = 320;
 constexpr int Height = 240;

ion/include/ion/timing.h

@@ -11,7 +11,7 @@ void msleep(uint32_t ms);
 
 /* millis is the number of milliseconds ellapsed since a random epoch.
  * On the device, epoch is the boot time. */
-uint64_t millis();
+volatile uint64_t millis();
 
 }
 }

ion/src/device/shared/drivers/display.cpp

@@ -44,15 +44,46 @@ void pullRect(KDRect r, KDColor * pixels) {
   pullPixels(pixels, r.width()*r.height());
 }
 
-void waitForVBlank() {
-  // We want to return as soon as the TE line is transitionning from "DOWN" to "UP"
-  while (Config::TearingEffectPin.group().IDR()->get(Config::TearingEffectPin.pin())) {
-    // Loop while high, exit when low
-    // Wait for zero
-  }
-  while (!Config::TearingEffectPin.group().IDR()->get(Config::TearingEffectPin.pin())) {
-    // Loop while low, exit when high
+bool waitForVBlank() {
+  /* Min screen frequency is 40Hz so the maximal period is T = 1/40Hz = 25ms.
+   * If after T ms, we still do not have a VBlank event, just return. */
+  constexpr uint64_t timeoutDelta = 50;
+  uint64_t startTime = Timing::millis();
+  uint64_t timeout = startTime + timeoutDelta;
+
+  /* If current time is big enough, currentTime + timeout wraps aroud the
+   * uint64_t. We need to take this into account when computing the terminating
+   * event.
+   *
+   * NO WRAP |----------------|++++++++++|------|
+   *         0           startTime    timeout   max uint64_t
+   *
+   * WRAP    |++++|----------------------|++++++|
+   *         0  timeout              startTime  max uint64_t
+   */
+  bool noWrap = startTime < timeout;
+
+  /* We want to return at the beginning of a high signal, so we wait for the
+   * signal to be low then high. */
+  bool wasLow = false;
+
+  uint64_t currentTime = startTime;
+  while (noWrap ?
+      (currentTime >= startTime && currentTime < timeout) :
+      (currentTime >= startTime || currentTime < timeout))
+  {
+    if (!wasLow) {
+      wasLow = !Config::TearingEffectPin.group().IDR()->get(Config::TearingEffectPin.pin());
+    }
+    if (wasLow) {
+      if (Config::TearingEffectPin.group().IDR()->get(Config::TearingEffectPin.pin())) {
+        return true;
+      }
+    }
+    currentTime = Timing::millis();
+    // TODO: sleep?
   }
+  return false;
 }
 
 void POSTPushMulticolor(int shift, int tileSize) {

ion/src/shared/dummy/display.cpp

@@ -3,5 +3,6 @@
 void Ion::Display::POSTPushMulticolor(int shift, int tileSize) {
 }
 
-void Ion::Display::waitForVBlank() {
+bool Ion::Display::waitForVBlank() {
+  return true;
 }