[client] x11: move sleep and calibration code outside of the event loop

Sleeping in the x11 event loop is not ideal as it will introduce latency
when processing other events, instead do this in the waitFrame handler.

client/displayservers/X11/x11.c

@@ -1041,72 +1041,15 @@ static void x11XInputEvent(XGenericEventCookie *cookie)
 
 static void x11XPresentEvent(XGenericEventCookie *cookie)
 {
-#define CALIBRATION_COUNT 200
-  uint64_t deltats  = 0;
-  uint64_t deltamsc = 0;
-
   switch(cookie->evtype)
   {
     case PresentCompleteNotify:
     {
-      x11DoPresent();
-
       XPresentCompleteNotifyEvent * e = cookie->data;
-
+      atomic_store(&x11.presentMsc, e->msc);
-      /* calibrate a delay to push our presentation time forward as far as
+      atomic_store(&x11.presentUst, e->ust);
-       * possible without skipping frames */
-      static int      calibrate = 0;
-      static uint64_t lastts    = 0;
-      static uint64_t lastmsc   = 0;
-      static uint64_t delay     = 0;
-      if (lastts)
-      {
-        deltats  = e->ust - lastts;
-        deltamsc = e->msc - lastmsc;
-
-        if (calibrate == 0)
-        {
-          if (!delay)
-            delay = deltats / 2;
-          else
-          {
-            /* increase the delay until we see a skip */
-            if (deltamsc < 2)
-              delay += 100;
-            else
-            {
-              delay -= 100;
-              ++calibrate;
-            }
-          }
-        }
-        else
-        {
-          if (calibrate < CALIBRATION_COUNT)
-          {
-            /* every skip we back off the delay */
-            if (deltamsc > 1)
-            {
-              delay -= 100;
-              calibrate = 1;
-            }
-
-            /* if we have finished, print out the delay */
-            if (++calibrate == CALIBRATION_COUNT)
-              DEBUG_INFO("Calibration done, delay = %lu us", delay);
-          }
-        }
-      }
-
-      lastts  = e->ust;
-      lastmsc = e->msc;
-
-      /* minor adjustments if we are still seeing odd skips */
-      if (deltamsc > 1)
-        delay -= 10;
-
-      usleep(delay);
       lgSignalEvent(x11.frameEvent);
+      x11DoPresent();
       break;
     }
   }
@@ -1187,7 +1130,72 @@ static void x11GLSwapBuffers(void)
 
 static void x11WaitFrame(void)
 {
+  /* wait until we are woken up by the present event */
   lgWaitEvent(x11.frameEvent, TIMEOUT_INFINITE);
+
+#define CALIBRATION_COUNT 400
+  const uint64_t ust = atomic_load(&x11.presentUst);
+  const uint64_t msc = atomic_load(&x11.presentMsc);
+
+  /* calibrate a delay to push our presentation time forward as far as
+   * possible without skipping frames */
+  static int      calibrate = 0;
+  static uint64_t lastts    = 0;
+  static uint64_t lastmsc   = 0;
+  static uint64_t delay     = 0;
+
+  uint64_t deltats  = 0;
+  uint64_t deltamsc = 0;
+  if (lastts)
+  {
+    deltats  = ust - lastts;
+    deltamsc = msc - lastmsc;
+
+    if (calibrate == 0)
+    {
+      if (!delay)
+        delay = deltats / 2;
+      else
+      {
+        /* increase the delay until we see a skip */
+        if (deltamsc < 2)
+          delay += 100;
+        else
+        {
+          delay -= 100;
+          ++calibrate;
+          deltamsc = 0;
+        }
+      }
+    }
+    else
+    {
+      if (calibrate < CALIBRATION_COUNT)
+      {
+        /* every skip we back off the delay */
+        if (deltamsc > 1)
+        {
+          delay -= 100;
+          calibrate = 1;
+          deltamsc = 0;
+        }
+
+        /* if we have finished, print out the delay */
+        if (++calibrate == CALIBRATION_COUNT)
+          DEBUG_INFO("Calibration done, delay = %lu us", delay);
+      }
+    }
+  }
+
+  lastts  = ust;
+  lastmsc = msc;
+
+  /* minor adjustments if we are still seeing odd skips */
+  if (deltamsc > 1)
+    delay -= 10;
+
+  struct timespec ts = { .tv_nsec = delay * 1000 };
+  while(nanosleep(&ts, &ts)) {};
 }
 
 static void x11StopWaitFrame(void)

client/displayservers/X11/x11.h

@@ -42,6 +42,7 @@ struct X11DSState
 
   int               xpresentOp;
   bool              jitRender;
+  _Atomic(uint64_t) presentMsc, presentUst;
   uint32_t          presentSerial;
   Pixmap            presentPixmap;
   XserverRegion     presentRegion;