vo_vdpau: simplify previous vsync timestamp calculation

The strange thing about this code was the shift parameter of the
prev_vs2 function. The parameter is used to handle timestamps before the
last vsync, since the % operator handles negative values incorrectly.
Most callers set shift to 0, and _usually_ pass a timestamp after the
last vsync. One caller sets it to 16, and can pass a timestamp before
the last timestamp.

The mystery is why prev_vs2 doesn't just compensate for the % operator
semantics in the most simple way: if the result of the operator is
negative, add the divisor to it. Instead, it adds a huge value to it
(how huge is influenced by shift). If shift is 0, the result of the
function will not be aligned to vsyncs.

I have no idea why it was written in this way. Were there concerns about
certain numeric overflows that could happen in the calculations? But I
can't think of any (the difference between ts and vc->recent_vsync_time
is usually not that huge). Or is there something more clever about it,
which is important for the timing code? I can't think of anything
either.

So scrap it and simplify it.

1 files changed, 8 insertions, 9 deletions

diff --git a/video/out/vo_vdpau.c b/video/out/vo_vdpau.c
index 58ac6b971f..8c50e44d6b 100644
--- a/video/out/vo_vdpau.c
+++ b/video/out/vo_vdpau.c
@@ -994,12 +994,13 @@ static int update_presentation_queue_status(struct vo *vo)
     return num_queued;
 }
 
-static inline uint64_t prev_vs2(struct vdpctx *vc, uint64_t ts, int shift)
+// Return the timestamp of the vsync that must have happened before ts.
+static inline uint64_t prev_vsync(struct vdpctx *vc, uint64_t ts)
 {
-    uint64_t offset = ts - vc->recent_vsync_time;
-    // Fix negative values for 1<<shift vsyncs before vc->recent_vsync_time
-    offset += (uint64_t)vc->vsync_interval << shift;
-    offset %= vc->vsync_interval;
+    int64_t diff = (int64_t)(ts - vc->recent_vsync_time);
+    int64_t offset = diff % vc->vsync_interval;
+    if (offset < 0)
+        offset += vc->vsync_interval;
     return ts - offset;
 }
 
@@ -1034,9 +1035,7 @@ static void flip_page_timed(struct vo *vo, int64_t pts_us, int duration)
     uint64_t ideal_pts = pts;
     uint64_t npts = duration >= 0 ? pts + duration : UINT64_MAX;
 
-#define PREV_VS2(ts, shift) prev_vs2(vc, ts, shift)
-    // Only gives accurate results for ts >= vc->recent_vsync_time
-#define PREV_VSYNC(ts) PREV_VS2(ts, 0)
+#define PREV_VSYNC(ts) prev_vsync(vc, ts)
 
     /* We hope to be here at least one vsync before the frame should be shown.
      * If we are running late then don't drop the frame unless there is
@@ -1065,7 +1064,7 @@ static void flip_page_timed(struct vo *vo, int64_t pts_us, int duration)
      */
     uint64_t vsync = PREV_VSYNC(pts);
     if (pts < vsync + vsync_interval / 4
-        && (vsync - PREV_VS2(vc->last_queue_time, 16)
+        && (vsync - PREV_VSYNC(vc->last_queue_time)
             > pts - vc->last_ideal_time + vsync_interval / 2
             || vc->dropped_frame && vsync > vc->dropped_time))
         pts -= vsync_interval / 2;