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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.
This commit is contained in:
wm4 2014-04-08 19:02:57 +02:00
parent 929793be7f
commit a748b62709
1 changed files with 8 additions and 9 deletions
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17
video/out/vo_vdpau.c
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@ -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;