[PATCH] alsaloop: reduce cumulative error caused by non-atomic samples calculation

[Ruslan Bilovol]

When doing loopback between two audio card with
same sampling frequency, I noticed slow increase
of pitch_diff.

When I changed order of get_queued_playback_samples()
vs get_queued_capture_samples(), I noticed same drift
of pitch_diff but if was decreasing this time.

This seems to be caused by non-atomic consecutive
snd_pcm_delay() invocation for playback then for
capture. snd_pcm_delay() measures delay between
read/write call and actual ADC/DAC operation.

So while we get this value for playback path in
get_queued_playback_samples(), next call to
get_queued_capture_samples() will happen a little
bit later so snd_pcm_delay() may return incorrect
value.

Be interleaving get_queued_{playback,capture}_samples()
order, we divide this small error between playback
and capture paths. I do not see any issues anymore
with one-way drift of pitch_diff.

Signed-off-by: Ruslan Bilovol <ruslan.bilovol at gmail.com>
---
 alsaloop/pcmjob.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/alsaloop/pcmjob.c b/alsaloop/pcmjob.c
index b252486..1b7925a 100644
--- a/alsaloop/pcmjob.c
+++ b/alsaloop/pcmjob.c
@@ -1951,8 +1951,16 @@ int pcmjob_pollfds_handle(struct loopback *loop, struct pollfd *fds)
 	}
 	if (loop->sync != SYNC_TYPE_NONE) {
 		snd_pcm_sframes_t pqueued, cqueued;
-		pqueued = get_queued_playback_samples(loop);
-		cqueued = get_queued_capture_samples(loop);
+
+		/* Reduce cumulative error by interleaving playback vs capture reading order */
+		if (loop->total_queued_count & 1) {
+			pqueued = get_queued_playback_samples(loop);
+			cqueued = get_queued_capture_samples(loop);
+		} else {
+			cqueued = get_queued_capture_samples(loop);
+			pqueued = get_queued_playback_samples(loop);
+		}
+
 		if (verbose > 4)
 			snd_output_printf(loop->output, "%s: queued %li/%li samples\n", loop->id, pqueued, cqueued);
 		if (pqueued > 0)
-- 
1.9.1