* snd_pcm_avail_delay() reporting inaccurate delay values
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@ 2025-07-31 12:36 ` GitHub issues - opened
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From: GitHub issues - opened @ 2025-07-31 12:36 UTC (permalink / raw)
To: alsa-devel
alsa-project/alsa-lib issue #468 was opened from kv2019i:
The alsa_conformance_test tool (https://chromium.googlesource.com/chromiumos/platform/audiotest/+/HEAD/alsa_conformance_test.md) has recently added support to use snd_pcm_avail_delay() to estimate the actual playback position. This has proven a very useful tool to test ALSA drivers on hardware (like that support by https://github.com/thesofproject/ ) where there is significant buffering in hardware (and hw_ptr can move in bursts).
When testing with high sampling rates, an issue was found where the estimated playback position seemed to go backwards in time, as observed by alsa_conformance_test. This was observed on multiple Intel platform using SOF firmware for DSP. Debugging the kernel driver delay() implementation showed, that driver never reported incorrect delay values, but somehow alsa_conformance_test still observed these. Example log:
```
alsa_conformance_test -P hw:sofnocodec,2 -d 1 -r 96000 -B 480 -c 2 -f S32_LE -p 480 --avail-delay --debug
...
TIME_DIFF(s) HW_LEVEL PLAYED DELAY DIFF RATE
[.. snip ]
0.000999950 960 3324 1476 96 96004.800240
0.000995762 864 3420 1380 96 96408.579560
0.001024820 672 3516 1284 96 93674.986827
0.000987638 621 3613 1187 97 98214.122988
[Notice] frame_diff -1 >= 4096.
0.000049322 576 3612 1188 -1 -20274.928024
# KV: the delay value of 1187 is never reported by driver and it causes the playback position to go backwards in time despite driver reporting the accurate delay correctly
0.000992228 480 3708 1092 96 96751.956204
0.000972403 864 3804 1476 96 98724.500027
0.000998287 768 3900 1380 96 96164.730183
0.000979150 768 3996 1284 96 98044.222029
0.000999349 672 4092 1188 96 96062.536711
0.001018172 480 4188 1092 96 94286.623478
0.000977025 960 4284 1476 96 98257.465264
0.001022131 768 4380 1380 96 93921.424945
0.000977795 768 4476 1284 96 98180.088873
0.001019073 601 4573 1187 97 95184.545170
[Notice] frame_diff -1 >= 4096.
0.000049433 576 4572 1188 -1 -20229.401412
0.000953494 480 4668 1092 96 100682.332558
0.000981225 960 4764 1476 96 97836.887564
0.001025294 768 4860 1380 96 93631.680279
```
When instrumenting alsa-lib snd_pcm_avail_delay() it became apparent the function is calling in middle of a DMA burst (when hw_ptr is moving fast ahead and the "avail" number is changing fast). alsa-lib does two syscalls to read avail and delay, the two values are not in sync and we end up reporting a bogus value. The error is not a big one, but this is enough to fail alsa_conformance_test test plans and push the calculcated standard rate error above acceptance criteria. This could also confuse media players who use this for video lip-sync, although in theory applications should handle this (a 0...3 frame error in delay reporting is not going to make a difference unless negative value breaks the application logic like happens here with alsa_conformance_test).
We can observe the error especially with higher rates (96, 176.4 and 192kHz). This is harder to hit with the more typical 48khz use-cases, and thus maybe has not been hit before.
Versions tested: 1.12.24, 1.12.11
Issue URL : https://github.com/alsa-project/alsa-lib/issues/468
Repository URL: https://github.com/alsa-project/alsa-lib
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