ALSA snd_pcm_writei() unexpected underrun

Technical Support
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1

I do not know if it is hardware-related, but I am going mad about spurious underrun errors on snd_pcm_writei() calls. The same code runs without issues on my laptop.

I use a blocking setup:

snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0);

Here is the snd_pcm_dump_sw_setup() output:

  tstamp_mode  : NONE
  tstamp_type  : MONOTONIC
  period_step  : 1
  avail_min    : 1764
  period_event : 0
  start_threshold  : 1
  stop_threshold   : 3528
  silence_threshold: 0
  silence_size : 0
  boundary     : 1849688064

While setting the hardware parameters, I log the results of the snd_pcm_hw_params_set_rate_near(), snd_pcm_hw_params_set_periods_near(), snd_pcm_hw_params_set_period_size_near() calls:

3719.1287 D [AlsaSound] SOUND: setupWithFreq sampling rate: 44100, dir: 0
3719.1288 D [AlsaSound] SOUND: number of periods: 2, dir: 0
3719.1289 D [AlsaSound] SOUND: period size: 1764 frames, dir: 0

Here is the relevant part from the filling loop, what is called repeatedly:

log.debug("play %d samples", n);
while ((ret = snd_pcm_writei(handle, playBuf, n)) != (long)n) {
    if (ret < 0) {
        log.warn("ALSA error: %s\n", snd_strerror(ret));
        if ((ret = snd_pcm_recover(handle, ret, 0)) < 0) {
            log.error("ALSA error after recover: %s\n", snd_strerror(ret));
            checkFatalAlsaError(snd_pcm_prepare(handle), "ALSA irrecoverable error: %s");
        }
    } else {
        log.warn("ALSA short write...?\n");
        break;
    }
}

Here is the log when everything is fine:

3751.3029 D [AlsaSound] Starting square sound, nsamples: 3528, nPeriods: 2, nFrames: 1764
3751.3030 D [AlsaSound] play 1739 samples
3751.3037 D [AlsaSound] play 1739 samples
3751.3046 D [AlsaSound] play 50 samples
3751.3048 D [AlsaSound] Stop sound

And sometimes I get this:

3752.8769 D [AlsaSound] Starting square sound, nsamples: 3528, nPeriods: 2, nFrames: 1764
3752.8770 D [AlsaSound] play 1739 samples
3752.8779 D [AlsaSound] play 1739 samples
3752.8782 W [AlsaSound] ALSA error: Broken pipe
ALSA lib ../../../alsa-lib-1.1.4.1/src/pcm/pcm.c:8323:(snd_pcm_recover) underrun occurred
3752.8792 D [AlsaSound] play 50 samples
3752.8793 D [AlsaSound] Stop sound

From the log timestamps it is visible that the underrun occurs within 2ms of the first write – what writes ~40ms of samples. The two shown examples are identical in any other way, the device is not playing sound and is prepare()'d. The writing thread is renice’d to priority -19.

What can be the problem, and the solution?

Please note that it is intentional that I write fewer samples at once than the period size. Also maybe it counts that I am doing some OpenGL ES work in another thread (running on normal nice value).

OpenEmbedded v2.8b4 - Linux kernel version 4.9.87, libasound2 version 1.1.4.1-r0

Thanks in advance!

2

You did not state whether or not the OpenGL ES work in another thread has any influence on your audio code underrunning or not.

3

Sorry for digging this old question. Did you get to realise what caused these unexpected underruns?
I have the same kind of error on an embedded system myself and would be happy to know about any findings you might have.

4

HI @tonesenna: Thanks for writing to the Toradex Community!

Could you please open a new Question and provide all the details (hardware version, software version, sample code, …) to reproduce the issue?

Thanks and best regards,
Jaski