[alsa-devel] Alsa-lib: Dmix slowptr latency behaviour

[Matin Momin]

Hi,

On my embedded device, I am using a push mode simple alsa pipeline
with following command

> arecord -D"dsp" -r48000  -c2 -fs32_le | aplay -D"dmix1"

where the asound conf looks this
-------------------------------------------------------------------
# This is the actual HW device
pcm.dsp {
        type hw
        card 1
        rate 48000
        channels 2
        format S32_LE
}
pcm.dmix1 {
        type dmix
        ipc_key 1024
        ipc_key_add_uid false
        ipc_perm 0666
        slave.pcm "dsp"
        slowptr true
}
-------------------------------------------------------------------

I measured the pipeline delay by using Audacity on my PC, and
measuring the difference in spike of audio recorded on the PC in a
silent environment (1st spike will be from the actual sound and second
will be from the output of my embedded device).

Here I get a delay of around 245 msecs.

Now, in the same configuration if I disable slowptr in dmix1, the
delay comes down to 160 msecs, which corresponds exactly to buffer
time (85 msecs).

I want to know whether this is expected or is it an aberration?

I have measured the userland time for the data to travel from
alsa-lib/src/pcm/pcm_hw.c: snd_pcm_hw_readi() to
alsa-lib/src/pcm/pcm_dmix.c:mix_areas() (By adding incremental numbers
to each period recorded by arecord and seeing how much time they take
to reach dmix). I cannot find any difference in absolute time
difference with both slowptr true and false. Also, the critical hw_ptr
and appl_ptr variables in pcm_hw and pcm_dmix also reflect same values
for slowptr true and false.

Is the above mentioned method to measure delay correct? If I don't see
a delay here, where else should I look?

The alsa-lib version is 1.1.0 and kernel is 3.8.13.

Thanks in advance.

Regards,
Matin