audio samples data flow from user mode to kernel mode

[Subhashini Rao Beerisetty]

On Sat, Jun 23, 2018 at 4:41 PM, <o at goosey.org> wrote:

>
>
> 23.06.2018, 14:52, "Subhashini Rao Beerisetty" <
> subhashbeerisetty at gmail.com>:
> > Hello All,
>
> Hello,
>
>
> > I’m trying to understand how audio samples transferred between user mode
> to kernel mode during playback and capture. I’m using aplay & arecord alsa
> utilities for playback and capture.
> >
> > Let us take a PCM wav file of sample rate 48000 and it has a total
> number of samples 480000 (Approx.Duration in seconds=10). Size of each
> sample is 8 bytes(two channels). After invocation of aplay from user mode,
> how does these audio samples gets copied to kernel mode? Can someone
> explain me on this?
> >
> > Is it possible to capture the timestamps for the first and last audio
> samples that arrive at the driver level?
> >
> > Can I consider the .trigger(for playback & capture) callback in
> SNDRV_PCM_TRIGGER_START  case is timestamp for the first audio sample?
> >
> > Similarly does .trigger callbacks SNDRV_PCM_TRIGGER_STOP gives the last
> audio sample timestamp?
>
> I think no, because the SNDRV_PCM_TRIGGER_START and SNDRV_PCM_TRIGGER_STOP
> functions only system call to the PCM transmitted.
> So, will want two different functions as playback / capture and the method
> depends on you use alsa or pulse.
>
> I think you want to:
>
> snd_mychip_playback_open
> snd_mychip_playback_close
>
> snd_mychip_capture_open
> snd_mychip_capture_close
>
Are these part of "struct snd_pcm_ops" .open & .close?  If so timestamps
captured at these API's gives the first and last audio sample timestamps?

Trigger funcs times are not real playback or capture time-stamps.

Regards

Ozgur

> Thanks,
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