[alsa-devel] Discarding the first few frames inside the kernel

[Takashi Sakamoto]

Hi,

On Jan 19 2016 03:21, Ankhit Vivekananda wrote:
> I am interfacing the ICS43432 MEMS microphone with McBSP interface. The
> data sheet of the ICS43432 microphone shows that it takes about 4.5ms
> (16kHz sampling rate) for startup. Because of this, I hear a transient
> noise at the beginning of every stream. Is there in any way I can discard
> the frames inside the kernel space itself before reading the samples using
> snd_pcm_readi() call?
>
> I was able to mute the first few frames after the readi() call, but would
> want to know if I could do it before the readi() call itself inside the
> kernel space.

If the data transmission starts in 'struct snd_pcm_ops.prepare' callback 
of the driver, we can wait what we want, because ALSA PCM core 
guarantees to call it in process context.

In userspace, this looks like (here I describe in alsa-lib API):
  - calling snd_pcm_hw_params()
    - snd_pcm_prepare() is called library internal
      * struct snd_pcm_ops.prepare() is called in kernel mode.
        * mute
        * start data transmission
        * wait for the 4.5ms or somewhat
        * return
      * return
    - return
  - PCM frame operation such as snd_pcm_writei()
    * snd_pcm_ops.trigger(TRIGGER_START) is called in kernel mode.
      * demute
    * PCM frames starts to be transferred from userspace.


Regards

Takashi Sakamoto