[alsa-devel] snd_pcm_ioplug_avail_update() confusion

[Takashi Iwai]

On Wed, 18 Oct 2017 08:03:57 +0200,
Rob Duncan wrote:
> 
> I'm working on an external IO plugin based on ioplug and I'm confused
> about the behavior of snd_pcm_ioplug_avail_update().  Specifically this
> call (src/pcm/pcm_ioplug.c:657):
> 
>     result = io->data->callback->transfer(io->data, areas, offset, size);
> 
> Why isn't there a subsequent call to snd_pcm_mmap_appl_forward() like
> there is in ioplug_priv_transfer_areas()?  It seems that without such a
> call the captured data will be lost, or have I missed something?

This is the code to only fetch the data for the capture stream at
avail update.  What happens there is to just fills the gap made by
updating the hw_ptr (right before this transfer call).  And, the
avail_udpate call itself doesn't change appl_ptr.

That said, the current behavior is intentional.


HTH,

Takashi