[PATCH 1/6] media: v4l2: Add audio capture and output support

[Shengjiu Wang]

On Mon, Jul 3, 2023 at 9:25 PM Takashi Iwai <tiwai at suse.de> wrote:

> On Mon, 03 Jul 2023 15:12:55 +0200,
> Hans Verkuil wrote:
> >
> > On 03/07/2023 14:53, Mark Brown wrote:
> > > On Mon, Jul 03, 2023 at 02:07:10PM +0200, Takashi Iwai wrote:
> > >> Shengjiu Wang wrote:
> > >
> > >>> There is no such memory to memory interface defined in ALSA.  Seems
> > >>> ALSA is not designed for M2M cases.
> > >
> > >> There is no restriction to implement memory-to-memory capture in ALSA
> > >> framework.  It'd be a matter of the setup of PCM capture source, and
> > >> you can create a corresponding kcontrol element to switch the mode or
> > >> assign a dedicated PCM substream, for example.  It's just that there
> > >> was little demand for that.
> > >
> > > Yeah, it's not a terrible idea.  We might use it more if we ever get
> > > better support for DSP audio, routing between the DSP and external
> > > devices if driven from the CPU would be a memory to memory thing.
> > >
> > >> I'm not much against adding the audio capture feature to V4L2,
> > >> though, if it really makes sense.  But creating a crafted /dev/audio*
> > >> doesn't look like a great idea to me, at least.
> > >
> > > I've still not looked at the code at all.
> >
> > My main concern is that these cross-subsystem drivers are a pain to
> > maintain. So there have to be good reasons to do this.
> >
> > Also it is kind of weird to have to use the V4L2 API in userspace to
> > deal with a specific audio conversion. Quite unexpected.
> >
> > But in the end, that's a decision I can't make.
> >
> > So I wait for that feedback. Note that if the decision is made that this
> > can use V4L2, then there is quite a lot more that needs to be done:
> > documentation, new compliance tests, etc. It's adding a new API, and that
> > comes with additional work...
>
> All agreed.  Especially in this case, it doesn't have to be in V4L2
> API, as it seems.
>
> (Though, the support of audio on V4L2 might be useful if it's closely
> tied with the a stream.  But that's another story.)
>

audio is a stream,  for this m2m audio case, V4L2 is the best choice
I found.

I know there is API change for V4L2,  but V4L2 is a good framework
for memory to memory,  I think it is worth to do this change.

if implement this M2M case in ALSA,  we may need to create a sound
card and open it twice for playback and capture,  it is complicated
to do this,  and I am not sure if there is any other issue besides these.
I can't find an example in the ALSA framework for this case.

best regards
wang shengjiu

>
>
> thanks,
>
> Takashi
>
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