[alsa-devel] [PATCH v2 3/3] ASoC: add generic dt-card support

[Jean-Francois Moine]

On Tue, 3 Feb 2015 16:47:48 +0000
Mark Brown <broonie at kernel.org> wrote:

> On Sat, Jan 24, 2015 at 08:30:27AM +0100, Jean-Francois Moine wrote:
> > Mark Brown <broonie at kernel.org> wrote:
> > > On Fri, Jan 23, 2015 at 07:34:56PM +0100, Jean-Francois Moine wrote:
> 
> > > > The simple card builder, 'dt-card' (maybe a better name would have been
> > > > 'graph-card'), acts just like the simple-card except that it does not
> > > > appear in the DT. Its creation is done by an audio controller.
> 
> > > Which audio controller?  There may be several CPU side audio interfaces
> > > in the same card.  For example people often want to have both low
> > > latency and high latency audio paths from the CPU into the hardware (low
> > > latency tends to increase power burn).  SoC centric system designs do
> > > sometimes also have PDM I/O, expecting to be directly connected to DMICs
> > > and so on, which results in a relatively large number of CPU interfaces.
> 
> > The audio controller which creates the card depends on the complexity
> > of the card. When there are many controllers, it is up to the designer
> > to define either a master audio controller or to instantiate a 'card'
> > device via the DT for doing the job.
> 
> So how does the simple controller interact with a more complex one given
> that it's somehow picking some controller node to start from?

A way to solve this problem could be to create only one card builder.
This creation could be explicit (created by the first active audio
controller) or implicit by the audio subsystem on the first controller or
CODEC creation.

Then, the card builder could scan all the DT looking for the audio
ports and create one or more cards according to the graph connectivity.

> > > > Well, forget about this. I never clearly understood why some widgets
> > > > and routes had to be defined at card level.
> 
> > > Please do try to understand the idea of representing simple components
> > > on the board and analogue interconects between devices - it's really
> > > important and not something that can be neglected.
> 
> > The problem is that this understanding would stay abstract: I have no
> > such a hardware. Anyway, if the representation can be done with the
> > simple-card, it may also be done with a graph of ports.
> 
> If you have a device with any sort of speaker or microphone, or any sort
> of external connector for interfacing with an external device like a
> headphone jack, then you have something that could be a widget.

I know what are the widgets and routes, I was just wondering why they
(especially the widgets) need to appear at the card level instead of
just being declared in the DAIs (from the platform or the DT).
And the same question may also be raised about the audio formats, clocks,
tdm's...

> > > That DT binding was done entirely in the context of video applications
> > > IIRC, this is the first time it's been discussed in this context.
> 
> > http://mailman.alsa-project.org/pipermail/alsa-devel/2014-January/070622.html
> > http://mailman.alsa-project.org/pipermail/alsa-devel/2015-January/086273.html
> 
> So there's been some in passing mentions, not really serious discussion
> though...

I may go back about the card builder, but Russell's idea about
declaring the tda998x audio parameters by a port as declared in a graph
of ports seems fine to me. This declaration should be compatible with
the use of the simple-card.

-- 
Ken ar c'hentañ	|	      ** Breizh ha Linux atav! **
Jef		|		http://moinejf.free.fr/