linux-kernel - Re: [PATCH v9 1/4] drm/i2c: tda998x: Add DT support for audio

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Date:	Fri, 9 Jan 2015 13:54:01 +0100
From:	Jean-Francois Moine <moinejf@...e.fr>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Jyri Sarha <jsarha@...com>, Mark Brown <broonie@...nel.org>,
	Dave Airlie <airlied@...il.com>,
	Andrew Jackson <Andrew.Jackson@....com>,
	alsa-devel@...a-project.org, devicetree@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 1/4] drm/i2c: tda998x: Add DT support for audio

On Fri, 9 Jan 2015 11:45:29 +0000
Russell King - ARM Linux <linux@....linux.org.uk> wrote:

> On Fri, Jan 09, 2015 at 12:30:36PM +0100, Jean-Francois Moine wrote:
> > In my original version, the audio-ports are a bitmap of the pins, the
> > bit 0 being the WS used for I2S. A fully wired tda998x would have been
> > as:
> > 
> > 	audio-ports = <0x03>, <0x04>, <0x09>, <0x11>;
> > 	audio-port-names = "i2s", "spdif", "i2s", "i2s";
> > 
> > With the new version, it would simply become:
> > 
> > 	audio-inputs = "i2s", "spdif", "i2s", "i2s";
> 
> How do you know which i2s inputs to enable?
> 
> Does it make sense for the audio inputs to be mixed like that?
> 
> You will need to enable one i2s for front L+R, and increasingly others
> for the additional channels.
> 
> I think we need to understand exactly how the 998x map I2S inputs to the
> HDMI channels to avoid making a mistake with the binding; remember, the
> binding isn't something that can be easily "bug fixed" at a later date
> as anything we come up with now has to be supported long term by the
> kernel.

The DT describes the hardware configuration.

A fully wired tda998x could be a chip with the audio pins connected to:
- a kirkwood-like audio device with one I2S and one S/PDIF output,
- two other audio devices with one I2S each.

The relation between an audio device and the associated pin is done by
the definition of the sound card.

With S/PDIF, only one stereo channel may be sent, but with I2S, up to 4
stereo channels may be sent. These channels are extracted by the
devices connected on the HDMI bus. There is no mixing.
An example could be the playing of a multi-language movie: each audio
channel carries one language. From the computer view, the playing
application sends each language to one sound card.

So, this means that the tda998x driver should check that S/PDIF and I2S
are not active at the same time, and it should also do a pin OR/AND on
I2S start/stop.

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