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

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Message-ID: <54AFDE8D.3000504@arm.com>
Date:	Fri, 09 Jan 2015 13:58:37 +0000
From:	Andrew Jackson <Andrew.Jackson@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Jean-Francois Moine <moinejf@...e.fr>
CC:	Jyri Sarha <jsarha@...com>, Mark Brown <broonie@...nel.org>,
	Dave Airlie <airlied@...il.com>,
	"alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 1/4] drm/i2c: tda998x: Add DT support for audio

On 01/09/15 13:07, Russell King - ARM Linux wrote:
> On Fri, Jan 09, 2015 at 01:54:01PM +0100, Jean-Francois Moine wrote:
>> On Fri, 9 Jan 2015 11:45:29 +0000
>> Russell King - ARM Linux <linux@....linux.org.uk> wrote:
>>> 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.
> 
> You're missing my point.
> 
> How does the driver know which of the I2S pins to enable in I2S mode?

[snip]

> My question is: how do we know which I2S inputs to enable, or are
> you suggesting that all I2S inputs should be enabled if operating in
> I2S mode irrespective of whether they may be active?
> 

Isn't it the case that for I2S:

* Word Select (WS) is always required to disambiguate left/right. So WS need
  not be specified in any device tree configuration.

* The audio inputs depend on a particular board but at least one is 
  required.  Fortunately, the TDA998x devices have all the same pin/input
  numbering so they /could/ be described as i2s0, i2s1, i2s2 and i2s3 as 
  per J-F's earlier email.  But tt isn't clear from my reading of the 
  TDA19988 datasheet (for example) whether one can skip channels (so
  does channel 0 always have to be present?).  If the channels must
  be enabled from 0 then one could simply specify the number of inputs.
  (The datasheets that I have don't indicate whether there's any 
  channel remapping performed internally by the TDA998x).

* What the driver does need to take account of is the number of inputs
  that are active so that the sound CODEC can be properly configured.

	Andrew

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