Message-ID: <20130810093137.GV23006@n2100.arm.linux.org.uk>
Date:	Sat, 10 Aug 2013 10:31:37 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Mark Brown <broonie@...nel.org>
Cc:	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
	Jean-Francois Moine <moinejf@...e.fr>,
	Liam Girdwood <lgirdwood@...il.com>,
	Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.de>,
	Rob Herring <rob.herring@...xeda.com>,
	alsa-devel@...a-project.org, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 1/1] ASoc: kirkwood: add DT support to the mvebu
	audio subsystem

On Sat, Aug 10, 2013 at 12:42:09AM +0100, Mark Brown wrote:
> On Fri, Aug 09, 2013 at 09:38:47PM +0100, Russell King - ARM Linux wrote:
> > On Fri, Aug 09, 2013 at 08:44:34PM +0100, Mark Brown wrote:
> 
> > > If someone wants to it should also be possible to convert the existing
> > > platforms without S/PDIF support over to DT, providing you don't mind
> > > changing the code once the DPCM and S/PDIF support is added and a bit of
> > > thought is put into where the S/PDIF output will fit into the bindings.
> 
> > Okay, so you're thinking that the I2S output will be enabled in the
> > absence of DPCM?  If so, that tells me that you don't understand my
> 
> When I say that it should be possible to convert the existing machines
> over to DT I'm talking about the machines that are supported with the
> drivers currently in mainline.  These machines presumably work just fine
> with the existing drivers which support the I2S only subset of the
> possible functionality in the hardware without using DCPM.  These are
> the the machines that could have DT added now without implementing the
> driver improvements to enable the extra hardware functionality.
> 
> When I say that they will need changing once DPCM and S/PDIF support is
> added what I mean is that those changes to the CPU side drivers should,
> as you correctly say, require all Kirkwood machine drivers to use DPCM
> even if only due to the handling of simultaneous startup of the S/PDIF
> and I2S interfaces.
> 
> The "you" in the above quote was intended to refer to people working on
> Kirkwood in general, not you personally unless you want to.
> 
> > What this means is that the conventional setup where you have _one_ DAI
> > link connecting the codec DAI to the CPU DAI won't work on Kirkwood
> > anymore, because there is no way to know which of the two outputs
> > should be enabled.  I avoided that in my patches, but you've objected
> > to that saying that it must use DPCM.  That makes the whole of kirkwood
> > entirely DPCM only, whether or not you have a single codec to connect.
> 
> Right, that's where the drivers should end up.  In terms of the device
> tree I would expect that it should be the case that there are distinct
> I2S and S/PDIF interfaces visible, or at least that there's some way to
> identify which interface on the SoC is being referred to - this is the
> thought about where the S/PDIF output will fit into the bindings that I
> mentioned being required.
> 
> So long as only I2S is used in the system (or at least any S/PDIF that
> is present is ignored at runtime) it should be possible to continue
> using the existing driver infrastructure until S/PDIF and DPCM support
> is added and then transition the existing drivers (both non-DT and any
> DT ones that get added or converted) over to that as part of the
> conversion.
> 
> In other words I was talking about a possible intermediate state where
> people are working with the DT bindings but the driver support for
> systems with S/PDIF in use is not there yet.  That's not required but it
> should be possible if there's interest in progressing the DT bindings
> while the driver is as it stands.

Right, so what you're proposing is to come up with a DT description for
the existing stuff, and then have to change (or at the very least augment)
that description later when the DPCM stuff goes in.

What should be done is to sort out DPCM, get that working and then sort out
the DT side of it, so that everyone gets only one transition to DT, not a
transition to a half-baked stop-gap DT and then have to go through another
round of DT changes.  "Because we can" is not a good enough reason not to
get this sorted properly.
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