linux-kernel - Re: [PATCH v5 1/7] sound: codec: wm8731: add rates constraints

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Message-ID: <20130715152228.GJ11538@sirena.org.uk>
Date:	Mon, 15 Jul 2013 16:22:28 +0100
From:	Mark Brown <broonie@...nel.org>
To:	Richard Genoud <richard.genoud@...il.com>
Cc:	Nicolas Ferre <nicolas.ferre@...el.com>,
	Liam Girdwood <lgirdwood@...il.com>,
	Bo Shen <voice.shen@...el.com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	alsa-devel@...a-project.org, devicetree-discuss@...ts.ozlabs.org
Subject: Re: [PATCH v5 1/7] sound: codec: wm8731: add rates constraints

On Mon, Jul 15, 2013 at 04:53:46PM +0200, Richard Genoud wrote:
> 2013/7/12 Mark Brown <broonie@...nel.org>:

> > This isn't going to work with systems which have a variable clock as the
> > input to the CODEC.  If it's imposing constraints the driver needs to
> > allow setting the clock to zero as a way of removing constraints (and
> > any existing drivers should be updated to do this if needed).

> Maybe I'm wrong, but I didn't find any system using variable clock
> with this codec.

The driver should be written with that possibility in mind even if there
were no users; it only takes a couple of lines of code.

> The sam9g20ek (soc/atmel/sam9g20_wm8731.c) is not using a crystal, but
> it's using a fixed clock anyway.
> But there's soc/pxa/corgi.c and soc/pxa/poodle.c that puzzle me.
> They seems to use a crystal, but they are setting a different sysclk
> depending on the rate.
> That seems wrong, but as I'm a newbie in ASoC...

Note that the CPU is clock master for those - it's going to be
outputting a clock based on the sample rate selected automatically.
These boards would be broken by your change as it stands.

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