linux-kernel - Re: [PATCH v2] ASoC: fsl_esai: Add ESAI CPU DAI driver

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Message-ID: <20140110132642.GM29039@sirena.org.uk>
Date:	Fri, 10 Jan 2014 13:26:42 +0000
From:	Mark Brown <broonie@...nel.org>
To:	Nicolin Chen <Guangyu.Chen@...escale.com>
Cc:	timur@...i.org, alsa-devel@...a-project.org, robh+dt@...nel.org,
	pawel.moll@....com, mark.rutland@....com,
	ijc+devicetree@...lion.org.uk, galak@...eaurora.org,
	rob@...dley.net, lgirdwood@...il.com, perex@...ex.cz,
	tiwai@...e.de, grant.likely@...aro.org, shawn.guo@...aro.org,
	devicetree@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2] ASoC: fsl_esai: Add ESAI CPU DAI driver

On Fri, Jan 10, 2014 at 09:03:39PM +0800, Nicolin Chen wrote:
> On Fri, Jan 10, 2014 at 12:04:39PM +0000, Mark Brown wrote:

> > This is about what I'd expect but then surely the next step is for the
> > driver to choose a defualt BCLK ratio - that's how most drivers work,
> > they try to generate the exact rate that is needed to clock the data.

> Does that mean I should call set_bclk() once in the startup() when !active
> to set a default bit clock rate to suit a common sample rate like 44100Hz?
> I'm a bit confused if so. Because the driver would call set_bclk() any way
> in the hw_params().

Right, any choice here needs to be deferred to hw_params() as you say.

> But your suggestion just reminds me of the slave mode in SSI driver as
> default mode. And I should patch ESAI to slave mode for default as well,
> shouldn't I?

master/slave selection is kind of orthogonal here - the two bits of
information that are normally needed are the MCLK to use (and its rate)
and the sample rate/format (which give you the BCLK that is needed).
Normally it's then possible to caculate a divider which generates BCLK
from MCLK.  Overriding is normally only needed if there are additional
constraints on BCLK due to something like limitations in one of the
devices or sample rates for the opposite direction if the BCLK is shared
but LRCLK isn't.

> > Are the bit clock shared between playback and capture?

> Only shared in synchronous mode and totally individual in asynchronous mode.
> Each of them can have their own HCK(MCLK) from different sources and derive
> their own SCK(BCLK).

OK, so in asynchronous mode there should be a good chance of a a default
choice working since it won't restrict the other direction.

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