linux-kernel - Re: [PATCH v4 2/4] dt-bindings: sound: add rockchip i2s-tdm binding

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Message-ID: <20210916122549.GF5048@sirena.org.uk>
Date:   Thu, 16 Sep 2021 13:25:49 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Nicolas Frattaroli <frattaroli.nicolas@...il.com>
Cc:     Liam Girdwood <lgirdwood@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Heiko Stuebner <heiko@...ech.de>,
        linux-rockchip@...ts.infradead.org, alsa-devel@...a-project.org,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/4] dt-bindings: sound: add rockchip i2s-tdm binding

On Wed, Sep 15, 2021 at 07:06:14PM +0200, Nicolas Frattaroli wrote:
> On Mittwoch, 15. September 2021 16:10:12 CEST Mark Brown wrote:

> > Why is this not part of the normal bus format configuration?  I don't
> > know what this is but it sounds a lot like I2S mode...

> This affects all TDM I2S modes, i.e. TDM Normal, TDM Left Justified and TDM 
> Right Justified.

> Without tdm-fsync-half-frame, we purportedly get the following output in TDM 
> Normal Mode (I2S Format):
> (ch0l = channel 0 left, ch0r = channel 0 right)

> fsync: 	_____________________________
>                      	            \____________________________
> sdi/sdo: ch0l, ch0r, ..., ch3l, ch3r,  ch0l, ch0r, ...
> 
> With tdm-fsync-half-frame, we purportedly get the following:
> 
> fsync: 	_____________________________
>                      	            \____________________________
> sdi/sdo: ch0l,  ch1l,  ch2l,  ch3l,   ch0r,  ch1r,  ch2r,  ch3r

> At least, according to the TRM. I do not have an oscilloscope to verify this 
> myself, and in the following paragraphs, I will elaborate why this seems 
> confusing to me.

fsync-half-frame is just normal TDM for I2S, the default mode is how DSP
mode normally operates.  I don't know that there's any pressing need to
support mix'n'match here, you could but it should be through the TDM
configuration API.

> So to answer the question, it's not part of the bus format because it applies 
> to three bus formats, and I'm completely out of my depth here and wouldn't 
> define three separate bus formats based on my own speculation of how this 
> works.

It is part of the bus format really.  I suspect the hardware is the kind
that only really implements DSP mode and can just fake up a LRCLK for
I2S in order to interoperate.

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