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

[Nicolas Frattaroli]

On Mittwoch, 15. September 2021 16:10:12 CEST Mark Brown wrote:
> On Sat, Sep 04, 2021 at 01:15:34AM +0200, Nicolas Frattaroli wrote:
> > +  rockchip,tdm-fsync-half-frame:
> > +    description: Whether to use half frame fsync.
> > +    type: boolean
> > +
> 
> 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.

The comment block "DAI hardware signal polarity" in soc-dai.h seems to imply 
that what the TRM says the tdm-fsync-half-frame mode is (if one inverts fsync 
polarity of those waveforms), is what is expected:

> * FSYNC "normal" polarity depends on the frame format:
> * - I2S: frame consists of left then right channel data. Left channel starts
> *      with falling FSYNC edge, right channel starts with rising FSYNC edge.
> * - Left/Right Justified: frame consists of left then right channel data.
> *      Left channel starts with rising FSYNC edge, right channel starts with
> *      falling FSYNC edge.

I don't know if this is only applicable to non-TDM I2S, and whether it's 
normal to have the channels interleaved like that in TDM.

I don't see any DAIFMT that does what this does in any case.

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.