Re: [PATCH v3 3/7] ASoC: sun4i-i2s: Add support for H6 I2S

[Maxime Ripard]

Hi Samuel,

On Thu, Sep 03, 2020 at 09:54:39PM -0500, Samuel Holland wrote:
> On 9/3/20 3:58 PM, Maxime Ripard wrote:
> > On Thu, Sep 03, 2020 at 10:02:31PM +0200, Clément Péron wrote:
> >> Hi Maxime,
> >>
> >> On Wed, 29 Jul 2020 at 17:16, Mark Brown <broonie@xxxxxxxxxx> wrote:
> >>>
> >>> On Wed, Jul 29, 2020 at 04:39:27PM +0200, Maxime Ripard wrote:
> >>>
> >>>> It really looks like the polarity of LRCK is fine though. The first word
> >>>> is sent with LRCK low, and then high, so we have channel 0 and then
> >>>> channel 1 which seems to be the proper ordering?
> 
> Which image file is this in reference to?
> 
> >>> Yes, that's normal.
> >>
> >> Thank you very much for this test.
> >>
> >> So I will revert the following commit:
> >>
> >> ASoC: sun4i-i2s: Fix the LRCK polarity
> >>
> >> https://github.com/clementperon/linux/commit/dd657eae8164f7e4bafe8b875031a7c6c50646a9
> > 
> > Like I said, the current code is working as expected with regard to the
> > LRCK polarity. The issue is that the samples are delayed and start to be
> > transmitted on the wrong phase of the signal.
> 
> Since an I2S LRCK frame is radially symmetric, "wrong phase" and "inverted
> polarity" look the same. The only way to definitively distinguish them is by
> looking at the sample data.
> 
> In "i2s-h6.png", the samples are all zeroes, so you're assuming that the first
> sample transmitted (that is, when the bit clock starts transitioning) was a
> "left" sample.
> 
> However, in "h6-i2s-start-data.png", there are pairs of samples we can look at.
> I'm still assuming that similar samples are a left/right pair, but that's
> probably a safe assumption. Here we see the first sample in each pair is
> transmitted with LRCK *high*, and the second sample in the pair is transmitted
> with LRCK *low*. This is the opposite of your claim above.
> 
> An ideal test would put left/right markers and frame numbers in the data
> channel. The Python script below can generate such a file. Then you would know
> how much startup delay there is, which channel the "first sample" came from, and
> how each channel maps to the LRCK level.
>
> It would also be helpful to test DSP_A mode, where the LRCK signal is
> asymmetric and an inversion would be obvious.

I had no idea that there was a wave module in Python, that's a great
suggestion, thanks!

You'll find attached the screenshots for both the I2S and DSP_A formats.
I zoomed out a bit to be able to have the first valid samples, but it
should be readable.

The code I used is there:
https://github.com/mripard/linux/tree/sunxi/h6-i2s-test

It's basically the v3, plus the DT bits.

As you can see, in the i2s case, LRCK starts low and then goes up, with
the first channel (0x2*** samples) transmitted first, so everything
looks right here.

On the DSP_A screenshot, LRCK will be low with small bursts high, and
once again with the first channel being transmitted first, so it looks
right to me too.

Maxime

Attachment: h6-i2s-samuel-wav-i2s.png
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Attachment: h6-i2s-samuel-wav-dsp-a.png
Description: PNG image

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