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

[Samuel Holland <samuel@sholland.org>]

Maxime,

On 9/10/20 9:33 AM, Maxime Ripard wrote:
> 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@kernel.org> 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.

Indeed, for H6 i2s0 with LRCK inversion in software, everything looks correct on
the wire.

It's still concerning to me that the BSP has no evidence of this inversion,
either for i2s0 or i2s1[1]. And the inversion seems not to be required for HDMI
audio on mainline either (but there could be an inversion on the HDMI side or on
the interconnect).

Even so, your research is sufficient justification for me that the code is
correct as-is (with the inversion). Thank you very much for collecting the data!

Cheers,
Samuel

[1]:
https://github.com/Allwinner-Homlet/H6-BSP4.9-linux/blob/e634a960316dddd1eb50f2a6cf237f2f1c6da3e6/sound/soc/sunxi/sunxi-daudio.c#L1062
where 1 == SND_SOC_DAIFMT_NB_NF, and there's no inversion in
sunxi_daudio_init_fmt().