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

[Samuel Holland <samuel@sholland.org>]

Maxime,

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.

> But the LRCK polarity is fine.
> 
> Maxime
> 

Samuel

----8<----
import wave
from struct import Struct

markers = (0x2, 0xe)
rate    = 8000
seconds = 10

struct  = Struct('<' + 'H' * len(markers))
nframes = seconds * rate
data    = bytearray(nframes * struct.size)

for i in range(nframes):
    frame  = [(m << 12) + (i % 2**12) for m in markers]
    offset = i * struct.size
    struct.pack_into(data, offset, *frame)

with wave.open('test.wav', 'wb') as wf:
    wf.setparams((len(markers), 2, rate, nframes, 'NONE', ''))
    wf.writeframes(data)