On Sun, Oct 03, 2021 at 05:57:34PM +0200, Jerome Brunet wrote:

On Sun 03 Oct 2021 at 01:43, Martin Blumenstingl [off-list ref] wrote:

quoted

Hello Jerome et al.,

on Amlogic Meson SoCs which use the AIU audio controller we have the
so-called "machine gun noise" (MGN) issue, reproducible for example in
Kodi. So far nobody was able to identify the cause of this issue. My
idea was to at least narrow down the issue so we can categorize it. For
that I wanted to get the SPDIF output from AIU to the HDMI controller
working.

Ok, I'll clarify (again) about this issue.

This issue is on AIU only (not the AXG series), on the i2s encoder
regardless of the output (HDMI, internal and external codec)

This encoder support either 2ch or 8ch mode, nothing else.
As long as only 2ch was supported, there was no problem.
In order to switch between 2ch and 8ch mode.  AIU_RST_SOFT_I2S_FAST and
AIU_I2S_SYNC shall be "poked" (check iu-fifo-i2s.c and
aiu-encoder-i2c.c). Without this, the switch is not done.

What these do, how and when it should be done is unclear. I tried
several combination. At the time I found something stable-ish but it was
not perfect.

At the time, It was completely new driver. Even if was not rock solid,
it was still progress and I opted to upstream it with an imperfect 8ch
support so people could help debug it. This was mentioned in the
original submission.

The other solution is to restrict to 2ch mode and remove
AIU_RST_SOFT_I2S_FAST and AIU_I2S_SYNC pokes. There will be no noise
anymore.

Hi, Jerome,

Thanks for the extensive clarification. I have compiled 5.15-rc3
commenting out the pokes for 8ch as you have suggested, both for
AIU_RST_SOFT_I2S_FAST and AIU_I2S_SYNC in both aiu-fifo-i2s.c
and aiu-encoder-i2c.c.

It works, the bug is gone just like you said it would be.

My suggestion is that, for the time being, until someone can solve
this properly, you restrict enablement of 8ch configuration under
an EXPERIMENTAL kernel option, with a big fat warning in the help
description, saying it will break 2ch.

Last but no least, thanks for helping Martin out.

Thank you,
Geraldo Nascimento

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