linux-kernel - Re: [PATCH] ASoC: wm8960: Remove bitclk relax condition

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Date:   Tue, 2 Mar 2021 13:19:04 +0000
From:   Charles Keepax <ckeepax@...nsource.cirrus.com>
To:     Shengjiu Wang <shengjiu.wang@....com>
CC:     <lgirdwood@...il.com>, <broonie@...nel.org>, <perex@...ex.cz>,
        <tiwai@...e.com>, <kstewart@...uxfoundation.org>,
        <allison@...utok.net>, <tglx@...utronix.de>, <info@...ux.net>,
        <ckeepax@...nsource.wolfsonmicro.com>,
        <patches@...nsource.cirrus.com>, <alsa-devel@...a-project.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ASoC: wm8960: Remove bitclk relax condition

On Tue, Mar 02, 2021 at 07:18:11PM +0800, Shengjiu Wang wrote:
> From: Daniel Baluta <daniel.baluta@....com>
> 
> Using a higher bitclk then expected doesn't always work.
> Here is an example:
> 
> aplay -Dhw:0,0 -d 5 -r 48000 -f S24_LE -c 2 audio48k24b2c.wav
> 
> In this case, the required bitclk is 48000 * 24 * 2 = 2304000
> but the closest bitclk that can be derived is 3072000. Since
> the clock is faster than expected, it will start to send bytes
> from the next channel so the sound will be corrupted.
> 
> Fixes: 82bab88910ee ("ASoC: codec: wm8960: Relax bit clock computation when using PLL")
> Fixes: 3c01b9ee2ab9 ("ASoC: codec: wm8960: Relax bit clock computation")
> Signed-off-by: Daniel Baluta <daniel.baluta@....com>
> Signed-off-by: Shengjiu Wang <shengjiu.wang@....com>
> ---

I think this is probably going to need a much more involved fix.
The problem is that there are systems that depend on this
behaviour, so you can't just flat out revert it. And to be fair
the I2S specification says that bit clock can run at a higher
rate than required for the data, so the behaviour is correct as
well.

Probably the best solution here is to add additional contraints
from the machine driver on which rates/bit depths/channels are
supported.

Thanks,
Charles