linux-kernel - Re: [PATCH] sound/soc/codecs/wm8978: add missing BCLK divider setup

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Message-ID: <aOUSZ2pnxRfxEPi4@opensource.cirrus.com>
Date: Tue, 7 Oct 2025 14:15:19 +0100
From: Charles Keepax <ckeepax@...nsource.cirrus.com>
To: Sune Brian <briansune@...il.com>
Cc: Liam Girdwood <lgirdwood@...il.com>, Mark Brown <broonie@...nel.org>,
        Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>,
        linux-sound@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sound/soc/codecs/wm8978: add missing BCLK divider setup

On Tue, Oct 07, 2025 at 08:48:40PM +0800, Sune Brian wrote:
> Charles Keepax <ckeepax@...nsource.cirrus.com> 於 2025年10月7日 週二 下午8:11寫道：
> > On Tue, Oct 07, 2025 at 07:22:10PM +0800, Sune Brian wrote:
> > > Charles Keepax <ckeepax@...nsource.cirrus.com> 於 2025年10月7日 週二 下午6:30寫道：
> > > > On Fri, Oct 03, 2025 at 05:13:04PM +0800, Brian Sune wrote:
> > > > > The original WM8978 codec driver did not set the BCLK (bit clock)
> > Its not missing its right there. That said your way is probably
> > slightly more standard these days, but we should take care of the
> > interaction between the two.
> 
> What my missing meant is if run with DEBUG flag on that case had never
> behave as expected.
> MCLK and LRCLK both is correctly outputted. While the current
> unpatched version will generate
> wrong BCLK complete break the codec. As such I proposed the BCLK patch.
> I had not investigate deep why it never calls but the "int div" is
> loaded and computed by where is a bit puzzling.
> And the loaded it simply with div on actual mclk/2/bit_per_channel is
> also incorrect.
> As mentioned in previous explanations, the clock register is a fix
> table on dividing # that is a LUT with restricted # allowed.

Yeah the existing code expects the machine driver to call
snd_soc_dai_set_clkdiv. I am guessing you are using something
like simple card that doesn't do this.

To be clear the bulk of your patch is good, updating this in
hw_params is probably more normal these days. But we need to
make sure the two paths don't interfere with each other. Think
of a system that is already calling snd_soc_dai_set_clkdiv() to
set BCLKDIV, after your patch BCLKDIV will be set twice
potentially to two different values and would generate no error
messages.

I think you have two options:

1) Remove WM8978_BCLKDIV from wm8978_set_dai_clkdiv. There are no
   upstream users that I can see, so this should be fine. This
   would mean an out of tree user of snd_soc_dai_set_clkdiv would
   now get an error so they know they need to fix something.
2) Only run your dynamic BCLK code if wm8978_set_dai_clkdiv
   hasn't been called. This would mean any out of tree users of
   snd_soc_dai_set_clkdiv would have no problems everything would
   keep working as before, but at the cost of a little complexity
   in the code.

I am happy with either approach so which ever you prefer is fine
with me.

Thanks,
Charles