linux-kernel - Re: [RFC PATCH v2 5/5] ASoC: apple: Add macaudio machine driver

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Message-ID: <YqIOwrRN8rPUF7Y8@sirena.org.uk>
Date:   Thu, 9 Jun 2022 16:16:18 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Martin Povišer <povik+lin@...ebit.org>
Cc:     Liam Girdwood <lgirdwood@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>, alsa-devel@...a-project.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mark Kettenis <kettenis@...nbsd.org>,
        Hector Martin <marcan@...can.st>,
        Sven Peter <sven@...npeter.dev>, asahi@...ts.linux.dev
Subject: Re: [RFC PATCH v2 5/5] ASoC: apple: Add macaudio machine driver

On Thu, Jun 09, 2022 at 04:09:57PM +0200, Martin Povišer wrote:
> > On 9. 6. 2022, at 15:33, Mark Brown <broonie@...nel.org> wrote:

> >> +		/*
> >> +		 * Primary FE
> >> +		 *
> >> +		 * The mclk/fs ratio at 64 for the primary frontend is important
> >> +		 * to ensure that the headphones codec's idea of left and right
> >> +		 * in a stereo stream over I2S fits in nicely with everyone else's.
> >> +		 * (This is until the headphones codec's driver supports
> >> +		 * set_tdm_slot.)
> >> +		 *
> >> +		 * The low mclk/fs ratio precludes transmitting more than two
> >> +		 * channels over I2S, but that's okay since there is the secondary
> >> +		 * FE for speaker arrays anyway.
> >> +		 */
> >> +		.mclk_fs = 64,
> >> +	},

> > This seems weird - it looks like it's confusing MCLK and the bit clock
> > for the audio bus.  These are two different clocks.  Note that it's very
> > common for devices to require a higher MCLK/fs ratio to deliver the best
> > audio performance, 256fs is standard.

> On these machines we are not producing any other clock for the codecs
> besides the bit clock, so I am using MCLK interchangeably for it. (It is
> what the sample rate is derived from after all.)

Please don't do this, you're just making everything needlessly hard to
understand by using standard terminology inappropriately and there's a
risk of breakage further down the line with drivers implementing the
standard ops.

> One of the codec drivers this is to be used with (cs42l42) expects to be
> given the I2S bit clock with

>   snd_soc_dai_set_sysclk(dai, 0, mclk, SND_SOC_CLOCK_IN);

That's very, very non-standard...

> I can rename mclk to bclk in all of the code to make it clearer maybe.
> Also the platform driver can take the bit clock value from set_bclk_ratio,
> instead of set_sysclk from where it takes it now. The cs42l42 driver I can
> patch too to accept set_bclk_ratio.

...indeed, set_bclk_ratio() is a better interface for setting the bclk
ratio - the CODEC driver should really be doing that as well.

> > This shouldn't be open coded in a driver, please factor it out into the
> > core so we've got an API for "set limit X on control Y" then call that.

> There’s already snd_soc_limit_volume, but it takes a fixed control name.
> Can I extend it to understand patterns beginning with a wildcard, like
> '* Amp Gain Volume’?

Or add a new call that does that.

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