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

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Message-Id: <7F1B2324-4376-4776-BA6D-FBC8892F10C2@cutebit.org>
Date:   Thu, 9 Jun 2022 17:27:12 +0200
From:   Martin Povišer <povik+lin@...ebit.org>
To:     Mark Brown <broonie@...nel.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 9. 6. 2022, at 17:16, Mark Brown <broonie@...nel.org> wrote:
> 
> 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.

OK

>> 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.

OK, adding that to my TODOs.

>>> 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.

OK