linux-kernel - Re: [RFC PATCH 3/5] HACK: ASoC: Tolerate N-cpus-to-M-codecs links

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Date:   Mon, 25 Apr 2022 15:11:14 +0200
From:   Martin Povišer <povik@...ebit.org>
To:     Mark Brown <broonie@...nel.org>
Cc:     Martin Povišer <povik+lin@...ebit.org>,
        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>
Subject: Re: [RFC PATCH 3/5] HACK: ASoC: Tolerate N-cpus-to-M-codecs links


> On 25. 4. 2022, at 14:55, Mark Brown <broonie@...nel.org> wrote:
> 
> On Mon, Apr 25, 2022 at 02:34:33PM +0200, Martin Povišer wrote:
>>> On 25. 4. 2022, at 14:25, Mark Brown <broonie@...nel.org> wrote:
> 
>>> If you register two separate DAIs (well, links) with the API without
>>> doing anything else the API will just expose them to userspace as two
>>> separate things with no indication that they're related.
> 
>> Sure, but what I am addressing here is a single DAI link with multiple
>> CPU DAIs, invoked in DT like this:
> 
>> 	dai-link@0 {
>> 		link-name = "Speakers";
>> 		mclk-fs = <256>;
>> 
>> 		cpu {
>> 			sound-dai = <&mca 0>, <&mca 1>;
>> 		};
>> 		codec {
>> 			sound-dai = <&speaker_left_woof1>,
>> 				<&speaker_right_woof1>,
>> 				<&speaker_left_tweet>,
>> 				<&speaker_right_tweet>,
>> 				<&speaker_left_woof2>,
>> 				<&speaker_right_woof2>;
>> 		};
>> 	};
> 
> You could parse this into two separate links for the benefit of the
> framewokr if you're using a custom machine driver (which I suspect you
> probably have to).

Yeah, this is parsed by the ‘macaudio’ machine driver from the series.

>>>> What about this interim solution: In case of N-to-M links we put in
>>>> the most restrictive condition for checking capture/playback stream
>>>> validity: we check all of the CPU DAIs. Whatever ends up being the
>>>> proper solution later can only be less restrictive than this.
> 
>>> That's not the issue here?
> 
>> Well to me it looks like it is. Because if I invoke the DAI link like
>> I quoted above, and the platform driver supports it, the playback/capture
>> stream validity check is the only place it breaks down. Notwithstanding
>> this may be the wrong API as you wrote.
> 
> I am surprised that doesn't otherwise explode TBH - at the very least
> I'd expect it to show two PCMs to userspace which if I'm understanding
> your description correctly isn't really what's going on.

I fill in a single snd_soc_dai_link, it exposes a single PCM and works
like a charm. That is as long as I patch the playback/capture check in
question.

I read that to be the clear intention of ASoC code: a DAI link becomes
one snd_soc_pcm_runtime.