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

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Date:   Fri, 22 Apr 2022 16:06:06 +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 4. 4. 2022, at 14:28, Mark Brown <broonie@...nel.org> wrote:
> 
> On Thu, Mar 31, 2022 at 02:04:47AM +0200, Martin Povišer wrote:
> 
>> +#if 0
>> 				dev_err(rtd->card->dev,
>> 					"N cpus to M codecs link is not supported yet\n");
>> 				return -EINVAL;
>> +#endif
>> +				cpu_dai = asoc_rtd_to_cpu(rtd, 0);
> 
> We need to figure out an interface for describing which CODEC/CPU
> combinations are connected to each other.  I'm not seeing a great way to
> do that right now, probably some side data table is going to be needed,
> or perhaps the CPU DAI drivers can be persuaded to only have one DAI
> actually register and claim to support more channels?  I'm not sure how
> a configuraiton like this is going to work at userspace level if the
> multiple CPU DAIs end up being visible...

To understand the issue better: How could the multiple CPU DAIs be
visible from userspace?

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.

As a reminder what happens on the Macs: the platform driver drives
all the CPU-side I2S ports that belong to the link with the same data,
so the particular CPU/CODEC wiring doesn’t matter.