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

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Message-ID: <YmaTHTKWAfM7FCcY@sirena.org.uk>
Date:   Mon, 25 Apr 2022 13:25:01 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Martin Povišer <povik@...ebit.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 Fri, Apr 22, 2022 at 04:06:06PM +0200, Martin Povišer wrote:
> > On 4. 4. 2022, at 14:28, Mark Brown <broonie@...nel.org> wrote:

> > 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?

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.

> 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?  

> 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.

Oh, that's not something I was aware of.  In that case this is the wrong
API - you should be using DPCM to map one front end onto multiple back
ends (Kirkwood does something similar IIRC, there will be other examples
but that's probably the simplest).  The back ends probably don't really
need to know that they're on the same physical bus (if indeed they are).

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