linux-kernel - Re: [RFC PATCH v2 0/7] Add audio support in v4l2 framework

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Message-ID: <ZOe74PO4S6bj/QlV@finisterre.sirena.org.uk>
Date:   Thu, 24 Aug 2023 21:21:52 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     Shengjiu Wang <shengjiu.wang@...il.com>,
        Hans Verkuil <hverkuil@...all.nl>,
        Shengjiu Wang <shengjiu.wang@....com>, sakari.ailus@....fi,
        tfiga@...omium.org, m.szyprowski@...sung.com, mchehab@...nel.org,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        Xiubo.Lee@...il.com, festevam@...il.com, nicoleotsuka@...il.com,
        lgirdwood@...il.com, perex@...ex.cz, tiwai@...e.com,
        alsa-devel@...a-project.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH v2 0/7] Add audio support in v4l2 framework

On Thu, Aug 24, 2023 at 07:03:09PM +0200, Takashi Iwai wrote:
> Shengjiu Wang wrote:

> > But there are several issues:
> > 1. Need to create sound cards.  ASRC module support multi instances, then
> > need to create multi sound cards for each instance.

> Hm, why can't it be multiple PCM instances instead?

I'm having a hard time following this one too.

> > 2. The ASRC is an entirety but with DPCM we need to separate input port and
> > output port to playback substream and capture stream. Synchronous between
> > playback substream and capture substream is a problem.
> > How to start them and stop them at the same time.

> This could be done by enforcing the full duplex and linking the both
> PCM streams, I suppose.

> > So shall we make the decision that we can go to the V4L2 solution?

> Honestly speaking, I don't mind much whether it's implemented in V2L4
> or not -- at least for the kernel part, we can reorganize / refactor
> things internally.  But, the biggest remaining question to me is
> whether this user-space interface is the most suitable one.  Is it
> well defined, usable and maintained for the audio applications?  Or
> is it meant to be a stop-gap for a specific use case?

I'm having a really hard time summoning much enthusiasm for using v4l
here, it feels like this is heading down the same bodge route as DPCM
but directly as ABI so even harder to fix properly.  That said all the
ALSA APIs are really intended to be used in real time and this sounds
like a non real time application?  I don't fully understand what the
actual use case is here.

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