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

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Message-ID: <CAA+D8AMriHO60SD2OqQSDMmi7wm=0MkoW6faR5nyve-j2Q5AEQ@mail.gmail.com>
Date:   Fri, 11 Aug 2023 19:05:43 +0800
From:   Shengjiu Wang <shengjiu.wang@...il.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     Takashi Iwai <tiwai@...e.de>, 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

Hi Mark, Takashi

On Thu, Aug 3, 2023 at 9:11 PM Shengjiu Wang <shengjiu.wang@...il.com> wrote:
>
> On Thu, Aug 3, 2023 at 1:28 AM Mark Brown <broonie@...nel.org> wrote:
> >
> > On Wed, Aug 02, 2023 at 10:41:43PM +0800, Shengjiu Wang wrote:
> >
> > > Currently the ASRC in ALSA is to connect to another I2S device as
> > > a sound card.  But we'd like to the ASRC can be used by user space directly
> > > that user space application can get the output after conversion from ASRC.
> >
> > That sort of use case would be handled via DPCM at the minute, though
> > persuading it to connect two front ends together might be fun (which is
> > the sort of reason why we want to push digital information down into
> > DAPM and make everything a component).
>
> Thanks.
>
> ASRC M2M case needs to run as fast as possible, no sync clock control.
> If use sound card to handle ASRC M2M case,  the user application
> should be aplay/arecord, then we need to consider xrun issue, buffer
> may timeout, sync between aplay and arecord,  these should't be
> considered by pure memory to memory operation.
>
> DPCM may achitect all the audio things in components and sound
> card,  it is good. but for the M2M case, it is complcated. not sure
> it is doable.
>

Beside the concern in previous mail,

DPCM needs to separate ASRC to be two substreams (playback and capture).

But the ASRC needs the sample rate & format of input and output first
then start conversion.

If the playback controls the rate & format of input,  capture substream
controls the rate & format of output,  as a result
one substream needs to get information(dma buffer address, size...
rate, format) from another substream, then start both substreams in the
last substream. How to synchronize these two substreams is a problem.
One stream can be released but another stream doesn't know .

So I don't think it is a good idea to use DPCM for pure M2M case.

So can I persuade you to consider the V4L2 solution?

Best regards
Wang Shengjiu