Re: Possibility of routing at the kernel level

From: Takashi Iwai <tiwai@suse.de>
To: Mark Farrugia <mark.farrugia@fiberdyne.com.au>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	alsa-devel@alsa-project.org,
	Tharindu De Silva <Tharindu.DeSilva@fiberdyne.com.au>,
	Fulup Ar Foll <fulup.arfoll@iot.bzh>,
	James O'Shannessy <James.OShannessy@fiberdyne.com.au>
Subject: Re: Possibility of routing at the kernel level
Date: Mon, 03 Sep 2018 15:01:16 +0200	[thread overview]
Message-ID: <s5hftyqhh0j.wl-tiwai@suse.de> (raw)
In-Reply-To: <CAPWhKM=qtg9knqFBdG9167GJg3Q=kpzudzk95pLCTX9nYpPTFg@mail.gmail.com>

On Mon, 27 Aug 2018 07:57:00 +0200,
Mark Farrugia wrote:
> 
> Hi all,
> 
> Following my initial proposition on Friday for kernel-space audio
> routing, we at Fiberdyne Systems have since published our initial
> source for such a solution, which we are calling ALSA Virtual Driver,
> or simply AVIRT.
> 
> Please find the source here for your perusal:
> https://github.com/fiberdyne/avirt
> 
> Currently we use module parameters for the kernel module, however, we
> will be porting configuration to another system, such as
> configfs/sysfs.
> 
> We are completely open to suggestions and constructive criticism.

Did you try snd-aloop driver?  I'm not advocating it (as it doesn't
mean a better latency), but it's often an option.

thanks,

Takashi

> 
> Regards,
> Mark
> 
> On Fri, Aug 24, 2018 at 4:27 PM Mark Farrugia
> <mark.farrugia@fiberdyne.com.au> wrote:
> >
> > Hi all,
> >
> > I am currently using the ALSA loopback driver to create a "virtual"
> > sound card to abstract underlying real sound cards.  I capture from
> > the loopback in user-space, and then subsequently playback to a real
> > sound card.
> >
> > The issue with this is that there is a significant latency introduced
> > (~100ms) due to the number of transitions of the stream from user to
> > kernel space. I am seeking a way to avoid this.
> >
> > My question is this: is there any way of routing PCM substreams at the
> > kernel level, so that we can avoid the need to pipe audio back up to
> > user-space with the loopback capturer?  If there is no current
> > solution that would achieve this, I would proposition to design and
> > implement a system that uses a top-level ALSA driver, which then
> > routes dynamically to one or many subscribed lower-level "audio path"
> > drivers.  The system would also have the option to loopback upon
> > itself.
> >
> > Does anyone see any reason that I couldn't (or shouldn't) attempt such
> > a solution? All feedback is welcome.
> >
> > Regards,
> > Mark
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