[alsa-devel] ASoC: simultaneous start of input/output channels

[Jonah Petri]

Hello Lars,

Thanks!  This is exactly the type of API I had overlooked.  Using
snd_pcm_link() gets the DMA timing pretty close, though not perfect.  It
might be good enough.

If I need further synchronization, I'll take a look at the link
architecture to see what I can use there.

Many thanks again,
Jonah

On Tue, Feb 23, 2016 at 5:42 AM, Lars-Peter Clausen <lars at metafoo.de> wrote:

> On 02/22/2016 05:10 PM, Jonah Petri wrote:
> > Hello!
> >
> > Our application is very low-latency full duplex audio on a Freescale SoC.
> > ALSA is configured with 2 64-frame buffers, with DSP happening on a
> > realtime user thread.
> >
> > With small buffer sizes, we're obviously very tight on timing, and I've
> run
> > into an issue which seems architectural in nature, and seemingly is
> worthy
> > of consulting the list.
> >
> > What we're seeing is a large temporal latency between the DMA completion
> > IRQ for the audio input DMA and audio output DMA. This delay is on the
> > order of 200 microseconds, but also somewhat variable (ok, "large" is
> > relative here, but it matters to us!).  In the ideal case, these two
> > interrupts would be perfectly aligned in time, allowing us to run DSP for
> > the full buffer's duration.  However, since the interrupts are offset by
> > about 20% of a buffer, we effectively lose that time for our DSP.  And,
> > since the timing is variable, and we're in a realtime application, this
> > produces scared engineers.
> >
> > Working backwards from the hardware drivers, I see that snd_soc_dai_ops's
> > trigger callback is used to start and stop the DMA.  The fundamental
> > "start" action comes down to a single register write which can easily
> start
> > both input and output streams in the same write.  However, it seems that
> > the snd_soc_dai_ops does not provide for simultaneous start of both input
> > and output streams.  Tracing back through the kernel interface and into
> > user space, it seems that starting both the input and output streams
> > necessitates two separate calls from userspace, which could explain the
> > delay, and some of the variability in timing.
> >
> > My question is:
> > * Did I miss some API or technique which could allow simultaneous start
> of
> > full duplex streams?
>
> Have a look at snd_pcm_link()
> (
> http://www.alsa-project.org/alsa-doc/alsa-lib/group___p_c_m.html#gac6c33091b049985baa6466e8fe93917e
> ).
> This links two PCM devices together to that operations like start/stop are
> done at the same time. Although this is just a soft-link, which means you
> send one IOCTL and the ALSA core will forward that to both trigger
> functions.
>
> > * If not, I would certainly appreciate advice on where we could fit this
> > into the existing architecture!  We would be glad to do the kernel work
> to
> > make this change, as this capability seems generally useful!
>
> If you can actually link them in hardware and launch both streams with the
> same register write you might need to adopt the internal link architecture
> to support this.
>
> - Lars
>
>