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

[Jonah Petri]

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?
* 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!

Thanks much for any help,

Jonah